The present invention relates to an accelerometer assembly and to a method of using the same. The accelerometer assembly finds use in the measurement of movement of a person and to treatment of the person.

Drop foot (sometimes also referred to as foot drop) is a condition caused by weakness or paralysis of the muscles involved in lifting the front part of the foot, which causes a person to drag the toe of the shoe on the ground or slap the foot on the floor. Drop foot can result from peroneal nerve palsy or from damage to the central nervous system such as stroke, spinal cord injury, traumatic brain injury, cerebral palsy and multiple sclerosis. The correction of the condition of drop foot in patients can be achieved by the electrical stimulation of the common peroneal nerve and contraction of the anterior tibialis muscle. Systems for correcting drop foot in subjects in this way, in particular Functional Electrical Stimulation (FES) systems, are known in the art and are commercially available. In known devices and methods of treating drop foot, the timing for the electrically stimulated correction is achieved using a switch that is located under the heel, typically in a shoe or other item of footwear. A reliable signal is generated which is synchronised to the gait cycle. However it is well known that users of such devices would prefer less cumbersome, alternative solutions. In particular, known devices generally require a pair of wires for providing a signal pathway from the foot switch to the neuromuscular stimulator, typically mounted on a belt around the waist of a person. Patients find such arrangements awkward to use.

It would be advantageous if an alternative to the aforementioned arrangement relying on a switch located beneath the heel of the subject could be found, in order to provide a less cumbersome device that is less intrusive and easier to use by the patient. Any effective alternative stimulation cycle must be synchronised to the gait cycle so one proposed method under investigation has been to measure movements of the subject, in particular the knee angle movements, and timing of movement events that are synchronised to the gait cycle to generate an alternative signal. Furthermore, it would be most advantageous if information could be obtained about the gait events, for example the detection of the position of the tibia of the subject, especially to determine when the tibia is vertical or substantially vertical. This information can then be used to stimulate the gluteal or hamstring muscles for the correction of different gait problems in a neurological patient.

A traditional method of measuring joint angles is to use goniometers. Mulder et al., "Artificial-reflex stimulation for FES-induced standing with minimum quadriceps force," Medical and Biological Engineering and Computing, vol. 28, pp. 483-488, 1990, and Veltink et al. "Detection of knee instability using accelerometers tfs and potential use in the control of FES-assisted paraplegic standing," 1993, pp. 1232- 1233, have used goniometers in an experimental test to measure knee angle and angular velocity. From this information, the transition from knee lock to unlock positions can be distinguished and the stimulation level of the quadriceps muscle of the subject can be controlled. However the use of goniometers has severe practical limitations, in particular when used in combination with Functional Electrical

Stimulation (FES) systems. First, the attachment usually requires double-sided tape for securing to the patient. Second, devices using the goniometer solution also tend to be bulky around the joint being measured. It has been reported that a goniometer can easily slide from the original position attached to the joint; therefore it needs to be frequently re-calibrated whenever a measurement is need (reference in this respect being made to E. C. Patrick et al., "Sensors for Use with Functional

Neuromuscular Stimulation," Biomedical Engineering, IEEE Transactions on, vol. BME-33, pp. 256-268, 1986). In addition, a goniometer can be easily broken and are vulnerable to damage. Further, goniometers generally have a high power consumption. A further problem lies in that goniometers are not easy to align with the joint axis which makes the result imprecise (reference being made to R.

Williamson et al. "Detecting absolute human knee angle and angular velocity using accelerometers and rate gyroscopes," Medical & Biological Engineering &

Computing, vol. 39, pp. 294-302, May 2001 ).

The gyroscope is another sensor that is able to perform kinematic

measurement. Gyroscopes are available as micro-machine sensors. This micro- machine sensor is tiny, inexpensive and lightweight. The output signal from the sensor is proportional to angular velocity that on integration can produce the angle information. Recently, a gyroscope has been used as a gait detector to control stimulation timing (I. P. I. Pappas et al., "A reliable gyroscope-based gait-phase detection sensor embedded in a shoe insole," IEEE Sensors Journal, vol. 4, pp. 268- 274, Apr 2004; S. N. Ghoussayni et al., "Experience in the use of a single Gyroscope as a sensor for FES foot drop correction systems," in 9th Annual Conference of the International FES Society Bournemouth, UK, 2004; and C. C. Monaghan et al., "Control of Triceps Surae Stimulation based on shank orientation using a uniaxial gyroscope " in 9th Annual Conference of the International FES Society Bournemouth, UK, 2004). Unlike the footswitch, this detector not only detects heel strike and heel rise but it is also capable of determining the position of the leg and foot of the subject during the gait cycle. However a gyroscope has one common problem, that is drift caused by integration (see P. H. Veltink et al., "Inertial sensing in a hand held dynamometer," 1996, pp. 502-503 vol.2). For a short time measurement, drift can be a significant problem even if there is little variation in the DC offset/zero frequency component. Monaghan et al., noted above, addressed this problem by having the system reset the detection system process each time before the next integration takes place.

Another possible motion sensor is an accelerometer. Accelerometers are also available as micro-machined sensors that are small, inexpensive, lightweight and have a low power consumption. The output signal of the accelerometer is proportional to the linear acceleration of the device. Accelerometers have many applications. Typically they are used to measure tilt/roll, vehicle skid, impact and detect vibration. Accelerometers have been employed in measuring the movement of a person, in particular their gait. In particular, in the measurement of gait, accelerometers have already been used to measure lower limb joints angles in realtime (A. T. M. Willemsen et al., "Real-time gait assessment utilizing a new way of accelerometry," Journal of Biomechanics, vol. 23, pp. 859-863, 1990); to evaluate everyday physical movement (C. V. C. Bouten et al., "A triaxial accelerometer and portable data processing unit for the assessment of daily physical activity," IEEE Transactions on Biomedical Engineering, vol. 44, pp. 136-147, Mar 1997) and to detect gait events during walking (A. T. M. Willemsen et al., "Automatic stance-swing phase detection from accelerometer data for peroneal nerve stimulation" IEEE Transactions on Biomedical Engineering, vol. 37, pp. 1201-1208, Dec 1990; R.

Williamson et al., "Gait event detection for FES using accelerometers and supervised machine learning," Rehabilitation Engineering, IEEE Transactions on Neural Systems and Rehabilitation], vol. 8, pp. 312-319, 2000; and A. Mansfield et al., "The use of accelerometry to detect heel contact events for use as a sensor in FES assisted walking," Medical Engineering & Physics, vol. 25, pp. 879-885, Dec 2003).

Integration with additional signal or signals is still required to calculate an angle from either angular acceleration or angular velocity when using an accelerometer and this integration begins to produce incorrect results due to drift within the measurement system. Willemsen et al. (noted above) have presented results for the calculated lower limb joints angles during walking, sitting and standing without the need to use integration. Williamson et al. (noted above) used a combination of two sensors, accelerometers and gyroscopes to estimate knee angle and angular velocity. The offset and drift from the gyroscopes were cancelled out by using auto-resetting and auto-nulling algorithms. In 1996, Veltink et al. (noted above) compared two methods, first using the integration signal of a gyroscope and second using tangential and radial acceleration from an accelerometer together with a differentiated signal of the gyroscope to estimate knee angle. The results of the study suggest that the second method gives more accurate results, while the first method has drift problems due to the signal integration. All existing approaches to the use of accelerometers have used such devices along a single axis only, combining, as described above, the outputs of the sensor with other devices, such as gyroscopes.

There are also known inertia! sensor systems that measure three dimensional translational and rotational movement. However, such systems suffer from the aforementioned disadvantages by being bulky and having a high power consumption. Unfortunately, as noted previously, these two factors represent major drawbacks in many applications, including a battery powered neuromuscular stimulator if it is to be reliable, yet also unobtrusive and aesthetic to the user. For example, a single axis rate gyro for example dissipates 30 mW (ADXRS150, 6mA at 5V). Recently, Simcox et al., "Performance of orientation sensors for use with a functional electrical stimulation mobility system," Journal of Biomechanics, vol. 38, pp. 1 185-1190, May 2005, used five sensor packs, each consisting of a single rate gyroscope and a pair of two axis accelerometers, arranged in the standard three dimensional way, to measure trunk and lower limb orientation. Jasiewicz et al., "Gait event detection using linear accelerometers or angular velocity transducers in able-bodied and spinal-cord injured individuals," Gait & Posture, vol. 24, pp. 502-509, Dec 2006, used the same sensor pack to detect heel rise and heel strike events during gait cycle.

There is a need for an improved device for measuring movement of a body or a subject, in particular one that is suitable for use in analysing the movement of a human or animal subject. There is a particular need for an apparatus for accurately analysing the gait of a person. It would be most advantageous if the device could operate in conjunction with a system for providing electrical stimulation to the person, for example an FES system, for example to assist in correcting drop foot. It would be preferable if the device could overcome some or all of the aforementioned problems associated with known systems and methods.

More generally, there is also a need for an improved device for measuring both the acceleration of a moving body and the angle of movement of the body. It would be particularly advantageous if such a device could employ accelerometers and dispense with other means, in particular gyroscopes, for determining both the acceleration and the angle of movement.

It has now been found that a plurality of accelerometers comprising at least three accelerometers can be arranged in a novel way, so as to detect complex movement patterns in a plane, in particular to provide a determination of both the acceleration and the angle of movement in the plane. In a preferred arrangement, two or more pairs of accelerometers, in particular two or more two-axes

accelerometers, are employed. A device incorporating the aforementioned arrangement of accelerometers, in particular multiple two-axes accelerometers arranged in such a manner, is particularly advantageous in analysing the movement of a person and is especially suitable for analysing the gait of the person. Such a device is also very well suited to providing an input signal for a system for electrically stimulating the subject, such as an FES system. Accordingly, in a first aspect of the present invention, there is provided an apparatus for detecting the movement of a body, the apparatus comprising three accelerometers, each accelerometer having an axis, the accelerometers being arranged such that the axes of the three accelerometers lie in a single plane or in parallel planes and are fixed in relation to a point, the accelerometers being arranged about the point to have their axes extending at non-zero angles to each other.

As indicated, the accelerometers may have their axes lying in a single plane. However, it is to be understood that the accelerometers may be displaced from one another and have their axes extending in different planes. In such a case, the axes of the accelerometers extend in planes that are parallel to one another. The operation of the present invention remains the same, with the same ability to determine the motion of the apparatus and any body to which it is attached.

Accordingly, references herein to the accelerometers having their axes lying in a single or the same plane are also references to the axes of the accelerometers lying in parallel planes.

If the accelerometers are arranged to have their axis lying in different, parallel planes, the separation between the parallel planes is preferably arranged to be as low as possible.

One embodiment of the apparatus of the present invention comprises three accelerometers. It has been found that the arrangement of three accelerometers with their axes lying in a single plane and extending at non-zero angles to each other around a point allows the acceleration of the apparatus, or a body to which it is attached, and the direction or angle of motion within the plane to be determined using just the output from the accelerometers and without relying on other components, as has previously been the case.

Uses of the apparatus of the present invention include the rapid, accurate and recordable determination of movement patterns of bodies for use in multimedia production in association with computer generated technologies, including, for example, reverse kinematics, virtual reality and augmented reality systems. The apparatus may be used to measure in real-time the position, in both translational and angular terms, of bodies for use in a wide range of electronic and computer gaming environments; sports training applications, such as training for golf, tennis and rowing, for example; and within any other environment in which humans and machines interact, for example the piloting of planes and helicopters and the driving of vehicles such as boats, cars and the like. Further, the apparatus finds use in the real-time measurement of the position and movement of a wide range of bodies, such as the movement of components of a robotic system. As will be described in more detail hereinafter, the apparatus finds particular use in the measurement, analysis and assessment of the motion of a human or animal subject, in particular to identify and assist in the correction of subjects having difficulties with movement, for example the condition of drop foot described above.

In one embodiment, the apparatus has three accelerometers, arranged as set out above. The output from three accelerometers arranged in a single plane may be used to determine both the acceleration and the angle of motion of the apparatus and a body to which it is attached within the plane. The apparatus may comprise more than three accelerometers arranged with their axes lying in the single plane, for example four, five, six, seven, eight or more accelerometers.

The three or more accelerometers are arranged in a fixed orientation with respect to each other, that is with their axes fixed relative to each other. The axes of the accelerometers extend at an angle to each other, in particular a non-zero angle. In this way, none of the axes of the accelerometers are parallel to one another. The angles referred to are the angles of the axes within the single plane. In one preferred arrangement, the accelerometers are arranged to have their axes extending evenly about the point in the plane. Thus, for three accelerometers, the axes of the accelerometers are arranged at 120' intervals, that is adjacent axes extend at an angle of 120 ^° to each other. However, the accelerometers may be arranged at irregular angles around the point. In particular, adjacent axes may extend at an angle of from 20 to 170° to each other, preferably at an angle of from 40 to 160°, more preferably from 60 to 150°, still more preferably from 80 to 140°, especially from 90 to 130 ^° . In one preferred embodiment, the apparatus of the present invention comprises four accelerometers arranged with their axes extending in a single plane. In particular, the four accelerometers are arranged into two pairs of accelerometers. In an especially preferred embodiment, the apparatus comprises a first pair of accelerometers and a second pair of accelerometers, the accelerometers of the first pair being arranged to detect movement along a first pair of axes, the axes of the first pair extending orthogonally to one another, the accelerometers of the second pair being arranged to detect movement along a second pair of axes, the axes of the second pair extending orthogonally to one another and extending at an angle to the axes of the first pair.

A particularly preferred arrangement is one in which the two accelerometers in each pair are provided by a two-axes accelerometer. Accordingly, in a preferred aspect, the present invention provides an apparatus for detecting movement of a body, the apparatus comprising a plurality of two-axes accelerometers, a first accelerometer being arranged to detect movement along each of a first pair of orthogonal axes, the second accelerometer being arranged to detect movement along each of a second pair of orthogonal axes, the orthogonal axes of the second accelerometer extending at an angle to the orthogonal axes of the first

accelerometer.

The apparatus of the present invention is particularly advantageous in many applications where determination of the movement of a body having a complex motion pattern is required. As noted, the apparatus allows accelerometers to be used to determine both the acceleration of the body and its angle or direction of motion. In particular, depending upon the number and arrangement of

accelerometers, embodiments of the apparatus of the present invention are suitable for determining motion along a single line, that is in the x direction, for determining motion in a plane, that is with motion in both the x and y directions, and for determining movement in three dimensions, that is motion with components in the x, y and z directions, where x, y and z are orthogonal.

The apparatus of the present invention comprises a plurality of

accelerometers, the plurality having at least three accelerometers, arranged with their axes in a single plane. Suitable accelerometers for use in the apparatus are known in the art and are commercially available. To render the apparatus compact and easy to use, it is preferred to use accelerometers incorporating microelectronic systems (MEMS) technology. Again, such devices are known in the art and are commercially available.

As noted, in one particularly preferred embodiment, the apparatus of the present invention comprises a plurality of two-axes accelerometers, such that a single two-axes accelerometer provides the accelerometers of each pair. Two-axes accelerometers are known in the art and are commercially available. Such accelerometers are characterised by having sensing elements arranged to detect motion of the device, in particular acceleration of the device, along two axes, the two axes being arranged perpendicular to one another. Any suitable two-axes accelerometer may be used in the apparatus of the present invention. A preferred accelerometer is one based upon microelectromechanical systems (MEMS) technology. Again, such MEMS devices are known in the art and are available commercially. One such device found to be particularly suitable for use in the apparatus of the present invention is the LIS 2L02AS4 MEMS Intertial Sensor, manufactured by ST Microelectronics, England.

Many accelerometers are provided with suitable interfaces to receive data from the sensing elements and generate an appropriate output signal for further processing. If the accelerometer is not provided with such an interface, one should be included in the apparatus.

The sensitivity of the accelerometers and the required output from the devices will be determined by the use being made of the apparatus of the present invention. The output of the accelerometers may be limited in terms of both the frequency and the sensitivity to acceleration. Again, these parameters will be determined by the use being made of the apparatus. For example, in the case of an apparatus for use in analysing the motion of a human or animal body, in particular the gait of a human subject, the output frequency of the accelerometers is preferably limited, in particular to a maximum output frequency of 750 Hz, more preferably to a frequency of 500 Hz. Output frequencies outside of the aforementioned ranges may be employed, if more suitable. Further, the sensitivity of the accelerometers to acceleration is preferably selected to suit the intended use of the apparatus. Again, for example, in the case of an apparatus intended for use in analysing the movement of the limbs of a human or animal subject, the sensitivity of the accelerometers is limited to +/- 12g, more preferably below +/- 10 g, still more preferably below +/- 8g. A sensitivity of about +/- 6g has been found to be particularly suitable for the apparatus when used to analyse the gait of a human subject, for example to detect and correct the condition of drop foot. At this sensitivity, the accelerometers remain sufficiently sensitive to monitor the movement of the limbs of the subject without the accelerations detected by the accelerometers being such that the signals are saturated. Accelerometer

sensitivities outside the aforementioned ranges may be employed, if more suitable.

The apparatus of a particularly preferred embodiment of the present invention comprises a plurality of pairs of accelerometers. The accelerometers in each pair are arranged with their sensing elements perpendicular to one another, such that motion along two orthogonally arranged axes is detectable. Further, the accelerometers are arranged such that the orthogonal axes of a first pair of accelerometers are at an angle to the orthogonal axes of a second pair of accelerometers. In this way, a complex pattern of motion of the apparatus or a body to which it is attached can be analysed and determined. In particular, such an arrangement allows for both translational movement and angular movement of the apparatus or the body to be detected and measured.

The apparatus may comprise two pairs of accelerometers arranged as hereinbefore described. The accelerometers of each pair are arranged such that their axes lie in a single plane and with the axes of the accelerometers in one pair lying in the same plane or in a parallel plane to the axes of the accelerometers of the second pair. In this way, the accelerometers are arranged about axes in both the x and y axes, with the accelerometers having no axis extending in the z direction. In this case, the apparatus is responsive to motion along a straight line and can provide an output to indicate the components of the motion along two orthogonal axes, for example horizontally and vertically. In this way, both the acceleration of the body and its angle of movement in the plane of the x and y axes may be determined. Embodiments of the present invention may also be employed to determine the motion of the apparatus, and a body to which it is rigidly attached, that is its acceleration and angle of travel, in three dimensions, that is in the x, y and z directions. The apparatus of such embodiments has a first set of three or more accelerometers arranged with their axes extending in a first plane, as set out above. In addition, the apparatus comprises a second set of two or more accelerometers having their axes extending in a second plane, the second plane extending at an angle to the first plane. In the case that the apparatus comprises two accelerometers in the second set with their axes in the second plane, the axis of one of the accelerometers of the first set extends in or parallel to the second plane.

More preferably, the apparatus comprises a first set of three or more accelerometers arranged with their axes extending in the first plane and a second set of three or more accelerometers with their axes extending in the second plane.

Higher numbers of accelerometers may be provided in either or both the first or second sets, such as four, five, six, seven or eight accelerometers. The second plane in which the second set of accelerometers are arranged may extend at any non-zero angle to the first plane, in particular an angle from 10 to 170°, more preferably from 30 to 150°, still more preferably from 50 to 120 ^° . It is particularly preferred that the second plane extends perpendicular to the first plane. In one embodiment, the apparatus comprises three pairs of accelerometers.

The three pairs of accelerometers are arranged as hereinbefore described, that is the accelerometers in each pair are arranged about orthogonal axes. Further, the pairs of accelerometers are arranged such that the axes of the accelerometers of each pair extend at an angle to the orthogonal axes of each other pair. The axes of the accelerometers in two of the three pairs are arranged to lie in the first plane, as described above, with the axes of the third pair of accelerometers arranged to lie in the second plane at an angle to the first plane, preferably perpendicular thereto. Three pairs of accelerometers allow the motion in and about a plane to be

determined and analysed, that is motion in the x, y and z directions. Similarly, the apparatus may comprise six pairs of accelerometers, the accelerometers of each pair being arranged as hereinbefore described and the axes of each pair extending at an angle to the orthogonal axes of each other pair of accelerometers. Four pairs of accelerometers arranged in this way allow the apparatus to be responsive to movement in three dimensions, that is in the x, y and z directions, with the proviso that there is no rotational motion about the z axis. In an alternative embodiment, the apparatus comprises nine accelerometers, with accelerometers arranged in the same orientation and parallel to one another providing duplicate outputs of the movement.

Higher numbers of pairs of accelerometers may be used, in particular to allow the apparatus to respond fully to movement about three axes. In particular, the apparatus may comprise six pairs of accelerometers, the accelerometers of each pair being arranged as hereinbefore described with the axes of each pair extending at an angle to the orthogonal axes of each other pair of accelerometers. The six pairs of accelerometers are arranged into three groups of two pairs. The axes of the first two pairs of accelerometers are arranged to lie in a first plane, the axes of the second two pairs of accelerometers are arranged to lie in a second plane perpendicular to the first plane, and the axes of the third two pairs of accelerometers are arranged to a lie in a third plane, perpendicular to the first and second planes. In this way, the apparatus is responsive to full motion about the x, y and z axes. As noted above, the accelerometers of the apparatus are preferably arranged in pairs, or a pair of accelerometers is comprised of a two-axes accelerometer, with the axes of the accelerometers in each pair arranged orthogonally to each other and at an angle to the axes of the other pair or pairs of accelerometers. The angle between the axes of one pair of accelerometers and the axes of another pair of accelerometers may be any suitable angle. The pairs of accelerometers may be arranged such that the angle between each axis of a pair and each axis of each other pair is the same or different. In a preferred arrangement, the angle between each axis of a first pair of accelerometers and each axis of a second pair is the same. In the case of an apparatus having two pairs of acceierometers, the angle between the axes of the first pair and the axes of the second pair is preferably from 25 to 75°, more preferably from 30 to 60°. A preferred angle is from 35 to 50 ^° , with an especially preferred angle being 45° (ττ/4 radians).

In the case of an apparatus having three pairs of acceierometers, the angle between the axes of a first pair and the axes of each of the second and third pairs is preferably from 50 to 150°, more preferably from 60 to 140°. A preferred angle is from 70 to 130°, with an especially preferred angle being 120 ^° (2π/3 radians).

In addition to the acceierometers, the apparatus of the present invention preferably comprises a processor for receiving the output signals of the

acceierometers. The processor is preferably arranged to process the signals received from the plurality of acceierometers to provide an indication of the motion of the apparatus and the body to which it may be attached. The processor may provide data relating to the motion in any desired form, for example generating a visual display of the motion of the body. Data storage means may be provided to allow data to be stored, for example for later retrieval and processing. In one preferred embodiment, the acceierometers are arranged in a single housing. The housing may also contain the processor and data storage means, if present in the apparatus. The housing is preferably provided with means for securing the apparatus to a body. Such means should allow the apparatus to be rigidly secured to the body, so as to prevent relative movement between the apparatus and the body during motion of the body and in use.

In one embodiment, the housing is preferably a portable housing, for example with suitable means to attach the housing to a body, such as the limb of a human or animal subject. The means for attaching the housing to the body are preferably releasable, for example for releasably attaching the housing to the limb of a subject, in particular a leg of the subject. A particularly compact embodiment comprises the processor and, optionally, data storage means disposed within the housing, together with the accelerometers.

In a further aspect, the present invention provides a method for measuring the movement of a body, the method comprising:

providing three accelerometers, each accelerometer having an axis, the accelerometers being arranged such that the axes of the three accelerometers lie in a single plane and are fixed in relation to a point, the accelerometers being arranged about the point to have their axes extending at non-zero angles to each other; and detecting using the accelerometers motion along the axis of each

accelerometer.

In a particularly preferred embodiment, the method of the present invention comprises:

providing a first pair of accelerometers and a second pair of accelerometers, the accelerometers of the first pair being arranged to detect movement along each of a first pair of axes, the axes of the first pair extending orthogonally to one another, the accelerometers of the second pair being arranged to detect movement along each of a second pair of axes, the axes of the second pair extending orthogonally to one another and extending at an angle to the axes of the first pair; and

detecting motion along each axis of each pair of accelerometers.

As noted, the method is particularly suitable for providing an indication of both the translational and rotational movement of the body. The method may provide a greater number of pairs of accelerometers, to allow a complex motion in two or more dimensions to be measured and analysed, as described above.

As described above, one preferred embodiment comprises providing the accelerometers arranged in pairs, the pairs of accelerometers are most preferably provided as two-axes accelerometers, the axes of which are arranged perpendicular to one another.

In still a further aspect of the present invention, there is provided a method for analysing the motion of a subject, the method comprising: providing on a limb of the subject an apparatus as hereinbefore described.

In particular, the method comprises providing a first pair of accelerometers and a second pair of accelerometers, the accelerometers of the first pair being arranged to detect movement along each of a first pair of axes, the axes of the first pair extending orthogonally to one another, the accelerometers of the second pair being arranged to detect movement along each of a second pair of axes, the axes of the second pair extending orthogonally to one another and extending at an angle to the axes of the first pair.

Preferred features of the method are as described hereinbefore.

As noted above, the method of the present invention finds wide application in situations where the motion of a body is to be determined, in particular its

acceleration and angle or direction of travel. As noted, in one aspect the method is particularly suitable for analysing or assessing the gait of a human subject, in particular to assist in the diagnosis or treatment of conditions affecting or impairing the motion of the subject, for example drop foot. In particular, the apparatus and method of the present invention find particular use in the treatment of drop foot and similar conditions, which treatment requires electrical stimulation of the nerves of the subject at the appropriate point in the gait of the subject. An effective stimulation cycle must be synchronised to the gait cycle of the subject, so as to stimulate the appropriate muscles at the correct point in the gait cycle and correct the adverse effects of the condition.

Accordingly, in a further aspect, the present invention provides a system for the neuromuscular stimulation of a subject, the system comprising:

a motion-detecting assembly for detecting the movement of the body of the subject, the assembly comprising an apparatus as hereinbefore described;

an electrical stimulator for generating an electrical stimulation of one or more nerves of the subject, the electrical stimulator being operable in response to signals received from the motion-detecting assembly. In still a further aspect, the present invention provides a method for the neuromuscular stimulation of a subject, the method comprising:

providing on a limb of the subject an apparatus as hereinbefore described; generating an output signal from the accelerometers corresponding to the movement of the subject;

generating an electrical signal for stimulation of at least one nerve of the subject on the basis of the output signal; and

providing the electrical signal to the at least one nerve of the subject to stimulate operation of at least one muscle of the subject.

The motion-detecting assembly and its operation are as hereinbefore described.

In a preferred embodiment, the apparatus and method of the present invention are employed to measure the gait cycle of the subject, in particular to monitor the angles of motion of the body of the subject, especially the knee angle of the subject. Electrical stimuli are provided to the nerves of the subject in response to the measured motion of the subject, to stimulate the muscles of the subject at the appropriate point in the gait cycle. The method and apparatus are particular suited to the correction of drop foot.

The electrical stimulator may be any suitable device for providing electrical stimuli to the body of the subject. Suitable devices are known in the art and are commercially available. One preferred device is a functional electrical stimulation (FES) system.

Embodiments of the present invention will now be described, by way of example only, having reference to the accompanying drawings, in which:

Figure 1 is a diagrammatical representation of the motion of a leg of a subject superimposed on a representation of three orthogonal axes, x, y and z; Figure 2 is a diagrammatical representation of the output of a first pair of accelerometers arranged at an angle Θ to the x and y axes;

Figure 3 is a diagrammatical representation of the output of a first and second pair of accelerometers arranged at an angle to the x and y axes and to each other;

Figure 4 is a representation of an arrangement of two two-axes

accelerometers for use in an apparatus and method of the present invention; Figure 5 is a diagrammatic representation of an arrangement of two-axes accelerometers according to an embodiment of the present invention for determining the motion of a body in three dimensions;

Figure 6 is a diagrammatic representation of an arrangement of six accelerometers in a plurality of planes; and

Figure 7 is a view of the arrangement of Figure 6 indicating the

accelerometers in one plane.

An example of the apparatus of the present invention employing two pairs of accelerometers and the method of its use in assessing the movement of the leg of a subject is illustrated in Figures 1 to 3. Turning to Figure 1 , there is shown a leg of a subject, generally indicated as

2, superimposed on three orthogonal axes, x, y and z. The leg 2 has an upper or thigh portion 4 and a lower or shin portion 6 separated by a knee joint 8. The upper and lower portions 4, 6 extend at an angle β to one another. As the leg 2 is flexed about the knee joint, the upper and lower portions 4, 6 move in a path with vectors b! and b ₂ respectively. An accelerometer assembly of the type described below may be arranged on the upper or lower leg portion 6, to allow the motion of the leg 2 to be monitored. The accelerometer assembly may be used to trigger a functional electrical stimulation (FES) system to correct or improve the gait of the subject. An accelerometer assembly is shown represented in Figure 2. The accelerometer assembly 10 comprises a first two-axes accelerometer 12 having a pair of orthogonally extending axes 14, 16 (that is the axes 14 and 16 are at an angle of TT/2 to each other). The accelerometer 12 is arranged to have its axes 14, 16 extending in a single plane, indicated by the x- and y-axes. The accelerometer 12 has outputs ST and s ₂ corresponding to motion along each orthogonal axis, 14, 16. The accelerometer 12 is shown arranged with the axis 14 extending at an angle Θ to the x-axis of the plane. The acceleration due to gravity, g, and its direction of action is also indicated in Figure 2.

To fully determine the motion of the body in the x/y plane, a second two-axes accelerometer 22 is arranged in the same plane as the first accelerometer 12, as shown in Figure 3. The two-axes accelerometer 22 has a pair of orthogonally extending axes 24, 26 (that is the axes 24 and 26 are at an angle of ττ/2 to each other). The second accelerometer 22 is arranged to have its axes 24, 26 extending . in a single plane, indicated by the x- and y-axes, the same as the first accelerometer. The second accelerometer 22 has outputs η and r ₂ corresponding to motion along each orthogonal axis, 24, 26. The accelerometer 22 is shown arranged with the axis 24 extending at an angle θ ₂ to the x-axis of the plane. Further, the axes 24, 26 of the second accelerometer 22 are arranged to extend at an angle a to the respective axes 14, 16 of the first accelerometer 12. Preferably, a is π/4, although other angle may be used.

The acceleration due to gravity, g, and its direction of action is also indicated in Figure 3.

Summarising the theory, Figure 3 shows the arrangement of the two two-axes accelerometers. The first two-axes device 12 has output signals Si and s ₂ . The first signal is zero when Θ is zero (si is horizontal) under the gravitational field alone (i.e. the accelerometer is not subjected to a translational movement). The second two- axes device 22 has output signals η and r ₂ rotated anticlockwise (but may also be rotated clockwise) relative to the first pair and fixed at an angle a. The output signal r. is zero when the whole assembly is rotated clockwise in Figure 3 so that θ=-α and it is held stationary.

Using complex notation where the vertical axis is imaginary (y-axis of Figure 3), for the output of the first accelerometer 12: s, - Re xe ΐθ + yie ¹ + gie ίθ

(1) where the signal s^ is in units of ms ^'2 and g is the acceleration due to gravity.

Similarly

s ₂ = Re xie ίθ ye ΐθ (2)

For the second accelerometer 22 inclined at an angle, a, the signals, n are rotated anticlockwise: r, = Re xe ^W +. yie ^W + gie ^W )e ^ia (3)

Re x ··i·e ϊθ - y■■e ΐθ - ge ίθ λ \e ia (4)

From these equations, the angle Θ, and accelerations ^x and^ can be determined as follows.

— j _| - r, cos a + r ₂ sin a

Θ = tan ^"

- s ₂ - r, sin a - r ₂ cos a _j The method uses estimates of a segment angle from accelerometers in the gravitational field. From a pair of body segments, for example the femur and tibia, the knee angle can be determined (Figure 1 ).

tan(p)= =^

More specifically, the output of the first accelerometer, s ₇ and s ₂ can be written as: j, = j cos0 - j)sin 0 - g sin #

(7)

and

s ₂ = -5i:sin# - j>cos# - gcos 0

(8) where

= horizontal acceleration in g

y = vertical acceleration in g

g = acceleration due to gravity (=1 ).

Each of the output consists of two acceleration components, static and dynamic. and ? represent the dynamic acceleration, whereas g represents the static acceleration. As there are three unknowns: * , ? and θ , at least three equations are needed to solve for all the unknowns. In the algorithm for use in relation to the apparatus, four equations are needed to solve the unknowns analytically. Therefore, a

The output of the second accelerometer, r, and r ₂ can be written as: r, = xcos0 ₂ -_ysin# ₂ -gsin# ₂ ^ and

r ₂ = -xsin0 ₂ -ycos0 ₂ -gcos# ₂

By multiplying Equation (7) with cos9 and Equation (8) with sin9, s ₁ and s ₂ can be written as: cos# = xcos ² 0- sin#cos#-gsin#cos#

and

s ₂ sinO = -xsin ² #- ^sin#cos#-gsin#cos0

By subtracting Equation (12) from Equation (11), is given by: j = s, cos0-s ₂ sinO (13)

By multiplying Equation (7) with sine and Equation (8) with cos9, s ₁ and s ₂ can be written as:

5, sin# = J sinflcostf-.ysin ² #-gsin ² Θ (14)

and

s ₂ cosO = -x sin 9 cos θ-y cos ,2 ² 0Q-gcos ,2 ² #n

(15)

By adding Equation (14) and Equation (15), ^y is given by: y = -S _\ S 0 - s ₂ cos0 - g

Now by multiplying Equation (9) with cos9 ₂ and Equation (10) with sin9 ₂ , then multiplying Equation (9) with sin9 ₂ and Equation (10) with cos9 ₂ , r _f and r ₂ can be written as: r, cos θ ₂ = x cos ² θ ₂ - y sin θ ₂ cos θ ₂ - g sin θ ₂ cos θ ₂

(17) and

r ₂ sin # ₂ = -x sin ² # ₂ - y sin # ₂ cos θ ₂ - g sin 0 ₂ cos 0

(18)

sin # ₂ = xsin # ₂ cos # ₂ - y sin ² 9 ₂ - g sin ² θ ₂

(19) and

r ₂ cos 9 ₂ = -xs\n 9 ₂ cos 9 ₂ - j^ cos ² θ ₂ - g cos ² θ ₂

(20) By subtracting Equation (18) from Equation (17), is given by: cos θ ₂ - r ₂ sin θ ₂

(21 )

By adding Equation (19) and Equation (20), ^y is given by: y - -r _x sin θ ₂ - r ₂ cos θ ₂ - g ^22)

To solve for Θ Equation (13) is equated to the negative of Equation (21 ),

The functions for cosa and sina reduce to ^ ^" ^ for a = π/4. Knowing Θ, equations (13) and (16) yield the horizontal and vertical accelerations in the x and y directions respectively.

Turning to Figure 4, there is shown a representation of key components of a device according to one embodiment of the present invention. The components, 102a and 102b, each comprise a printed circuit board 104a, 104b, on which is mounted a two-axes accelerometer 106a, 106b. The two-axes accelerometers are commercially available components (LIS2L02AS4 accelerometers, ex. STM Electronics, England) employing MEMS technology. The accelerometers are mounted on their respective boards in conventional manner, together with associated commercially available processors 108a, 108b. The two-axes accelerometers 106a, 106b correspond to the accelerometers

12, 22 shown in Figure 3. As shown in Figure 4, the first two-axes accelerometer 102a is mounted in a first orientation on the respective board 104a, with its orthogonal axes indicated and having the outputs s-i and s ₂ . The second two-axes accelerometer 102b is similarly mounted, with its orthogonal axes as indicated and having outputs η and r ₂ . As can be seen, the axes of the second accelerometer 102b are at an angle of ττ/4 radians to the axes of the first accelerometer 102a.

When assembled, the two printed circuit boards are mounted to have the axes of the first accelerometer extending in the same or a parallel plane to the axes of the second accelerometer.

Turning to Figure 5, there is shown an arrangement of three pairs of two-axes accelerometers for determining the motion of a body in three dimensions, that is in the x, y and z directions. The assembly, generally indicated as 302, comprises a first sensor unit 304 having first pair of two-axes accelerometers 306, 308 having outputs Si , s ₂ , η and r ₂ from their respective orthogonal axes, as discussed in detail above. The axes of the accelerometers 306, 308 extend in the x/y plane. Similarly, a second sensor unit 310 having a second pair of two-axes accelerometers 312, 314 are provided having outputs Si , s ₂ , r, and r ₂ from their respective orthogonal axes, as discussed in detail above. The axes of the accelerometers 312, 314 extend in the x/z plane. Further, a third sensor unit 316 comprises a third pair of two-axes

accelerometers 318, 320 are provided having outputs Si , s ₂ , n and r ₂ from their respective orthogonal axes, as discussed in detail above. The axes of the accelerometers 3 8, 320 extend in the y/z plane.

A simplified embodiment of the device shown in Figure 5 may be provided, in which the redundancy between the individual accelerometers is employed to reduce the total number of accelerometers. Thus, each pair of individual accelerometers in which the axes are parallel may be reduced to a single accelerometer, thus resulting in a total number of individual accelerometers of nine, in place of the twelve shown in Figure 5. Alternatively, the arrangement shown in Figure 5 may be employed, with those accelerometers that are oriented the same or are parallel to one another providing duplicate information regarding the movement.

A further simplification of the device may be made by reducing the total number of single accelerometers to six, with the accelerometers being arranged in three dimensions such that the axes of the accelerometers are arranged

symmetrically about a point, in particular with the angle between the axes of any pair of accelerometers is 120°.

Determination of the motion of a body to which the device shown in Figure 5 or its simpler alternative is attached is as generally set out above. Finally, referring to Figure 6, there is shown one possible arrangement of six accelerometers, numbered 1 to 6, about a point in three dimensions. The

accelerometers are shown with their axes arranged equally around the point at an angle of 120° to one another. For ease of reference a single plane containing the axes of accelerometers 1 , 4 and 5 is indicated in Figure 7.

Uses for the apparatus and method of the present invention, in particular the specific embodiments described above, include: i) Rapid, accurate and recordable determination of relative body angle

information for use within multimedia content production using a variety of computer generated technologies, such as, but not limited to, reverse kinematics, virtual reality and augmented reality; ii) Rapid, accurate and real-time measurement of positional (translational and angular) information for use: a. within a variety of electronic games formats that enable environmental data, such as human position location data, to be integrated with the electronic game environment. Examples of such applications include, but are not limited to, the Wii™game from Nintendo™ and the EyeToy™ game from Sony™; within sports training environments, such as, but not limited to, golf, tennis, rowing, to record body position information; within environments where human machine interaction may have critical safety aspects and the manner in which some control operations may require to be subsequently analysed, such as, but not limited to, aeroplane, helicopter, boat or car/ coach/ bus driving/ piloting;

(iii) Rapid, accurate and real-time measurement of positional (translational and angular) information for use in mechanical/ robotic control environments.