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2 .5 .1 A d v a n ta g e s o f A c c o m m o d a tin g R e s is ta n c e D evices

O'Hagan et al. (1995) lists the advantages of accommodating resistance devices over other types of strength testing devices (Table 2.9):

Table 2.9: Advantages of Accommodating Resistance Devices (O'Hagan et al, 1995) 1 'High' and 'low' resistance loads can be used

2 The resistive force precisely matches the strength curve of the applied force 3 The imposed resistance is passive-reactive

4 They are small and weigh little relative to the high resistive loads provided.

5 They often do not provide resistance to eccentric actions. In some, but not other, circumstances this may be seen as a disadvantage._______________________________________________________

2 .5 .2 T h e B io k in e tic E r g o m e te r

Garg et a l (1988) used a Biokinetic ergometer to simulate one-handed starting of a lawn-mower engine. This is "an electromagnetic dynamometer operating in a quasi velocity-regulated mode ... the resistance is proportional to the intensity of effort applied". They state that the operating velocity increased above the selected regulation velocity in proportion to the magnitude of the applied force. The device consisted of a handle attached to a flexible tension line wound around a drum connected via a one-way clutch and belt drive to a d.c. generator. "When the velocity of rotation ... exceeds the selected velocity, a reactive ... force ... is produced which is equal to the applied mechanical force." This description implies that no net force is transmitted to the device at this point, i.e. no acceleration can occur, i.e. it is truly isokinetic. However, they specifically state that they used the device "as opposed to an isokinetic device" "to allow for the acceleration pattern encountered in a normal human motion".

They measured force and distance using a load cell and a potentiometer attached to the handle and rope. Their results are summarised in Table 2.10. Typical force-time and velocity-time curves are shown in Figure 2.14. Four different conditions, simulating typical pulls made to start different lawn mowers, were used. Each pull was 1.1 m long. They also measured static strength in the direction of pull at the start position of each pull. 50 males and 49 females acted as subjects. Peak velocities reached 3.3 m-s“^ Static strengths were significantly greater than dynamic strengths which can be attributed to length-tension differences and the force-velocity effect, since the peak dynamic force did not occur at the start of the pull, and the mean will have included all of the weaker parts of the pull. Handle location had significant effects on peak and mean strength.

Age was also found to have a significant effect with a general tendency for strength to decrease with increasing age. Linear regression of peak and mean strength against peak velocity resulted in r values of 0.85 and 0.84 respectively. They consider that one-

handed dynamic pulling strength could be adequately estimated from peak or mean velocities. This, of course, will be device dependent.

Table 2.10: Results obtained by Garg et a l (1988). All values are the means of four conditions

Variable Males Females

Peak force 302.8 N 185.0 N

Peak velocity 2.4 ms"^ 2.0 m s-i

Work done 204.8 J 115.2 J

Time of peak force 0.25 s 0.20 s

Cycle time 0.80 s 0.87 s

Time between peak F and peak V 0.07 s 0.13 s

Peak dynamic / peak static strength 55% 58%

Mean dynamic / mean static strength 34% 39%

2.4 300 2.0 r (/) É 240

8

I

Figure 2.14: Figures 2 and 3 from Garg et a l (1988) (combined): Variation with time of dynamic pulling strength and velocity of pull for typical male (---) and female (— ) pulls

Garg and Beller (1990) used this device to examine how one-handed dynamic pulling strength was affected by 'speed of puli', start height of the handle, and angle of pull. This was done to help design manual-start gasoline powered engines. They clearly state that the device is not isokinetic, but then quote mean velocities attained for the three different resistances to motion selected as if they were constant velocities (see Table 2.11). They quote small standard deviations for the velocities at their three settings, but give very much wider ranges, which suggests that there are errors in the data. Also, it is clear from their description of the device that the relationship between force exerted and 'speed' is non-linear.

Table 2.11: Table 2 from Garg and Beller (1990). Original legend:

"Observed mean and peak speeds and pulling angles". Data from nineteen males

Variable Set value

Mean

Observed values

SD Range

Mean speed (m s“*) Slow 0.68 0.04 0.4 - 0.9

Medium 0.97 0.03 0.6 - 1.3

Fast 1.14 0.05 0.6 - 1.6

Peak speed (m-s“‘) Slow 0.96 0.05 0.8 - 2.3

Medium 1.43 0.05 0.8 - 2.3

Fast 1.87 0.07 1.0 - 2.8

Pulling angle (°) 15 15.6 0.26 14 - 21

25 25.8 0.34 17 - 30

35 34.9 0.11 31 - 36

Garg and Beller (1994) also used this device to examine the effect of speed and box width on lifting strength. In particular, it was believed that "the use of different lifting speeds, actual boxes in place of a handle bar, and mean strength in place of peak strength, might have significant effects on isokinetic lifting strength and, therefore, on its relationship with static strength and maximum acceptable weight", and that use of boxes instead of a handle bar would provide a more appropriate simulation of actual lifting tasks. For all the dynamic tasks, subjects lifted tote boxes from the floor to a bench at 0.8 m high at a frequency of 0.2 lifts min"^

They claim they measured isokinetic strength, when they had previously explicitly stated that the device is not isokinetic. Moreover, the 'cycle times' (in fact the time to lift the load to 0.8 m), by definition constant for an isokinetic device, ranged from 1.02 to 2.90 s for a single 'isokinetic' velocity of 0.41 m s"^ with standard deviations around 0.45 s (see Table 2.12). For a 0.8 m lift, these translate into velocities ranging from 0.28 to 0.78 m s"\ Also, the mean 'cycle' time increased as the box width increased!

They recommend, that as "the complexities associated with isokinetic strength" are not fully understood, static strengths and maximum acceptable weights should be used for job design and evaluation. The various dynamic tests do have all manner of

complexities associated with them, but this does not indicate that they should be abandoned since the complexities of static and psychophysical tests are also legion! Maximum acceptable weight decreased as box size increased, but it can be shown from their data that load moment stayed approximately constant. Peak static strength was 12% higher than mean static strength, which probably reflects the use of the protocol recommended by Caldwell et a l (1974) for isometric testing. Mean and peak static strength decreased with horizontal distance at a greater rate than maximum acceptable weight did, but total load moment stayed approximately constant. Peak 'isokinetic' strength was 53% higher than mean strength for all three box sizes, which shows that

the relationship between mean and peak strength was roughly constant, which would be anticipated given pulls of the same type against the dynamometer described.

Table 2.12: Table 6 from Garg and Beller (1994). Original legend: "Lifting cycle time(s)". Data from nine males

Cycle time (s)

Strength type Box size Mean SD Range

Isokinetic lifting at 0.41 m s“^ Small 1.78 0.42 1.02 - 2.18 Medium 1.93 0.48 1.10 - 2.50 Large 2.18 0.49 1.30 - 2.90 Isokinetic lifting at 0.51 m s“^ Small 1.47 0.28 0.94 - 1.94 Medium 1.54 0.36 0.94 - 2.12 Large 1.65 0.36 0.94 - 2.22 Isokinetic lifting at 0.6 m s"‘ Small 1.23 0.22 0.82 - 1.50 Medium 1.30 0.25 0.86 - 1.70 Large 1.43 0.29 1.02 - 1.98 Maximum acceptable weight Small 1.03 0.27 0.66 - 1.58 Medium 1.10 0.30 0.71 - 1.62 Large 1.20 0.30 0.78 - 1.75

2 .5 .3 T h e G rieve h y d r o d y n a m o m e te r

Grieve (1984) used a water-filled dynamometer to measure power outputs of dynamic pulls. An annular piston, fitted with a baffle plate to restrict flow, was pulled through a water-filled tube. He reported that the resistance to movement was proportional to the square of velocity, and that, to prevent the water boiling, the area had to be sufficient to avoid pressure below the piston dropping to below saturated vapour pressure during a pull. Position was sampled at 130 Hz, and velocity, force and power output derived from these measurements.

Grieve and van der Linden (1986) used the same dynamometer to investigate the effect of height on force, speed and power output in a single horizontal concentric pull by the upper limb. They measured both force and position, at a sampling rate of 250 Hz, for resistances of 15, 120, 600 and 5000 kg m"\ resulting in peak velocities ranging from 2.2 m-s"‘ to 0.3 m-s"^ respectively. Contrary to their expectations, they found no significant differences in either static or dynamic performance at eye height, shoulder height, and elbow height, which they concluded was probably not due to a common factor but to a complex interplay of limitations. They described the conditions at the handle during the pull using a force-velocity-position surface. They confirmed the importance of horizontal reach distance on the dynamic performance, and showed that the total work done in a concentric pull increased with the resistance.

Grieve (1993) constructed a new hydrodynamometer from a 2 m high tube 200 mm in diameter. Resistance could be set by using bungs to close holes in a leaky piston. Subjects lifted a handle from a rest 400 mm from the floor. Using optical switches. Grieve (1993) used the time taken to lift through the 0.7 - 1.0 m range to calculate mean

power output from calibration tables. This range was selected because it encompassed the region of greatest static lifting strength, and occurred after the initial accelerative phase of the lift, when device and muscle activation effects would be important.

Fothergill (Fothergill, 1992; Fothergill et al, 1996) modified this hydrodynamometer by adding force, displacement and velocity transducers. He measured one and two handed exertions by 9 males and 9 females lifting from 0.4 m to head height at each of three resistances. Isometric lifting strength was measured at six body landmarks between head height and knee height. Gender, number of hands, resistance and height caused significant variation in both strength and power normalised to body weight. Power increased with number of hands. He linked the observation that the dynamic forces measured correlated poorly with body weight with the discussion by Pheasant (1977) of the 'live' and 'dead' axes of force. Since on this device the line of force passes through the foot base, it corresponds closely with the live axis, meaning that the factors limiting strength are musculoskeletal rather than the distribution of body weight.

Fothergill (1992)'s subjects started to lift with an over arm grip (wrists pronated). Cine film of two subjects lifting against a range of resistances showed how lifting techniques changed as the resistance to lift increased. Leg and back extension were completed when the hands reached hip height. At low resistances subjects lifted smoothly using abduction and flexion of the shoulders to shoulder height, followed by lateral rotation of the upper arms at about head height, concluding with extension of the elbows and flexion and adduction of the shoulders as the hands approached full reach height. At high resistances the subjects suddenly, when the hands were at about chest height, completely adducted the shoulders and fully extended the wrists to give an underarm grip. At the same time they also flexed the knees and hips to lower the body. This allowed the lift to continue with an upward thrust generated with the legs and elbow^and shoulder , It would appear that the cause of this change of strategy was a lack of strength in the lateral rotators of the shoulders and the need to reduce the loads on the wrists and elbows.

2 .5 .3 T h e O m n itro n h y d ra u lic d y n a m o m e te r

Hortobagyi e ta l (1989) used a Omnitron hydraulic dynamometer This device is effort- dependent, using oil forced through adjustable valve openings to provide the resistance. It is not clear what the measurement they took was, except that it was "the peak score of the 10 trials". It is therefore not clear whether this was the instantaneous peak force recorded or a mean. Russell et a l (1992) examined the reliability of the Omnitron using an upper body testing protocol. Their data suggested that there is a learning effect when testing novice subjects. Using measures of intraclass reliability they found that the reliability could be maximised by using at least two tests of at least two repetitions, but was very high when a single test session was used.