Clasificación de actores y contrapropuestas

2.6.1 Speed o f lifting

Anderson and Chaffin (1986) state that virtually all proposed techniques of safe manual handling recommend that the load be lifted in a slow and controlled manner. The purpose of this is to reduce moments of inertia and to facilitate the ability of the individual to react to unforeseen circumstances.

In contrast to this, Grieve (1970) had previously described weightlifting as the defeat of gravity, saying that the earlier a lifter engages gravity, the more spectacular the short term advantage, and that a long-drawn out struggle is to be avoided at all costs. He contrasted lifting by exerting a steady force slightly greater than the load with exerting a single jerk causing the weight to coast to the desired height. The first method would take a very long time and be fatiguing, and limit maximum weight lifted to the upwards strength of the weakest posture. He suggested that real lifting is a compromise between the two methods. He demonstrated that the exact way that impulses are applied and decay are vitally important, and that an explosive effort as early as possible is more effective at gaining height than the same total impulse over time. On this basis he criticised the suggestion that isometric strength has relevance to dynamic lifting. Grieve (1975) found that in isoinertial lifts performed as quickly as possible an impulsive force was applied to the load. In most cases, this peaked within 100 ms of lift-off. The ratio of the peak force to the weight of load decreased as the load

increased. For lighter loads the force at the feet fell below body weight later in the lift. For heavier loads this force did not fall below body weight. In crouch lifting, forces at the feet developed more than 100 ms before lift-off and peaked at lift-off or soon after, meaning that the body travelled upward faster than the load until the force at the hands peaked. In stoop lifts the load travelled faster than the body throughout most of the lift with both starting from rest at lift-off. Much higher lower-body velocities were

acquired in a crouch-lift for a given force than were acquired in a stoop-lift.

Mital and Karwowski (1985) selected a Mini-Gym speed of 0.75 m-s~^ as "the speed of actual lifting movement, determined in a separate experiment", but do not give details of this other experiment. Mital et al. (1986a) claim that 0.75 m-s“^ is approximately the actual speed of lifting of manual lifting tasks, but fail to substantiate this. Aghazadeh and Ayoub (1985) chose this speed because Pytel and Kamon (1981) had used

0.73 m s"\ and their experience of studying films of lifting actions was that a mean duration of lift from floor to 1.27 m high was 1.7 s (i.e. a mean speed of 0.75 m s"^). Kumar et al. (1988) criticise Pytel and Kamon (1981) for instructing their subjects to start their motion with a jerk which "could be difficult to control, could be unsafe, and also would tend to artificially inflate the peak strength values due to the inertial effects".

The inertial effects are not specified and are difficult to imagine with a device as small as the Mini-Gym. They also claim that increased lifting speed "could cause a greater hazard to a person than is now indicated by static strength values" (sic). They offer no evidence for this beyond claiming that static strengths measured in optimum postures are an indicator of maximal capability. This implies that they do not understand the force-velocity curve (Chapter 2.1) which causes the force exerted to decrease as the velocity attained by the person increases, thereby preventing the over-exertion injuries which can occur in maximal static exertions.

Stevenson et a l (1990a) note that the very fast type of lifting elicited by the ILM contravenes the normal recommendation that loads be lifted in a slow and controlled manner. Because the lifts were fast, high and very variable forces and accelerations were obtained. They inferred from this that predictions of maximal isoinertial lifting capacity based upon static strength are inadequate.

Garg and Beller (1994) found the mean speed of lifting maximum acceptable weights was greater than the mean speeds attained for any of their three resistances. They concluded that since 'speed' affects 'isokinetic' lifting strength, job-specific 'isokinetic' strength measurements should be made at the speed used to lift the load. They comment that data on lifting speeds of heavy loads is lacking and that it is not clear how object characteristics affect speed of lifting. The reduction in strength as 'speed' increased led them to concur with the recommendation that heavy loads should be lifted slowly and smoothly. They argue that there is a conflict between subjective perceptions and

'isokinetic' lifting capability because high-speed lifting is perceived as less stressful than low-speed lifting. Perhaps they should have concluded that the physical stress the subjects rated was the force produced, which also decreased as speed increased. 2 .6 .2 T h e f o r c e / velocity a n d p o w e r / velocity re la tio n s h ip s

Grieve and van der Linden (1986) note that in a movement which begins and ends at rest, substantial portions of the movement may be accomplished before peak output is achieved and optimal use will be made of the force-velocity characteristics of a fully active muscle group for only a brief period. They also found that peak

hydrodynamometer power output occurred earlier in the pull as resistance increased. Kumar et al (1988) found that peak strength decreased with increasing speed of lift and occurred progressively higher and later during the lifting cycle. Further examination of their data shows that peak strength occurred more rapidly in faster lifts for males, but more slowly for females. Presumably this is a function of both the force-velocity curve and the rate of recruitment of motor units in the relevant muscles.

Timm (1988) found that as speed increased, isokinetic peak force, peak force

work decreased whereas height of peak force and mean power increased as speed

increased. These patterns were consistent across all the test speeds. Two-way ANOVAs showed significant differences for all parameters except height of peak force. There was a general trend of decreasing force and work as speed increased, but power

increased with speed. These findings are consistent with both the force-velocity and the power-velocity curves. Post-hoc analysis failed to show significant differences between heights of peak force across test speeds and the age spectrum even though the height of peak force increased with test speed and with subject age while peak force decreased in both instances.

Garg and Beller (1990) found that 'speed', height and angle of pull all had significant effects on mean and peak dynamic pulling strengths, with speed having a much greater effect than either height or angle. Strength decreased as a function of speed and handle height and showed a peak at an angle of 25° to the horizontal. Body part (elbow, shoulder or back) and 'speed' had significant effects on Ratings of Perceived Exertion (RPE), but the effects of handle height and angle of pull were of no practical

significance. RPE for the three body parts decreased as pulling speed increased and overall ratings of comfort increased. The shoulder was the most stressed body part, followed by the elbow and back. Speed affected comfort the most, and angle the least. They concluded that it is important to know speed, height and angle of pull when determining job physical strength requirements, especially for high speed pulling tasks. They note that increasing mean speed from 0.7 to 1.1 m-s"^ can reduce strength exerted by more than 50% and that their finding that peak dynamic strength occurred

progressively earlier as speed of pull increased contradicted Kumar et al. (1988) who reported that peak isokinetic strength occurred later in the cycle as velocity of lift increased. They concluded that pulling tasks should be performed at slow speeds to maximise strength and minimise over-exertion injuries. This conclusion ignores the power requirement of the task, since greater powers can be obtained at higher velocities, and power is probably the most useful input measure to the device. Their assumption that slower speeds are safer because strength is greater is contradicted by their finding that RPE decreased as speed increased. They recommend that the pulling force required to start a gasoline powered engine should be reduced proportionately if a high cranking speed is required to start the engine, again assuming that power output is fixed, rather than velocity dependent, as is actually the case.

Garg and Beller (1994) found that increases in both speed of lifting and box width decreased 'isokinetic' lifting strength significantly, with speed having the larger effect. RPE of the low back decreased with speed of lifting and increased with box width. The RPEs of maximum acceptable weight, static strength and 'isokinetic' strength at

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Figure 2.15: Fig.3 from Bosco et al. (1995) (redrawn). According to the original legend: "Average force (F) (squares) and average power (P) (dots), developed during hüf-squat exercises performed with various loads (from 35% to 210% of the subject's body mass) are shown according to the average vertical velocity (VO for male (filled symbols) and female (open symbols) ]\xmptvs."

Bosco et al. (1995) used data collected in half-squat exercises over a range of weights to derive force-velocity and power-velocity relationships (Figure 2.15). Males had higher values of force and power than females. They found significant sex differences in force, but not in velocity and power for heavy loads.

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Figure 2.16: Fig. 4 from Bosco et al. (1995) (redrawn). Original legend: "Power ratio (men : women in percentages) found in half-squat exercise according to the loads used (from 35% to 210% of the subject's body mass, n = 7)."

The greatest gender differences in force, velocity and power were found at light loads. The female : male power ratio increased almost linearly as the load relative to body

mass increased (Figure 2.16). They considered that the velocity of maximum knee extension is more important than the force in characterising sex differences.

Fothergill et al (1996) found one-handed exertions to be slower than two-handed exertions, and, as expected, increases in resistance decreased the speed of the exertions. Resistance level weakly affected position of peak power, but not peak force. Peak one- handed strength and power occurred at lower heights than two-handed strength and power (2.5% and 2.0% of stature difference), but in all cases was about midway between knee and knuckle height. Differences in lifting strength between resistances were greatest at knee height, and decreased as the lift progressed above knee height. He also found that peak hydrodynamometer power occurred at a lower height for the highest of three resistances than for the other two resistances (a difference of 3% of stature). Both power and velocity decreased as the resistance increased.

2 .6 ,3 L i f t stra teg y

Stevenson et a l (1987) examined the effect of lift strategy on prediction of performance on a freestyle box lifting task. The most common strategy (30% of males, 60% of females, 59 subjects) involved a relatively straight back, with the box lifted to waist height, and then thrust to the height of the target platform. Only 8% used a straight back and a smooth continuous motion to the destination. They obtained correlations of 0.66 for males and 0.78 for females between free-style ELM performance and box lifting capacity using the most common lifting strategy. They concluded that constrained task protocols are not valid measures of task performance, that the ILM in its present form should not be used as a predictive test of lifting ability and that using the ELM as an indicator of general body strength would require the removal of as many constraints as possible. They found that ILM measurements, including maximum lift score and

kinetic profile, account for no more than 60% of the variance in task performance scores and argue that better predictions would require a two dimensional lift envelope to make the testing system more closely resemble actual lifting tasks.

Table 2.13: Dynamic measures obtained by Stevenson et a l (1990a) from ILM lifts to 1.83 m performed by 33 female and 99 male soldiers. Mean overhead grip reach is 118% stature (Pheasant, 1986)

Parameter Height of parameter as percent stature

Anatomical landmark

% total lift time Males Females

Maximum force 46% Mid thigh 3% 5%

Maximum power 64% Waist height 16% 20%

Maximum velocity 77% Chest height 20% 28%

Minimum force 103% Head height 33% 46%

Second maximum force 113% Above head height 60% 72%

Stevenson et a l (1990a) mention briefly a pilot study where 20 subjects were filmed performing the 1.83 m ILM test. As a result they characterise an ELM lift as consisting

of an initial leg / back extension pulling phase, a wrist changeover manoeuvre, and a final arm extension pushing phase. They also link the heights at which certain events occurred during the lift to fixed percentages of stature (Table 2.13). They linked their finding that minimum acceleration / force occurred at just above head height to the end of the wrist changeover manoeuvre, which they describe as a prolonged process

occurring between the points of maximum velocity and minimum acceleration.

Peak velocity must occur when acceleration drops to zero, i.e. at the point at which the subject does not have sufficient strength to impart additional momentum but can exert only enough force to support the weight stack. This is consistent with a height of 77% stature. When this point is reached the subject must change grip and apply an upward force greater than the weight of the stack before the velocity drops to zero. Success on the ILM therefore depends on imparting as much momentum as possible in the early part of the lift to carry the subject through this wrist changeover. This fits completely with the suggestions of Grieve (1970) on the defeat of gravity in weight lifting.

Charteris et a l (1994) used an ultrasonic motion monitor to replicate the isoinertial lifts of Stevenson et a l (1990a). They describe the lift of a 25 kg bar sliding in vertical tracks as involving "an accelerative pull, a hitch of the wrists to get the bar above the grip, and a push to full overhead stretch". They found that all the displacement, velocity, force, power and energy parameters were sensitive to the motion adjustments in the bar associated with the wrist-hitch. They found similar dynamic patterns to those found by Stevenson et a l (1990a). They also found that a free-lift of a bar bell was very similar to the uni-planar isoinertial lift.