DETECTION OF MOVEMENTS OF THE HUMAN FOREARM DURING AND AFTER CO-CONTRACTIONS OF MUSCLES ACTING AT THE ELBOW JOINT

We report here observations on the effects of muscle contraction histo ry on thresholds for the detection of movements at the elbow joint of human subjects. Detection thresholds were measured in the direction of flexion or extension to movements of the relaxed forearm at a speed o f 0.2 deg s(-1) with the elbow at 90 deg. 2. As reported previously, t hresholds for movements in the direction of extension were lower than in the direction of flexion after a conditioning co-contraction of elb ow muscles with the arm flexed by 30 deg from its mid-position (hold-s hort). After a co-contraction with the arm held extended by 30 deg (ho ld-long), thresholds were lower in the direction of flexion. 3. Here w e have made two additional observations. Thresholds for movements of t he passive forearm after a co-contraction at the 90 deg test position (hold-test) were low, both in the direction of flexion and extension. Secondly, when thresholds were measured while subjects were carrying o ut a co-contraction of forearm muscles (15-20% maximum voluntary contr action), thresholds were much higher. 4. It is concluded that muscle c ontraction history is an important factor to consider when making meas urements of movement thresholds at the relaxed elbow joint. It is spec ulated that during an active contraction increases in muscle spindle d ischarges evoked by fusimotor activity lead to the rise in movement de tection threshold.