The effect of load uncertainty on anticipatory muscle activity in catching.

To investigate how the CNS copes with load uncertainty in catching, anticipatory postural adjustments (APAs) in one-handed catching of balls of known and unknown weights were compared. Twenty-nine (n = 29) men (mean age = 21.1 years) participated, all of whom had engaged in a sport activity requiring hand-eye coordination. Participants' muscle activity in the biceps brachii, triceps brachii, wrist flexor group, and bilateral erector spinae at L4-5 was recorded using electromyography (EMG) while they caught visually identical balls of four different weights (0.5, 1.33, 2.17, and 3.0 kg). EMG integrals were computed for the 1 s prior to ball drop (pre-drop period), and the interval between ball drop and catch (drop period). Uncertainty about ball weight had no effect on APA activity during the pre-drop period. During the drop period, however, load uncertainty did influence APA activity in the biceps brachii, triceps brachii, and the wrist flexor muscles (i.e., the effect of ball weight on APA magnitude depended on the presence or absence of load knowledge). In the known ball weight condition, participants exhibit greater APA magnitude with increases in ball weight. In contrast, under the unknown ball weight condition, APA magnitude was relatively consistent across ball weights and at a level similar to the APA magnitude for an intermediate weight (i.e., the second heaviest ball of four) in the known weight condition. In catching balls of unknown weights, the CNS appears to scale APA magnitude to afford the greatest chance of catching the ball, regardless of the weight.