Abstract:
Apparatus is disclosed for monitoring strain and/or load applied to the body of a vertebral mammal. The apparatus includes means ( 10, 11 ) for measuring position of the body relative to an inertial frame of reference and for providing first data indicative of the position, wherein the means for measuring position includes at least one inertial sensor such as an accelerometer. The apparatus includes means ( 12, 15 - 19 ) for measuring muscle activity and for providing second data indicative of the muscle activity. The apparatus also includes means for storing the data and means for processing the data to provide a measure of strain and/or load to the body. The strain and/or load is typically applied to the back of the mammal. A method of monitoring strain and/or load applied to the body of the vertebral mammal is also disclosed.

Description:
RELATED APPLICATIONS 
     The present application is a National Phase entry of International Application Number PCT/AU2005/000743, filed May 25, 2005, which claims priority from, Australian Application Number 2004902785, filed May 25, 2004, the disclosures of which are hereby incorporated by reference herein in their entirety. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to apparatus and a method for monitoring strain and/or load applied to the body of a vertebral mammal. The present invention is particularly suitable for monitoring strain and/or load applied to the back of the mammal including the lumbar spine and it will be described herein in this context. Nevertheless, it is to be understood that the present invention is not thereby limited to such applications. In this document use of the words strain and/or load in relation to the body of a mammal includes a reference to movements, muscle forces and loads including gravitational loads acting on the body of the mammal. 
     Preventing back injuries presents a major workplace safety challenge. According to the Bureau of Labour Statistics in the United States, more than one million workers suffer back injuries each year. Back injuries account for one in every five workplace injuries or illnesses. Moreover, one-quarter of compensation indemnity claims involve back injuries, costing industry billions of dollars on top of the pain and suffering borne by employees. 
     A documented cause of compensable workplace injuries is due to manual handling tasks such as lifting, placing, carrying, holding and lowering of materials. Statistics show that four out of five injuries are to the lower back and three out of four of such injuries occurred while an employee was lifting. One difficulty in preventing and/or treating lower back injury is due to a lack of facility to objectively measure movements and stresses placed on the lower back over time and therefore the risks that are associated with use of the lower back when undertaking a task or activity that presents a risk of injury. 
     The present invention may provide a mechanism for avoiding or at the very least minimizing incidence of back injuries and/or may assist rehabilitation of existing injuries by monitoring movements, associated muscle activities and loads experienced by the lumbar spine. 
     Because preventing a back injury is far preferable to repairing the injury, the present invention may monitor loads placed on a person&#39;s back while that person is undertaking a task or activity and may assess whether that load presents a risk of injury. The monitoring may be performed in real time to provide useful feedback to the person so that the person may modify a task or activity being undertaken in a manner that may reduce the load. This may lead to a reduction of the load and a consequent reduction in risk or occurrence of an injury. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention there is provided apparatus for monitoring strain and/or load applied to the body of a vertebral mammal, said apparatus including: 
     a sensor for measuring acceleration of said body relative to an inertial frame of reference and for providing first data indicative of said acceleration, wherein said measuring sensor includes at least one inertial sensor; 
     a memory for storing said acceleration data at least temporarily; and 
     a processor for processing said acceleration data to provide a measure of strain and/or load to said body, wherein said processor is adapted to derive angular position from said acceleration data. 
     The apparatus may further include structure for measuring muscle activity and for providing second data indicative of the muscle activity. 
     According to a further aspect of the present invention there is provided a method of monitoring strain and/or load applied to the body of a vertebral mammal, said method including: 
     measuring acceleration of said body relative to an inertial frame of reference and providing first data indicative of said acceleration, wherein said measuring is performed by at least one inertial sensor; 
     storing said acceleration data at least temporarily; and 
     processing said acceleration data to provide a measure of strain and/or load to said body, wherein said processing includes deriving angular position from said acceleration data. 
     The method may further include measuring muscle activity and providing second data indicative of the muscle activity. 
     Throughout the description and claims an inertial frame of reference denotes a frame of reference in which Newton&#39;s laws of motion apply. When no force is being exerted on an object then the object will move inertially. A frame of reference that moves with such an object is an inertial frame of reference. An inertial sensor denotes a sensor that responds to inertial forces such as forces that relate to acceleration of a system or that give rise to a change in velocity. 
     At least one or each inertial sensor may include at least one accelerometer. The accelerometer may measure linear acceleration of the body or body part with which it is associated. At least one or each accelerometer may include structure for measuring acceleration simultaneously along one, two or three orthogonal axes. Displacement data may be derived from at least one or each accelerometer by a process of integration as is well known in the art. Alternatively or additionally data may be derived from one or more accelerometers to provide angular displacement or position relative to a reference such as a direction defined by gravity. The apparatus may include structure for deriving angular position from the acceleration data such as by calculating a forward tilt angle and a side tilt angle. The apparatus may include structure such as a gyroscope for deriving rotational position of the body part. 
     Each accelerometer may detect a change in acceleration of a small mass mounted within a micro chip on a PCB board. As the PCB board, and the accelerometer move from one position to another, the mass experiences an acceleration at the start of the movement as well as a deceleration as the movement ceases. The accelerometer may convert movement of the mass into a voltage signal (typically in mV) that represents data in its most raw form. Span and offset adjustments may convert the voltage signal to a G force value, by way of calibration constants. A first calibration constant (p) is known as a ‘multiplier’ or ‘gain’ constant and may be derived by means of simultaneous equations wherein signal values equate to G force values. A second calibration constant (o) is known as an offset constant. Once calculated, the calibration constants (p) and (o) may be programmed into software and may become a permanent fixture of the programming. There may be two calibration constants for each channel and three channels per sensor. 
     Angular displacement of the accelerometer may be calculated by multiplying the raw signal value by the gain constant (p) and adding the offset constant (o). The resulting value may represent the G force acting on one axis. For a resultant G force in three dimensions, three axes trigonometry may be used, wherein x is the horizontal axis, y is the vertical axis and z is the ‘through page’ axis. Using 3D Pythagoras and an inverse tangent formula, two angles may be derived to give a position for the accelerometer. One accelerometer in isolation may only give a direction of movement, but when there are two accelerometers, the difference between angles of the two accelerometers may represent a change in position (in degrees) of one accelerometer compared to the other accelerometer. This may allow the apparatus to calculate angular position of the lumbar spine, at any moment in time, within a three dimensional axis. 
     The following expressions may be used to derive angular changes from accelerometers.
 
 ep+o= 1 g
 
 fp+o= 1 g
 
where:
 
e=millivolts for 1 g
 
f=millivolts for −1 g
 
p=gain (multiplier)
 
o=offset
 
Solving p and o:
 
     
       
         
           
             
               
                 
                   
                     ep 
                     + 
                     o 
                     - 
                     fp 
                     - 
                     o 
                   
                   = 
                     
                   ⁢ 
                   
                     2 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     g 
                   
                 
               
             
             
               
                 
                   
                     
                       ( 
                       
                         e 
                         - 
                         f 
                       
                       ) 
                     
                     ⁢ 
                     p 
                   
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                   p 
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                       e 
                       - 
                       f 
                     
                   
                 
               
             
             
               
                 
                   
                     ep 
                     + 
                     o 
                   
                   = 
                     
                   ⁢ 
                   
                     1 
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                     g 
                   
                 
               
             
             
               
                 
                   o 
                   = 
                     
                   ⁢ 
                   
                     
                       1 
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                       g 
                     
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                     ep 
                   
                 
               
             
             
               
                 
                   or 
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                     fp 
                     + 
                     o 
                   
                   = 
                     
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                   o 
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     Note: values for p and o should be calculated for each axis.
 
 x   mV   p   x   +o   x   =x   g  
 
 y   mV   p   y   +o   y   =y   g  
 
 z   mV   p   z   +o   z   =z   g  
 
     The above 3 equations show for the 3 axes the span and offset adjustment which converts millivolts to g. 
     The magnitude and tilt (forward/side) for the resultant vectors may be calculated as follows. 
     Magnitude:
 
 r   g =√{square root over ( x   g   2   +y   g   2   +z   g   2 )}
 
The magnitude represents the vector sum in three dimensions of the resultant G force.
 
Forward Tilt:
 
             θ   =       tan     -   1       (       z   g           x   g   2     +     y   g   2           )           
The forward and side tilt angles θ, β give the rotational position of the accelerometer relative to the z and x axes respectively.
 
Side Tilt:
 
     
       
         
           
             β 
             = 
             
               
                 tan 
                 
                   - 
                   1 
                 
               
               ( 
               
                 
                   x 
                   g 
                 
                 
                   
                     
                       z 
                       g 
                       2 
                     
                     + 
                     
                       y 
                       g 
                       2 
                     
                   
                 
               
               ) 
             
           
         
       
     
     The structure for measuring position may measure displacement in a lateral or side to side flexion plane. The structure for measuring position may also measure displacement in an extension or front to back flexion plane. The structure for measuring position may include means for measuring rotation. A measure of rotation may be derived from one or more accelerometers, muscle activity and/or one or more gyroscopes. 
     The structure for measuring muscle activity may include means for measuring surface electromyography (EMG) to establish electrical activity within a muscle. A measure of EMG may be correlated with muscle activity and/or muscle force. The structure for measuring muscle activity may be used to calculate muscle fatigue. The apparatus may include a muscle fatigue algorithm for calculating fatigue level. 
     The structure for measuring muscle activity may be adapted to measure activity of muscles in the lumbar back, the abdominal region or other muscles that correlate with load on the lumbar spine. In some embodiments activity of muscles may additionally be measured in the biceps of the subject. Measurement of activity in antigravity muscles in an upper limb such as the biceps may assist in resolving ambiguous readings of muscle activity from the back muscles particularly when performing a lifting operation from a fully bent over position. The activity of erector spinae/multifidus and biceps brachii muscles may be measured via recordings of EMG. 
     The apparatus of the present invention may include means for measuring skin stretch. The means for measuring skin stretch may include a helical coil. When the coil extends, its impedance may change in a reliable and repeatable way. The change in impedance may be measured in any suitable manner and by any suitable means such as means of a bridge circuit. 
     The memory may receive data from at least one sensor. Each sensor may include or be associated with an analog to digital (A to D) converter. Alternatively, at least one or each sensor may output analog data. The memory device may include or be associated with one or more A to D converters to convert the analog data to a digital domain prior to storing the data. The apparatus may include a digital processor for processing the data. The processor may process the data in real time to provide biofeedback to the person being monitored. The digital processor may include an algorithm for evaluating risk of back injury. The digital processor is programmed with the algorithm perform calculations based on risk assessment principles. The risk assessment may include an evaluation of risk components associated with individual data provided by each measuring means. The risk components may include profile data associated with the person being monitored. Profile data may include personal data and family history which may have a bearing on risk of back strain and/or injury. The cumulative evaluation may be used to provide bio-feedback by various means and may be used as a warning of impending risk and/or retraining system for rehabilitation of an existing injury. The risk components may be combined in accordance with risk assessment principles to provide a cumulative evaluation of risk of back strain and/or injury. The risk components may be combined in a linear or non-linear fashion, eg. weightings may be attributed to the risk components that reflect the contribution that each component makes to the overall assessment of risk. The data storing means may store data in digital format for later analysis and/or reporting. In some embodiments, the memory device includes a memory structure for storing the digital data such as a memory card, a memory stick, or the like. In at least one embodiment, the memory structure is removable to facilitate downloading the data to a remote processing device such as a PC or other digital processing engine. 
     The system of the present invention may include a user interface means. The user interface means may include a display screen and one or more controls such as buttons or the like to allow the user to interact with the data storing means. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       A preferred embodiment of the present invention will now be described with reference to the accompanying drawings wherein: 
         FIG. 1  shows transducer pads and an EMG electrode assembly associated with the apparatus of the present invention; 
         FIGS. 2A and 2B  show placement protocols for transducers relative to a persons back; 
         FIG. 3A  shows a block diagram associated with an accelerometer transducer; 
         FIG. 3B  shows a block diagram associated with a gyroscope transducer; 
         FIG. 4  shows a block diagram associated with an EMG transducer; 
         FIG. 5  shows a block diagram associated with a flexion transducer; 
         FIGS. 6   a  to  6   d  show a circuit diagram associated with accelerometer and gyroscope transducers; 
         FIGS. 7   a  to  7   d  show a circuit diagram associated with an EMG transducer; 
         FIGS. 8   a  to  8   d  show a circuit diagram associated with a flexion transducer; 
         FIGS. 9   a  to  9   c  show a flow diagram of processing software associated with the apparatus of the present invention; 
         FIGS. 10   a  and  10   b  show a flow diagram of software for calculating body orientation; 
         FIGS. 11   a  and  11   b  show a flow diagram of software for calculating position of a lower back in a lateral flexion plane; 
         FIGS. 12   a  and  12   b  show a flow diagram of software for calculating position of a lower back in an extension flexion plane; 
         FIGS. 13   a  and  13   b  show a flow diagram of software for calculating rotation of a lower back; 
         FIGS. 14   a  and  14   b  show a flow diagram of software for calculating EMG level and muscle fatigue; 
         FIGS. 15   a  and  15   b  show a flow diagram of software for calculating vibration; 
         FIG. 16  shows a flow diagram of software for calculating velocity and acceleration; 
         FIGS. 17   a  and  17   b  show a flow diagram of software for calculating cumulative risk of back injury; and 
         FIGS. 18   a  to  18   c  show a sample questionnaire for determining profile data for a person being monitored. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a pair of transducer pads  10 ,  11  and an EMG electrode assembly  12 . Pads  10 ,  11  and EMG assembly are connected to a data logger (not shown) via detachable plug in cables  13 ,  14 . Transducer pads are positioned on the back of a person being monitored as described with reference to  FIGS. 2A and 2B . 
     The transducer pads  10 ,  11  are attached to the skin in any suitable manner such as via adhesive tape. Each transducer pad  10 ,  11  incorporates one or more accelerometers. Preferably angular changes of lumbar spine orientation are measured by use of four accelerometers, two of which are placed at the upper lumbar spine and other two, at the lower lumbar spine. 
     If one accelerometer is used in each transducer pad it should be mounted such that it is located centrally over the spine. Each transducer pad  10 ,  11  may be at least 10 cm wide and may include a gyroscope to provide rotational data. 
     EMG electrode assembly  12  includes a plurality of EMG electrodes  15 - 19 . Electrodes  16 ,  17  are adapted to measure muscle activity on the left side of the person&#39;s back and electrodes  18 ,  19  are adapted to measure muscle activity on the right side of the person&#39;s back. Electrode  15  is a reference electrode for each circuit associated with EMG electrodes  16 - 18 . 
     Referring to  FIG. 2A  use of the monitoring apparatus requires locating of landmarks on the lower back to ensure reliable readings. The landmarks may be located using the following procedure:
         1. The subject should be instructed to stand upright but relaxed;   2. If hair on the lumbar region of the subject has not been clipped, hair clippers may be used to remove excessive hair from the region;   3. An alcohol wipe may be used to clean the skin in the region thoroughly (to remove any oil on the skin);   4. The PSIS (Posterior Superior Iliac Spine) should be located and marked on the left and right sides such that each is the size of an olive;   5. A small horizontal line should be drawn across the middle of each PSIS ‘olive’;   6. The two horizontal marks should be joined with one extended horizontal line (Line “A”) stretching across the spine;   7. After confirming that the subject is in their starting position, a distance of 150 mm up from line “A” should be measured with a soft measuring tape pressed against the subjects skin, gently following skin contours, and marked with a small horizontal line (Line “B”);   8. Two less prominent lines ‘C’ and ‘D’ should be marked. Line “D” should be 10 mm down from line “A” and Line “C” should be 10 mm up from Line “B”.   9. The subject should be instructed to stand with feet shoulder width apart and the distance between the lines “A” and “B” verified;   10. The subject should be instructed to bend forward to their marker, keeping their knees straight.   11. The subject should be instructed to return to their starting position.   12. Adhesive sheets should be placed as follows:       

     (a) An adhesive sheet should be prepared by removing the backing; 
     (b) The subject should be instructed to bend forward to their marker. When the patient is in this position, the adhesive sheet should be placed such that it&#39;s top edge aligns with line “A” (see  FIG. 2A ). The subject should be instructed to return to their starting position; 
     (c) When the subject is upright, the second adhesive sheet should be prepared and placed such that its bottom edge aligns with Line “B”. 
     The apparatus of the present invention should be accurately fixed to the back of the person being monitored to minimize reading errors. Referring to  FIG. 2B  care should be taken in fixing the transducer pads and EMG electrode assembly. Fixation preferably should adhere to the following precautions: 
     1. Care should be taken not to rotate the accelerometers in relation to each other. If the coil in between them is twisted, it&#39;s readings may become unstable. 
     2. Accelerometer two should be placed lightly on the centre of the spine, with its bottom edge aligned with Line “B”. It should be pressed just hard enough to ensure it stays in position. 
     3. Accelerometer one should be placed lightly on the centre of the spine, with its top edge aligned with Line “A”. It should be pressed just hard enough to ensure it stays in position. 
     4. The subject should be instructed to bend forward to their marker and hold. Whilst the patient is in this position, placement of both accelerometers may be verified. If okay, each accelerometer board should be pressed down firmly to ensure good adhesion. The subject should be instructed to return to their starting position. 
     5. The subject should be instructed to bend forward to their marker slowly, and return to their starting position. When this is occurring, placement and adhesion of the accelerometer pads should be verified. 
     The diagram shown in  FIG. 3A  includes accelerometer  30 , low pass (averaging) filter  31  and analog to digital (A to D) converter  32 . Accelerometer  30  preferably includes a 3D or 3 axes accelerometer such as a Kionix KXM52. Alternatively, accelerometer  30  may comprise a pair of 2D accelerometers such as a device type ADXLZ10E. Each transducer pad  10 ,  11  may include an accelerometer such as accelerometer  30 . Each accelerometer  30  provides outputs that are proportional to acceleration being applied to its respective axes. As gravity provides a constant acceleration of 9.8 m/s2 directly downward, a stable point of reference can be used to determine inclination. By using a minimum of 3 axes, a 3 dimensional vector pointing in the direction of acceleration may be generated. By using two accelerometers, one in each transducer pad, the angle between the accelerometers may be calculated using appropriate software. 
     Low pass filter  31  provides an averaging function for the accelerometer signal by removing sudden changes in acceleration which may cause aliasing errors. A to D converter  32  provides a digital representation of the accelerometer signal suitable for processing via a digital processor. 
     The diagram shown in  FIG. 3B  includes gyroscope  33 , low pass (averaging) filter  34  and A to D converter  32 . Gyroscope  33  preferably includes a device type ADXRS300. 
     The EMG diagram shown in  FIG. 4  includes input amplifier  40 , filter  41 , precision rectifier  42 , low pass (averaging) filter  43  and A to D converter  44 . Amplifier  40  includes a plurality of inputs for receiving signals from EMG electrodes  15 - 19 . Amplifier  40  includes highly sensitive inputs for amplifying the very small signals produced from electrodes  15 - 19 . Low pass filter  43  removes unwanted portions of the frequency spectrum. Filter  41  may include a band pass filter. A to D converter  44  provides a digital representation of the EMG signal suitable for processing via a digital processor. 
     The flexion diagram shown in  FIG. 5  includes oscillator  50 , resistor  51 , amplifier  52 , precision rectifier  53 , low pass (averaging) filter  54  and A to D converter  55 . Oscillator  50  provides an AC excitation voltage for a flexion sensor  56  such as a helical coil of wire. The coil of wire forms an inductor. Many variables affect the value of the inductance such as coil length. By stretching and contracting the coil the inductance is varied. The impedance of the coil is proportional to inductance according to the formula:
 
 X   L =2π fL  
 
wherein X L =impedance, f=oscillator frequency, and L=the coil inductance.
 
     Oscillator  50  should be stable in amplitude and frequency. Resistor  51  may be arranged to form with the flexion sensor  56 , a voltage divider. As the value of resistor  51  is constant, the input voltage to amplifier  52  is controlled entirely by the changing impedance of the flexion sensor  56 . Low pass filter  54  is an averaging filter and is used to remove sudden changes in the impedance of the flexion sensor  56 . A to D converter  55  provides a digital representation of the flexion impedance signal suitable for processing via a digital processor. 
     The diagram shown in  FIGS. 6   a  to  6   d  includes accelerometer package  30 A. Accelerometer package  30 A may be a device type KXM52-L20 or equivalent. The output of each channel (x, y, z axes) of accelerometer  30 A is connected to a respective low pass filter  31 A,  31 B,  31 C. The output of each filter  31 A,  31 B,  31 C is connected to a respective input of A to D converter  32 A. A to D converter  32 A may be a device type DS2450S. The diagram also includes a gyroscope circuit  33 A including an angular rate sensor such as a device type ADXRS300. The gyroscope circuit  33 A provides orientation reference data for calculating changes in body position. The output of gyroscope circuit  33 A is connected via low pass filter  34 A to an input of A to D converter  32 A. The output of A to D converter  32 A is in a digital format and is connected to a digital processing circuit (not shown) such as a microprocessor. The digital output may be connected to a PC via a suitable interface. 
     The EMG circuit shown in  FIGS. 7   a  to  7   d  includes input amplifiers  40 A and  40 B. Each input amplifier  40 A,  40 B receives an input from a respective pair of electrodes  15 - 16  and  15 - 17  associated with EMG electrode assembly  12 . The output of input amplifier  40 A passes via low pass and high pass filters  43 A,  41 A to rectifier and integrator  42 A. The output of rectifier and integrator  42 A is applied to one input of A to D converter  44 A. The output of input amplifier  40 B passes via a similar low pass and high pass filter and rectifier and integrator and is applied to a respective input of A to D converter  44 A. The output of A to D converter  44 A is in a digital format and is connected to a digital processing circuit as described above. A second EMG circuit similar to that shown in  FIGS. 7   a - 7   d  is used to interface pairs of electrodes  15 - 18  and  15 - 19  associated with EMG electrode assembly  12  to the digital processing circuit. 
     The flexion circuit shown in  FIGS. 8   a  to  8   d  includes crystal controlled oscillator  50 A. The output of oscillator  50 A is applied to respective inputs (In 1 -In 3 ) of flexion channels  81 ,  82 ,  83 . Flexion channel  81  includes a resistive divider comprising resistors R 6 , R 49 . One arm of the divider is connected to a channel of a flexion coil (not shown). An output of the divider is connected to amplifier  52 A. The output of amplifier  52 A is connected via rectifier circuit  53 A to low pass filter  54 A. The output of filter  54 A is connected to one input of A to D converter  55 A. Flexion channels  82 ,  83  are configured similarly to flexion channel  81  and will not be described in detail. The outputs of flexion channels  82 ,  83  are connected to respective inputs of A to D converter  55 A. The output of A to D converter  55 A is in a digital format and is connected to a digital processing circuit as described above. 
     Digital data from the accelerometer, EMG and flexion circuits is processed in the digital domain via a digital processing engine such as a suitably programmed microprocessor or the like. The microprocessor may be included with the data logger or it may be located remotely from the data logger. Data may be processed in real time to provide real time feedback to the person being monitored. The feedback may include a measure or evaluation of risk of back strain and/or injury based on the data collected during a predetermined time frame(s). Alternatively, the data may be processed offline to provide an assessment of activities logged during a preceding time frame(s) such as a day&#39;s activities with respect of risk of back strain and/or injury. 
     The digital processing engine may be programmed with suitable software for evaluating risk of back strain and/or injury. The software may determine risk components associated with data provided by the accelerometer, EMG, flexion and other circuits as appropriate. The risk components may be combined in accordance with risk assessment principles to provide a cumulative measure of risk of back strain and/or injury. The risk components may be combined in a specific algorithm to give a risk score that is accumulated per unit of time, such as per second, for an entire time that a monitoring device is worn by a person. The accumulated or aggregate score may be matched to the wearer&#39;s personalized preset thresholds. When the aggregate or accumulated score reaches personalized risk thresholds, biofeedback (vibrational, visual and/or auditory) may be triggered. There may be more than one theme of biofeedback. For example, feedback may be based on an hourly aggregate score and on a further aggregate score calculated over a shorter time frame such as a sliding five minute window. The different themes have regard to the fact that risk of injury may arise from a variety of contributing events including events associated with a short burst or bursts of relatively intensive activity as well as events associated with longer periods of less intensive activity that present significant risk when accumulated over a period such as an hour or more. 
     Flow charts illustrating examples of software for processing data from the accelerometer, EMG and flexion circuits are shown in  FIGS. 9 to 17 . The software may be adapted for calculating a cumulative score indicative of risk of back strain and/or injury. 
     In  FIG. 9   
     t=period of time to which a data sample relates 
     S=Sample Rate 
     CUM 1   t =Period of time in seconds of the 1 st  cumulative feedback window. 
     CUM 1 #=Number of data samples in the 1 st  cumulative feedback 
     CUM 1 #=6,000 
     CUM 1   s =Result of Data Scores for the period CUM 1   t    
     CUM 1   th =Threshold Score for the period CUM 1   t    
     CUM 2   t =Period of time in seconds of the 2 nd  cumulative feedback window. 
     Number of data samples in the 1 st  cumulative feedback window. eg. for 60 minutes window at sample rate of 20 Hz. 
     CUM 2 #=72,000 
     CUM 2   s =Result of Data scores for the period CUM 2   t    
     CUM 2   th =Threshold Score for the period CUM 2   t    
     In  FIG. 10   
     ACCMIN=Minimum usable value for accelerometer output 
     ORIU=Range of degrees in which body is upright 
     ORILF=Range of degrees in which body is lying on front 
     ORILB=Range of degrees in which body is lying on back 
     ORIUD=Range of degrees in which body is upside down 
     In  FIG. 11 
 
POS LF   =A° 
 
MAX-If
 
     Where: 
     POSLF=Position in the Lateral Flexion Plane 
     A°=Relative Angle of the Lower Back in the Lateral Flexion Plane 
     MAX-if=Maximum Range of Movement to date in the Lateral Flexion Plane 
     ACCMIN=Minimum usable value for accelerometer output. 
     In  FIG. 12 
 
POS LF   =B° 
 
MAX-f
 
     Where: 
     POS F =Position in the Flexion/Extension Plane 
     B°=Relative Angle of the Lower Back in the Flexion/Extension Plane 
     MAX-f=Maximum Range of Movement to date in the Flexion/Extension Plane 
     ACC MIN =Minimum usable value for accelerometer output. 
     In  FIG. 13   
     POS ROT =Position in the Flexion/Extension Plane 
     C.°=Relative Angle of the Lower Back in the Flexion/Extension Plane 
     MAX -ROT =Maximum Range of Movement to date in the Flexion/Extension Plane 
     ACC MIN =Minimum usable value for accelerometer output. 
     In  FIG. 14   
     EMG J =Self Calibrating personal maximum reading for EMG. 
     EMG k =Root Mean Squared value of raw EMG signal. 
     EMG L =EMG normalised value. A percentage of EMG J    
     EMG FAT =EMG Fatigue Score. 
     EMGFATt EMG FATt =Period of time used for Fatigue Analysis 
     EMG M =EMG Fatigue Level—low grade 
     EMG N =EMG Fatigue Level—high grade 
     EMG RES =Result of EMG Fatigue Analysis 
     EMG SC =EMG Score 
     Note: This diagram is repeated for each muscle group being measured. 
     In  FIG. 15   
     ACC MIN =Minimum usable value for accelerometer output 
     VIB LFT =Level of vibration for the lateral flexion plane at time t. 
     VIB Ft =Level of vibration for the flexion/extension plane at time t. 
     VIB UDt =Level of vibration for the vertical plane at time t. 
     VIB SC =Final result of the vibration analysis. 
     VIB t =Period of time used for vibration analysis. 
     In  FIG. 16   
     POS LF =Position in Lateral Flexion Plane at time t. 
     POS F =Position in Flexion/Extension Plane at time t. 
     POS ROT =Position in Rotation Plane at time t. 
     POS ROT =Position in Rotation Plane at time t. 
     SPE LF =Speed in Lateral Flexion Plane. 
     SPE F =Speed in Flexion/Extension Plane. 
     SPE ROT =Speed in Rotation Plane 
     ACC LF =Acceleration in Lateral Flexion Plane. 
     ACC F =Acceleration in Flexion/Extension Plane. 
     ACC ROT =Acceleration in Rotation Plane. 
     In  FIG. 17   
     t=period of time to which a data sample relates 
     S=Sample Rate 
     CUM 1   t =Period of time in seconds of the 1st cumulative feedback window. 
     CUM 1 #=Number of data samples in the 1st cumulative feedback window. eg. for 5 minutes window at sample rate of 20 Hz. 
     CUM 1 #=6,000 
     CUM 1   s =Result of Data Scores for the period CUM 1   t    
     CUM 1   th =Threshold Score for the period CUM 1   t    
     CUM 2   t =Period of time in seconds of the 2 nd  cumulative feedback window. 
     CUM 2 #=Number of data samples in the 1 st  cumulative feedback window. eg. for 60 minutes window at sample rate of 20 Hz. 
     CUM 2 #=72,000 
     CUM 2   s =Result of Data scores for the period CUM 2   t    
     CUM 2   th =Threshold Score for the period CUM 2   t    
     PRO=Profile Data Result 
     remainder as defined in previous documents 
     The software processes raw data received from the accelerometer, EMG and flexion circuits shown in  FIGS. 6 to 8 . The software also processes data from a Gyroscope circuit as well as profile data associated with the person being monitored. The profile data may allow calculation of parameters and risk thresholds to be tailored to personal habits and factors that may have a bearing on risk of back strain and/or injury such as age, sex, weight, height, family history, fitness level, occupation etc. Profile data may be obtained by means of a questionnaire or the like. A sample questionnaire for this purpose is set out in  FIG. 18 . 
     Finally, it is to be understood that various alterations, modifications and/or additions may be introduced into the constructions and arrangements of parts previously described without departing from the spirit or ambit of the invention.