CONTRACTILE PROPERTIES OF RAT GASTROCNEMIUS-MUSCLE DURING STAIRCASE, FATIGUE AND RECOVERY

Slowing of relaxation is one of the anticipated changes in the contrac tion of fatigued skeletal muscle. However, interpretation of the mecha nism(s) contributing to slowed relaxation may be affected by the measu rement technique employed. In this study, relaxation was measured in t hree ways: (i) traditional half-relaxation time; (ii) peak rate of rel axation; and (iii) late relaxation time, measured from 50 to 25 % of p eak developed tension. When rat gastrocnemius muscle was stimulated in directly in situ at 10 Hz, developed tension increased in 10 s to 185 %, then decreased to 39 % after 1 min with little additional change ov er the next 4 min. After 10 s of inactivity, developed tension was 60 % of the initial value, but did not recover further over the next 20 m in. The half-relaxation time transiently decreased at the start of sti mulation, then by 20 s was considerably prolonged. Within 10 s of reco very, half-relaxation time returned to prestimulation values but becam e prolonged again by 10 min of recovery. The peak rate of relaxation w as proportional to the developed tension at all times except 2.5-10 s of 10 Hz stimulation, at which time acceleration of relaxation was evi dent, and 15-20 s of the 10 Hz stimulation when it was relatively decr eased. The late relaxation time increased during the repetitive stimul ation, returned to control level early in recovery, then increased aga in, by 5 min of recovery. The diverse responses indicated by these ind ices of relaxation potentially discriminate different mechanisms which contribute to slowing of relaxation in fatigue, a point which would b e missed if a single method of measurement of relaxation was employed.