CONSTANT VELOCITY IN LIFTING AS A CRITERION OF MUSCULAR SKILL*

Abstract

The derivation of a measure of skill from changes in velocity throughout a lifting movement was studied. The distances traversed by the wrists in lifting n 10 kg weight to the height of 1-67 in were recorded photographically with the aid of neon lights flashing at 0.1 sec intervals. The values of velocity, acceleration and force were computed at successive stages throughout each movement.

A shift from constant acceleration to constant velocity in movement was significant after training. The estimates of this trend were made by comparing within movement: (a) the mean acceleration with the mean acceleration in the central phase of movement, (b) the kinetic energy developed with the square of mean velocity, (c) the maximum with the mean velocity.

The following effects were shown: as lifting became faster during training the peak velocity approached a constant; the peak velocity was not appreciably increased but the velocity in the initial and final phases of movement was progressively raised towards the peak; this was most pronounced for the mean velocity of lifting 1 -6 m/see and coincided with the greatest work output per litre of expired air. With even faster lifting the constant acceleration reappeared together with a sharp decline of the work output per litre of expired air.

With constant velocity in lifting which involved a vast number of muscles, a reduced force, used with the maximal velocity, reflects the characteristic relation between velocity of contraction and force. With constant acceleration great force coincides with great velocity, suggesting that the elastic element of the muscle engages the contractile mechanism and that a time lag between the motor discharge and its sensory feedback occurs which takes the velocity of the limbs out of control.

On the behavioural level the correspondence, approaching one to one, between the maximum and the mean velocity in movement was statistically a sensitive measure of progress in acquisition of skill.

Notes

*The experiment has been suggested by Dr. O. C, J. Lippold from the Department of Physiology, University College, London.