Visual Timing of Muscle Preactivation in Preparation for Landing

Lee (1976, 1980a, 1980b) proposed that the inverse of the rate of dilation of an image across the retina (tau), caused by an approaching surface, directly specifies time to contact and that preparatory actions are triggered at specific tau-margins. This study investigated the possibility that such a visually regulated control mechanism is responsible for muscle preactivation prior to landing impact. Muscle preactivation enables the momentum exchange to be controlled prospectively resulting in soft, injury-free landings. Six subjects performed 10 drops from 3 heights (0.72 m, 1.04 m, and 1.59 m) onto a force plate. Comparison of traces from the force plate and electromyography (EMG) traces from the rectus femoris muscle enabled calculation of the time before ground contact at which preactivation occurred. This time to contact lengthened significantly as the height from which subjects dropped increased. Tau-margin values were calculated for the mean preactivation time for each subject at each height. A within subjects analysis of variance (ANOVA) revealed no significant difference between tau-margins for the three heights. Mean tau-margins were 0.592 s, 0.522 s, and 0.583 s for the low, medium, and high heights, respectively. The results support muscle preactivation as being lawfully grounded at the ecological scale, rendering putative cognitive mechanisms unnecessary.