Kinematic and electromyographic responses to perturbation of a rapid grasp.

Kinematic and electromyographic responses of the thumb and index finger to load-induced extension of the thumb during a rapid precision grasp of the thumb and finger were studied in four human subjects. Angular position of the index finger metacarpophalangeal (MP) and proximal interphalangeal (PIP) joints was recorded along with the linear position of the thumb tip (TH). Myoelectric activity was recorded from flexor pollicis longus, flexor digitorum superficialis, first dorsal interosseous, and extensor digitorum communis. Loads that extended the thumb were applied ranging from 125 ms prior to movement onset to movement onset. For loads applied earlier than 50 ms before movement onset, the amount of flexion of TH was reduced compared with control trials. Contact between the finger and thumb was attained nonetheless due to an altered trajectory of the fingertip that was generated by reduced flexion of PIP and increased flexion of MP. These finger responses appear to be functional in that contact of the finger pulp on the more distal pulp of the thumb was preserved. With loads delivered near onset of the grasp, there was increased PIP flexion, rather than reduced PIP flexion. These responses to later loads occurred despite greatly reduced magnitudes of TH flexion compared with loads delivered well before onset of the gasp movements. Thus reduced PIP flexion observed with early loads was not simply the result of finger biomechanics. The thumb flexor muscle increased activity 45-55 ms after onset of the load, whereas responses of the finger flexors began 65 ms after load onset. Response magnitudes decreased as loads were introduced nearer to movement onset. Measured reaction times of the finger muscles to thumb extension stimuli averaged 154 ms, which indicated that the responses of the finger muscles were not voluntary responses to the thumb extension. Afferent information generated by perturbation of the thumb during a grasp movement can influence the activity of intrinsic and extrinsic muscles to yield apparently functional compensations in the closing movements. However, temporal limitations exist that appear to offer greater constraints on the use of afferent signals for controlling rapid movements than for sustained grasp.