AFTER-EFFECTS ON STIFFNESS AND STRETCH REFLEXES OF HUMAN FINGER FLEXOR MUSCLES ATTRIBUTED TO MUSCLE THIXOTROPY

1. While the subject maintained a weak contraction in his finger flexo r muscles, holding the metacarpophalangeal joints in 45 deg flexion, t est torque pulses were applied which caused rapid finger extension mov ements and electromyographic (EMG) stretch reflex responses. Before ea ch test pulse the fingers were passively flexed or extended ('postshor t' and 'post-long' trials) for about 10 s. The EMG and joint deflectio n responses in the two types of trial were compared after averaging. 2 . In the 'post-long' trials, the EMG reflex response showed a comparat ive increase in latency, with a reduction of the short-latency (M1) co mponent and an enhancement of the medium-latency (M2) component. 3. Th e angular deflections were larger, and the turning points of the defle ctions, which indicated the start of the mechanical reflex responses, occurred later in the 'post-long' trials. These differences were not s een when the torque pulse was immediately preceded by a strong, brief isometric finger flexor contraction in the test position. 4. Immediate ly following the return to the test position the background finger fle xor EMG activity was larger in the 'post-long' trials, a difference wh ich gradually subsided over 15-20 s. A strong, brief contraction in th e test position also eliminated this inter-trial difference. 5. The re sults are interpreted as manifestations of thixotropic after-effects i n intra- and extrafusal muscle fibres. It is proposed that the M1 comp onent of the stretch reflex is largely a response to the 'initial burs t' of impulses in primary spindle afferents. The results have implicat ions in relation to current debate concerning both the mechanisms resp onsible for segmentation of EMG stretch reflex responses and the post- contraction effects on muscle stiffness and stretch reflexes.