MUSCLE RESISTANCE TO SLOW RAMP WEAKLY DEPENDS ON ACTIVATION LEVEL

The mechanical response of human m. flexor pollicis longus to slow (3. 2 degrees/s) linear stretch by 5.5 degrees was measured during sustain ed (45-60 s, 9-13.5 p.p.s.) unfused tetanus evoked by electrical stimu lation. The stiffness increased during unfused tetanus. At the late ph ase of unfused tetanus it was 1.8 +/- 0.2(mean +/- S.D.) times greater than at the early phase. The sensitivity of the isometric tension lev el to a short change in a stimulation frequency also increased. At the late phase of unfused tetanus force oscillations increased 1.2 +/- 0. 2-fold during slow stretch or shortening and immediately reached a sma ller amplitude after the cessation of length change. This was probably related to the friction and thixotropy in muscles. Muscle resistance to slow ramp depended only weakly on activation level. In the late pha se of unfused tetanus the stiffness per unit force was 1.5 = 0.4 times greater at 9-13.5 p.p.s. than at 20-25 p.p.s. Thus, the relative valu e of muscle stiffness was greater for smaller activation levels typica l for maintenance of posture. The enhancement of muscle stiffness duri ng sustained unfused tetanus and a weak stiffness dependence on the ac tivation level indicated a non-additivity of processes occurring in ac tive muscle. (C) 1997 Published by Elsevier Science Ltd.