Modulation of reflex responses of flexor digitorum superficialis muscle to fingers extension

The motor commands elaborated as reaction to an unexpected perturbation may be associated with short or long loop reflexes. We investigated the modulation of these responses during a reactive grasping task handling the predictability of the magnitude of perturbation. Subjects were required to place their forearm supine on a table and maintain a normal relaxed position of the hand. They were instructed to react grasping a handle pulled by loads of 750 g or 1250 g. Subjects performed two sessions of 5 trials for each load (predictable condition) and one session of 10 trials with a randomized distribution of the loads (unpredictable condition). Kinematic data were captured by an electrogoniometer attached to the second phalanx of the middle finger while the electromyography of Flexor Digitorum Supeficialis muscle was recorded by a couple of surface electrodes. For each trial we measured the latencies of muscle activations, the angular range and the duration of the fingers extension. By performing a two-ways ANOVA with a repeated measures design, we found a significant effect of predictability only for the latency of the short loop reflex (p< 0.05) with longer response times for unpredictable (37±3 ms) than predictable conditions (29±2 ms). Instead, a highly significant effect of load was detected only for the angular range during the extension (p < 0.01; 1250 g: 7.4±1.4°; 750 g: 4.1±1.2°). These results indicated that during a reactive grasping task the predictability of load may modulate the latency of the short loop reflexes.