Page:Philosophical Review Volume 2.djvu/371

No. 3.] a research into the development of voluntary motor ability in children, with respect to (1) maximum rate of rhythmically repeated movement, (2) precision of movement, (3) bilateral development. Under the first head, experiments on adults show that a person's rate, as tested for shoulder, arm, wrist, and finger joints, suffers slight variations due to local and subjective conditions, is greatly diminished by fatigue, and unaffected by amplitude of movement. Experiments on some eight hundred school children show that the mean rate of growth of rate between six and sixteen is from .15 to .3 taps on a Morse key per second; that periods of accelerated and retarded growth appear; that the hand outgrows the arm; and that the mean rate of boys slightly exceeds that of girls. Precision was tested with two movements, one familiar (writing); the other unfamiliar (probing). The results indicate that (1) in normal individuals precision is subject to greater variation than maximum rate; (2) absolute size of errors decreases most rapidly between the ages six and nine; (3) errors are greater in the unfamiliar movement; (4) there is little difference between the sexes. From these experiments and those on rate, the following results are obtained concerning bilaterality. (1) In right-handed persons the right hand and arm excel the left in strength, rate, and precision. (2) The effects of effort through either upper extremity are probably shared by both. This is concluded from the facts that (a) between the ages tested the right hand does not outgrow the left in rate-ability; (b) the mean error in precision-tests is reduced by a greater absolute amount for the left hand; (c) the amount and duration of exertion with one hand depend upon previous or simultaneous exertion with the other, and (d) the maximum rate of a joint is possibly affected by immediately previous exertion of the corresponding joint on the other side. (3) Corresponding joints have generally the same periods of accelerated and retarded growth, but there is usually bilateral asymmetry of development. The fact that, in general, periods of rapid growth involve increasing asymmetry shows that asymmetry is a normal attendant of growth. In addition the following theoretical conclusions are drawn from the experiments on rate, (1) The maximum rate of voluntary rhythmically repeated movement is probably a critical test of voluntary control. It is apparently identical with in nervation rate, and a test of (a) the power of arrest and reversal, (b) the general condition of the central nervous system, and (c) the condition of motor centres involved. (2) The periods of acceleration, decline, and recovery of rate-ability in children