GB2422790A - Measurement of physical fitness - Google Patents

Abstract

A method of obtaining a fitness index is proposed, representing the ability of one's own muscles to move one's own mass. A suitable movement is that of standing up from a knees-bent position, against gravity. A suitable measure of the motion is its vertical component of peak acceleration. The acceleration may be measured directly, as by an accelerometer attached to a suitable part of the body, or indirectly from the vertical components of displacement or velocity as functions of time. It may also be calculated using Newton's laws, from the mass involved in the motion and the force exerted by the muscles; this approach allows the fitness index to be obtained from a modified form of conventional bathroom scales.

Description

Measurement of physical fitness This invention is concerned with human

health, and in particular with the quantitative assessment of physical fitness.

In health screening, several numerical measures are used as approximate indicators of body condition. The most basic of these is body mass (in common parlance, weight'); for present purposes it is measured by bathroom scales, in kilograms. However, a healthy tall person may be much heavier than a healthy short person, and so the body mass is an ambiguous measure of health. More useful is the body-mass index', given by dividing the mass in kilograms by the square of the height in metres; values of 20- 2 5 are regarded as healthy. Bathroom scales are available that give both the mass and the body-mass index. These require that the height be inserted and stored, for each user; the simple calculation is then done internally, and displayed side-by-side with the mass.

Although the body-mass index is only a start, it does give a convenient quantitative figure-of-merit for a well-proportioned body.

Extensive clinical studies have established general relationships between the body-mass index and the proportion of the body mass that represents fat, and these relationships begin to have statistical significance if account is also taken of age and gender. So bathroom scales are available that allow age and gender to be inserted and stored, for each of several users; these scales then add to the display a statistically likely estimate of the body fat percentage. Such an estimate, of course, is not entirely particular to the individual. To mitigate this weakness some scales allow the insertion of a subjective fitness category'; this, in effect, prejudges the outcome, and is of little real value.

At the individual level, in particular, ambiguity is introduced if a person undertakes a regime of exercise, and so increases muscle mass; because muscle is more dense than fat, the body-mass index - contrary to expectation - may actually increase. If the exercise also burns off some fat, the body-mass index may stay much the same, or at best yield only slight reductions not truly representative of the benefit achieved.

As a partial solution to these problems, bathroom scales are now available purporting to measure the individual body fat percentage. The principle is to apply a small alternating electric voltage (typically at 50 kHz) between conducting pads on the footplate of the scales; a minute electric current (typically less than 1 mA) then passes up one leg, across the lower abdomen, and down the other leg. The measurement of the current then allows the calculation of the electrical impedance of this path. The impedance depends largely on the water content encountered along the path. Because the water content of fat is small, the impedance provides an estimate of the dimensions and properties of the fat-free component of the body. A second measurement at a different frequency may refine this estimate. Then statistical generalizations, coupled with inserted data on height, waist, age, gender, fitness and race, may allow the impedance estimate to be interpreted as a total body water percentage and a total body fat percentage, and to yield estimates of muscle mass, bone mass, and metabolic rate.

These in turn may be combined in some weighted fashion to yield a single number representing a quantitative figure-of-merit for personal physique'.

This state of the art is exemplified in bathroom scales and other healthmonitoring equipment marketed by Tanita Corporation, Salter Housewares Ltd, and other manufacturers. The patent record includes European Patent 0 455 014 and US Patents 415 176, 6473 641, 6 477 409, 6 480 736, 6 487 445 and 6 532 385.

Despite the advanced state of the art, new developments are desirable. In particular, the impedance measurement is subject to much personal variation and noise' associated with fat distribution, time of day, state of health, intake of foods and liquids, sweating, cross-sectional area of limbs, reproductive cyclicity, and other factors unrelated to physical fitness as such. The reliance on statistical norms is unwelcome in a personal context. The need to specify a fitness category is a clear weakness. The impedance measurement is invalidated by metallic implants in the body, and is not compatible with pacemakers.

Accordingly it is an object of the present invention to provide method and means for the assessment of a fitness index which is entirely personal, and which is directly and clearly meaningful in the context of everyday physical life.

It is a further object to provide such method and means to individuals who have pacemakers, metal inserts, or local abnormalities of the fluid circulation systems in the body.

It is a further object to provide a fitness index in the form of a single number representing a meaningful physical reality.

These objects are achieved in a numerical measure of fitness representing the ability of one's muscles to accelerate one's own body mass. The measure is obtained by the following steps in combination: a) defining an initial position of the body from which the body is to be moved to a final position; b) causing the motion by muscular effort, as quickly as possible; c) assessing the peak acceleration achieved during the motion; and d) recording and displaying this peak acceleration.

The acceleration may be assessed directly by an accelerometer, or indirectly from the displacement or velocity of the motion as a function of time, or by calculation from measured masses and measured forces participating in the motion.

The invention is now described with reference to the drawings, in which: Figure 1 shows, in schematic side and front views, an initial body stance that may be used in the practice of the invention; Figure 2 illustrates an accelerometer incorporated into headware; Figure 3 illustrates an accelerometer held in firm contact with a part of the spine; Figure 4 graphs, as functions of time, the displacement, velocity and acceleration of a point on the body during the muscle-powered motion; Figure 5 is a counterpart of Figure 4 for the case in which the peak acceleration is numerically larger than the earth's gravity g Figure 6 illustrates how the displacement measure of the motion may be recorded as a function of time, using a video camera and a vertical scale; Figure 7 depicts an alternative technique for measuring the displacement as a function of time, using a cord attached to the body; Figure 8 illustrates how the acceleration may be derived from measurements of force and mass, using simple bathroom scales; Figure 9 shows a form of readout panel for displaying the results of the fitness measurement; Figure 10 shows schematically the initial and final positions of the body when the measurements are made with bathroom scales modified according to the invention; Figure 11 illustrates an alternative embodiment modified for greater stability; Figure 1 2 illustrates a further alternative offering greater stability; Figure 1 3 shows an alternative arrangement particularly suited to a commercial application and/or a situation where the ceiling is more rigid than the floor.

A familiar objective, in the attempt to achieve physical fitness, is to increase muscular strength while decreasing body mass. The present invention seeks to provide a simple measure of fitness that may be used to quantify progress in this endeavour.

In the prior art, muscular strength may be assessed directly as the ability to lift weights or to overcome resistance, or indirectly by estimating the muscle mass from impedance measurements as described above. The body mass may be measured directly. The present invention therefore adopts as its simple measure of fitness the ratio of the muscular strength to the body mass. However, it concentrates, at least initially, on situations where the muscular strength is applied to move the body mass against gravity. Thus if one bends the knees from a standing position, and then stands erect, the muscles of the legs and lower torso exert a force to raise the centre of mass of the body through a certain vertical distance against gravity. If, in the knees-bent position, one places the hands on the knees and assists the upward movement using the muscles of the arms and upper torso, the body rises more quickly. In a gross sense, the body mass M is being raised against gravity by a composite muscular force of vertical component F; then Newton's second law predicts a vertical acceleration of a = F/M.

The proposed simple measure of fitness therefore has the nature and dimensions of an acceleration.

This measure is intuitively satisfying. Fitness does not lie in muscular force alone, nor in mass alone, but in a healthy proportion between them. A fit person whose muscles and mass are in healthy proportion - whether the person is of small build or large - can climb stairs quickly; each stair requires an upwards acceleration, and the measure of that acceleration is the measure of that healthy proportion. An unfit person whose mass has increased without a proportionate increase in muscles again whether of small build or large - indicates the scale of the problem by the time needed to rise from the couch.

This invention therefore adopts as a measure of fitness the peak vertical acceleration of which the body is capable.

Figure 1 illustrates, in highly schematic form, one suitable stance for the measurement of the acceleration. In the side view of Figure 1 a, the knees 1 are bent, so that the thighs 2 and the calves 3 may be in partial contact. The heels 4 may be off the ground 5, the weight of the body being borne on the soles 6. The back 7 and the neck 8 support the head 9. The hands 1 0 are placed firmly on the knees 1; as shown more clearly in the front view of Figure 1 b, the elbows 11 protrude outwards, to allow a favourable angle between the forearm 1 2 and the upper arm 1 3. The acceleration is provided by the simple operation of standing up, as quickly as possible.

Within limits, the stance of Figure 1 may be varied to suit the individual; the stance to be adopted is simply the one that yields the largest acceleration. For any one individual, this simple criterion may suggest that the stance be modified as the body shape and/or the fitness improve. The limits on this freedom are just that there should be only one movement (upwards) from the initial stationary position, and that, for a total-body fitness measurement, the hands and the knees should remain in contact until they are separated naturally by the action of standing up.

The acceleration may be measured in any of several ways. It may be measured directly, as by an accelerometer. It may be measured as the first derivative of a velocity sensor, or as the second derivative of a displacement sensor. It may be obtained from the time taken to stand up. Or it may be obtained by calculation, using the forces acting on the ground during and before the action of standing up. These approaches are discussed in turn.

Recent advances in the design and manufacture of accelerometers make it attractive to measure the acceleration directly. The issues then become: how and where to attach the accelerometer to the body, and how to ensure that the measured variable properly represents the vertical component of the acceleration, against gravity.

Figure 2 shows one implementation, in which the accelerometer 21 is built into the top of a piece of headware 22, in such a fashion as to keep the accelerometer in contact with the top of the head 23, with its sensitive axis generally vertical. The headware 22 may take any suitable form, exemplified by a bobble cap, a helmet, or a scrum cap' of the type used by rugby forwards. The power supply and the output of the accelerometer may be taken along wire connections such as 24 to an electronics unit 25 incorporating a display 26; conveniently, this unit may be wall-hung. Alternatively battery-powered electronics may be incorporated into the headware, with or without a wireless link, and the output may be rendered in speech.

The acceleration measured by the apparatus of Figure 2 differs somewhat from that experienced by the centre of mass of the body (which is typically in the region behind the navel); the shock-absorbing' properties of the spine and the joints and the skin delay the motion and so decrease the acceleration. However, with a motion as large as that of standing up, this effect is relatively small; also, of course, it does not affect comparative values. A similar cushion' provided by the hair may be minimized by forming the underside of the accelerometer housing into a suitable tripod.

The normal balance controls of the body tend to restrain any swaying of the head during the action of standing up. However, as a refinement, inaccuracies associated with the deviation of the sensitive axis of the accelerometer from the vertical may be tackled by replacing the single accelerometer 21 by an array of three orthogonal accelerometers.

If these are provided with electronics effective down to zero frequency, the array can calculate its own orientation (relative to the vertical) in a quiet moment before and after the action of standing up, and calculate the vertical component of the acceleration during the motion. Encapsulated units including the three accelerometers and all the necessary electronics are becoming commercially available.

Figure 3 shows an alternative implementation of the accelerometer (or array of three accelerometers), adapted to be attached to the body in the vicinity of the spine. The housing of the accelerometer unit 31 is formed with one or more dimples' 32 shaped to engage with one or more of the protrusions of the skin associated with the vertebrae.

An elasticated belt 33 keeps the unit 31 locked' on to the spine, without materially affecting the muscular motion involved in standing up. Fastening the belt around the waist (for example, by strips 34 of the material having the trade mark Velcro') locates the accelerometer unit in a good position to measure the acceleration of the body near its centre of mass. The type and disposition of the electronics may be as described for Figure 2; in the preferred embodiment illustrated, the electronics are attached to the belt, with wire connections as at 36 and the display (inverted) as at 35.

Figure 4 illustrates in notional form the vertical motion of the body (for example, at its centre of mass, or elsewhere) as a function of time. Line 41 of the top graph (vertical displacement) shows zero displacement in the quiet period before the standing-up, and then the rise to the fully erect position. Line 42 shows the vertical velocity, which is the first derivative of the displacement; the velocity increases to a maximum at the time of fastest displacement, and then returns to zero as the body becomes erect. Line 43 shows the corresponding vertical acceleration, which is the first derivative of the velocity and the second derivative of the displacement; it shows an initial peak 48 representing the muscular effort, followed by a negative trough representing the final slowing under the combined action of gravity and the limiting extension of the muscles.

This curve 43 represents the waveform expected from the accelerometer 21 in Figure 2 or 31 in Figure 3.

It follows that the desired measurement of acceleration may also be obtained by replacing the accelerometer 21 or 31 by a velocity sensor or a displacement sensor, followed by one or two differentiations of the output with respect to time. However, the identification of the vertical component of the motion, from these sensors, may not be as practical as with the accelerometer arrays described above.

Figure 4 also illustrates that the desired measurement of acceleration may be inferred by timing the action. Thus the total time to stand up is seen on the displacement curve as the time from the start point 44 to point 45 on the waveform (44 to 46 on the time axis); this contains information on the initial muscular acceleration, but is also dependent to some extent on the height. The time measurement from the same start point to the peak time 47 on the velocity waveform 42 reduces this dependence. The same measurement may be made more simply as the time duration of positive upwards acceleration on waveform 43. Or, advantageously, the measurement may be moved even further forward in time by measuring the time to the peak 48 of the acceleration waveform. If the start point 44 is indistinct (as is often the case in practice), the time measurement may be made as the breadth of the acceleration peak at some convenient percentage of its maximum value; this is suggested in the figure, for 70%, as the time interval 49.

Figure 5 is the counterpart of Figure 4 for an individual sufficiently muscular to produce an acceleration numerically greater than that of gravity (g) - in which case the feet leave the ground during the operation of standing up. The details of this are not required for present purposes, but the broad message is clear: the desired acceleration measurement is best made at or near the first peak 51 of the acceleration waveform.

The same conclusion emerges from a consideration of the cushioning' effect of the body's shock-absorbing components, and of the delays and minor bounces that they may induce.

Therefore, at this stage of the discussion, the preferred technique for measuring the acceleration is the use of an accelerometer disposed as in Figure 2 or Figure 3, combined with electronics capable of detecting, measuring and storing the output value corresponding to the first major peak of upwards acceleration after initiation. Such accelerometers are available with guaranteed sensitivity; coupled with appropriate electronics, this means that the system may be calibrated directly in units of acceleration (mis2).

However, the technique of constructing and measunng the time waveforms of Figure 4, while less direct and less easy to calibrate, is not to be dismissed altogether; for particular realizations, it may have compensating advantages.

Figure 6 illustrates an example appropriate to the gym environment, in which there is usually a video camera available, with a direct connection to a computer display. If the action of standing up is videoed with a vertical scale 61 in the frame (as shown schematically in Figure 6), the vertical position of a marker on the body (such as at 62 or 63) may be identified from frame to frame; this then allows the automatic construction and display of the displacement waveform 41 in Figure 4, by plotting this vertical position against a time scale defined by the frame rate. The corresponding velocity and acceleration waveforms may be computed by standard techniques, and the numerical value of the acceleration peak displayed. Although this may seem a rather convoluted way of deriving the acceleration, it does have the advantage that it requires no physical connection to the body.

Figure 7 illustrates a simple physical alternative for constructing the displacement waveform 41 of Figure 4, when an attachment to the body is acceptable. A light inextensible cord 71, attached to an inextensible halter 72 round the neck, passes through a hole into a suitable box 73 (on which the user may stand), round a pulley wheel 74 and on to a drum 75 fitted with a light spiral spring of the form used, for example, in self-retracting dog leads. The action of standing up then causes the cord 71 to be withdrawn from the drum 75 against light tension just sufficient to keep the cord taut. A motion sensor 76 attached to the pulley 74 delivers an output related to the displacement or velocity of the cord 71; for example, this sensor may be a rotary digitizer whose output is subsequently differentiated against time and displayed as a measure of the peak acceleration. Clearly many variations on this approach are feasible within the scope of the invention.

Figure 8 now turns to a method of obtaining the acceleration by calculation, using the forces acting on the ground during and before the action of standing up. In many situations this is the preferred method of practising the invention.

Initially, the technique may be described in terms of a traditional spring-operated bathroom scale 81, standing on firm ground 82. When an individual stands on the scale, the spring is compressed to a degree proportional to the body weight Mg (a force, measured in newtons), and the pointer turns accordingly. However, the scale of the pointer assumes a reasonable local value for the earth's gravity (such as g = 9.81 mIs2), and effectively divides the weight by g it then displays the reading as the body mass M in kilograms. After the individual has completed the adoption of the knees-bent stance of Figure 1 (as at 83), and everything is static, the pointer reading is the same. All is now ready for the action of standing up. As the standing-up commences, the pointer moves momentarily to a higher reading; the greater the acceleration, the higher the reading. It then falls back through the static reading to a lower reading, before finally returning to the static reading as the body ceases to move. These changes of pointer reading, of course, correspond to the acceleration graph 43 of Figure 4.

The pointer swings are rapid, and it is difficult to catch the maximum reading by eye. In this simple illustration with a spring-operated scale, the maximum could be caught by recording the pointer movement with a video camera, and by playing back the recording frame by frame. Then if the pointer reading is R0 in the static situation, and R1 at the transient maximum, the desired peak of acceleration is given by a = g(R11R0 -1). The fitness index may then be expressed in any of several forms. First, it may be computed absolutely, using the local value of g (typically 9.81 mIs2 as above); the index is then the numerical value of a, in units of metres per second squared (for example, 7.8 m/s2).

Second, it may be computed merely as (R1/R0 -1), with the derived value giving the fitness index in units of g (for the same example, 0.8g). Third, the peak acceleration may be quoted as a percentage of g, as given by 100 (R1/R0 -1); the same example would then yield 80%. In the space age, the second of these forms (0.8g) has a certain cachet, and may be preferred.

It should be noted that the calibration of the pointer scale is immaterial; the fitness indicator has the same value whether the scale is marked in kilograms or pounds (or stones and pounds converted into pounds) . As a refinement, the mass and the vertical motion of the footplate (known from the compliance of the spring) may be taken into account in the calculations, as may be the estimated mass of that part of the feet expected to move as one with the footplate.

The same principles may now be applied to modern bathroom scales. These replace the old mechanical spring system by a number of strain gauges (typically four, one at each corner of the footplate), and replace the mechanical pointer by an electronic digital readout. Whereas the old mechanical pointer showed variations in the reading if the user moved on the scale, the electronic units are able to monitor any variations as a time waveform, to calculate a mean value corresponding to the true static value, and to hold this single value for display. Such a treatment may be preserved in apparatus constructed according to the present invention, so that the apparatus may display the normal weight' measurement in addition to the fitness indicator. However, for the purposes of the invention, the electronic circuitry is adapted to provide two additional manipulations. These are the detection of the peak of the time waveform, and the holding of the peak value; these operations are simply done using analogue or digital comparators well known in the art. With effective values for R0 and R1 thus stored in the electronics, the fitness index may then be computed internally from the equation given above, and the result displayed on the readout panel side-by-side with the body weight'.

Figure 9 illustrates such a readout panel. The body mass obtained from the static measurement is shown, as is the fitness index (here, in units of g). The height may be input using buttons, and this allows computation of the body-mass index, displayed as shown.

The fitness indicator of the invention may also be incorporated into scales having one or more adaptations for measuring body impedance, as described earlier. Then, as clinical data accumulate, the peak muscular force (given by g(R7-R0) in the situation described above) may be interpreted in terms of muscle mass; advantageously, this partpersonalized value of muscle mass may be the one used in estimating the body fat and the body water from the impedance measurements. These estimates may be displayed in an extended panel similar to Figure 9. The visual indications may be in different colours; one or more of them may also be rendered in speech. Provision may also be made for the display of one or more previous values, and/or the percentage change since a previous value; these neatly identify the acceptable situation in which the body mass increases but the fitness index increases equally or more.

The system of muscular forces within the body is very complex. The major simplification used in this implementation of the invention is to concentrate on the forces acting on the ground, rather than on those within the body. The placement of the scale - between the body and the ground - effectively allows the measurement of the forces acting on the ground. This implementation also yields the advantage of requiring no wired or other connection to the body.

The scales of the invention must be able to read forces larger than those acting on conventional scales; for example, to accommodate a user with an acceleration sufficient to lift the feet off the ground, the range of the scale must be at least double the user's static reading. This requirement is lessened somewhat by the fact that an individual of very large mass is unlikely to be capable of such an acceleration, but it does mean that a warning light or beep should be provided to alert any user who exceeds the available range.

The chassis' of the scale must be robust. The use of strain gauges (or other force- measuring devices of low intnnsic compliance) is to be preferred over mechanical springs. Ideally, scales should be used on rigid ground or a firm floor; where they are used on a thick carpet they may be provided with feet to ensure that little further compaction is possible.

Figure 1 0 illustrates a scale in operation. The scale is shown at 1 00; as discussed in connection with Figure 8, it has the basic form familiar from bathroom scales of the prior art, particularly those using strain gauges rather than mechanical springs. However, as noted above, it is of more robust construction, and with a greater measuring range; in the interests of stability, it may also be somewhat larger. The electronics unit 1 01 within the scale is capable of performing the additional measurements and manipulations described in connection with Figure 8, and of driving the display 1 02 in accordance with the discussion of Figure 9. Figure 1 Oa shows the user 1 03 in the static knees-bent stance of Figure 1 a, ready for the action of standing up. Figure 1 Ob shows the user on completion of the action of standing up. The static measurement of body mass may be made either during the initial static condition of Figure 1 Oa or during the final static condition of Figure 1 Ob, or in both, or as a separate static operation before starting the acceleration measurement. The strain gauges may be protected from excess stress by robust mechanical stops; as a refinement, an automatic cut-out system may disable the system (and beep or show error' on the display) if any of the strain gauges is forced out of its linear range by misuse, by inappropriate placement of the feet on the footplate, or by a seriously irregular floor.

The parts of the body do not all move equally during the action ofstanding up; indeed, the heels may even move down while most of the body moves up. However, the centre of mass of the body does move up in a clear and reproducible fashion, over a distance large compared with the noise' of other random motions of the body. From the stance depicted in Figure 1 Oa, the action of standing up employs a first group of muscles in the legs and lower torso, and a second group in the arms and upper torso. After a satisfactory measurement of the acceleration achieved from this stance, a second measurement may be made without contribution from the arms; the hands may be placed on the hips rather than on the knees, or the arms may simply hang loose. In this way the muscle strength of the legs and lower torso may be assessed separately, and that of the arms and upper torso estimated approximately by subtraction. However, movement of the arms to provide a reaction mass for the muscles of the torso - while it may provide an additional measurement useful in some studies - jeopardizes the main acceleration measurement, and should not be used in establishing the fitness index.

Figures 11-1 3 illustrate variations of the arrangement particularly appropriate in professional fitness centres. In Figure 11 a the scale itself, shown in side elevation at 11 0, is basically as in Figure 1 Oa, though it may be extended somewhat in the forward (rightwards) direction. A rigid vertical member 11 2 (which may be a tube) is fastened securely to the footplate 111; lateral struts or braces (not shown) may be used to guarantee that the member 11 2 and the footplate 111 move substantially as one.

Telescoping into the vertical member 11 2 is an extension member 11 3, which may be positively and securely locked in place at 11 4. Extending laterally each side of the top of member 11 3 are handles 11 5, also shown at 11 5 in the plan view b above. The display unit 102 (Figure 1 0) with its readout panel 91 (Figure 9) may be moved from its position on the footplate in Figure 1 0 to the top of member 11 3 for easier viewing.

The revised stance appropriate to this embodiment is shown at 11 6. The position of the arms 11 7 is now different, with the hands 11 8 grasping the handles 11 5. The vertical position of the telescoping member 11 3 may be adjusted and locked at 114, to optimize the assistance given by the arms in the action of standing up. Otherwise the operation is as described in the discussion of Figures 8 and 1 0, except that the automatic cut-out system mentioned above may require more deliberate placement of the feet. The supplementary measurement without use of the hands may be done with the hands on the hips or at the sides, as before, or the stability of the rising motion may be aided by allowing the hands to grasp a light plastic member (shown in plan view c at 11 9) able to slide freely, substantially without friction, on vertical member 11 3.

Figure 1 2 shows another arrangement to aid stability. The scale itself (11 0 in the side elevation of Figure 1 2a and the plan view of Figure 1 2b) is as in Figure 11, except that it may be made wider than the lower body. A hoop-shaped side rail 1 20 is secured rigidly on each side of the footplate; the vertical members 1 21 of these side rails may be made telescopically adjustable to accommodate users of different height and/or arm length. Hand grips 1 22 may be provided on the horizontal part of the side rails. An additional optional feature is a cross-member 1 23 adding stability to the side rails. It also provides a rest or stop for the buttocks or upper thighs during the adoption of the initial knees-bent stance of Figure 1 a; this assures greater repeatability of the stance, and facilitates a moment of initial equilibrium for a static measurement of body mass.

The vertical height of the rest 1 23 may be made adjustable, and the adjustment fine- tuned for maximum acceleration. The display unit and readout panel may remain on the footplate, or be moved to a location on the side rails, such as 1 24. A mechanical or electromechanical locking system (not shown) may be incorporated to lock the footplate to the baseplate except when a reading is to be taken; an unlock button may be provided where it can be operated by a thumb, as at 1 25.

Figure 1 3 adds another variant that may be appropriate in some settings. In this, the entire apparatus is suspended from a rigid beam, rather than placed on the floor. A light cage 1 32, fitted with a rigid base 1 33 and rigid handrails 1 34, is suspended from the beam 1 30 with clearance from the floor 1 31. The suspension member 1 35 carries a single strain gauge, depicted schematically at 1 36. Hand grips may be provided at 1 37, a buttock rest at 1 38, and the display unit at 1 39.

In the implementations of Figures 11-1 3, of course, the increased mass of the superstructure must be taken into account in the determination of the body mass.

It should be stressed that the invention is not limited to any particular stance shown in the figures. Further, the muscle-powered motion need not be against gravity; in particular it may be horizontal, against only the inertia of the body or body part being tested. Further again, it may be used to measure the strength of individual muscles, or groups of muscles smaller than those specifically tested in the descriptions above.

Whatever the circumstances in which the invention is practised, it should be remembered that the general advice to all persons undertaking muscular activity is to warm up' first. Further, it should be stressed that the rapid standing-up required in the practice of the invention may involve a momentary reduction of blood pressure in the brain. It is known that a prolonged depletion of blood supply to the brain carries a significant medical risk. However, the upward accelerations and downward decelerations involved in the practice of the invention are small and short-lived compared to many encountered by fit persons in the course of normal life particularly by those participating in cross-country running, contact sports or theme-park rides.

For the fit, therefore, the fitness index of the invention may be supplemented by a muscle stamina index, defined as the number of times an acceleration may be repeated before its numerical value falls to a given percentage (perhaps 70%) of its peak.

For the unfit, however, it may be wise to exclude from the practice of the invention all individuals who, by reason of age or medical condition, should avoid any risk of even a momentary diminution of blood pressure in the brain. In those situations where this exclusion may be unwelcome (for example, in the monitoring of physiotherapeutic progress in rehabilitation centres), the basic principle of the invention may still be applied in modified form. Instead of measuring an acceleration of the body mass upwards, against gravity, one may measure a deceleration downwards, from approximately free fall. For this, the user initially stands erect, and then starts moving towards the knees-bent position as quickly as possible. However, the motion must be arrested before the knees are fully bent - so that the deceleration is produced by muscular action rather than by the impact of the thighs on the calves. To the degree that the user is able to master this action, the computational techniques set out in this description may be adapted and applied to the calculation of a modified fitness index.

Claims (11)

1. Claims 1. A method of obtaining a numerical assessment of physical

   fitness, such assessment representing the ability of one's own muscles to accelerate one's own body mass, and being obtained by the following steps in combination: a) defining an initial position of the body from which the body is to be moved to a final position; b) causing the said movement by muscular force, as quickly as possible; c) measuring the peak acceleration achieved during the movement; d) recording and displaying this peak acceleration.
2. 2. The method of claim 1, in which the said acceleration is achieved against gravity, and in which the said movement, the said muscular force, and the said peak acceleration are all represented by their vertical components.
3. 3. The method of claim 1, in which the acceleration is measured directly, as by an accelerometer.
4. 4. The method of claim 1, in which the acceleration is measured indirectly, using measurements of body displacement or body velocity as functions of time.
5. 5. The method of claim 1, in which the acceleration is achieved against gravity, in which the reaction force supporting the body against gravity is measured both in the stationary condition and during movement, and in which the acceleration of the body is calculated according to Newton's laws.
6. 6. Apparatus for obtaining a numerical assessment of physical fitness, comprising means to measure the peak acceleration achievable when using one's own muscles to move one's own mass.
7. 7. The apparatus of claim 6, in which the muscular force and the body movement are represented by their vertical components, and the said means measures the vertical component of the peak acceleration.
8. 8. The apparatus of claim 6, in which the said means comprise one or more accelerometers.
9. 9. The apparatus of claim 6, in which the said means comprise one or more sensors to measure the displacement or velocity of the body during movement, and in which the peak acceleration is derived from such measurements of displacement or velocity as functions of time.
10. 1 0. A method of obtaining a numerical assessment of physical fitness, such assessment representing the ability of one's own muscles to accelerate one's own body mass, and being obtained by the following steps in combination: a) interposing a force-measuring unit between the body and a rigid support; b) obtaining, from the said unit or otherwise, a measure of the static weight of the body; c) actuating the muscles of the body to cause the body to move vertically, as rapidly as possible; d) taking a measure of the changing apparent weight of the body during its vertical movement, as indicated by the said force-measuring unit; e) manipulating the measurements of steps (b) and (d) to obtain a value of the peak acceleration achieved in step (C); and (f) presenting the said peak acceleration as the desired assessment of physical fitness.
11. 11. Method and apparatus for obtaining a numerical assessment of physical fitness substantially in accord with the description and drawings herein.