DISSOCIATION BETWEEN METABOLIC AND CONTRACTILE RESPONSES DURING INTERMITTENT ISOMETRIC-EXERCISE IN MAN

This study examines the temporal changes in high-energy phosphate and metabolite levels, and in force-generating capacity, during and after voluntary submaximal repetitive isometric exercise (RIE). Eight male s ubjects performed one-legged RIE with the knee extensors at 40 % maxim um voluntary contraction (MVC) target force (duty cycle: 6 s contracti on, 4 s rest) in a 48 cm bore whole body 1.5 T superconducting magnet. Phosphocreatine (PCr), inorganic phosphate (P-i), ATP and pH were mea sured every 9 s. Force-generating capacity was repeatedly measured usi ng MVC force and electrically stimulated contractions (sequential trai n of impulses of 1-100 Hz). During RIE, MVC declined gradually by 56 /- 5 % (mean +/- S.E.M.). Electrically stimulated force also declined, with a disproportionally large drop in low-frequency force, seen as a decline from 0.76 +/- 0.02 to 0.33 +/- 0.02 in 20:50 Hz force ratio. The PCr decline during RIE was 65 +/- 9 %, in most subjects seen as a rapid initial drop followed by less or no further decline to exhaustio n. pH declined in parallel by 0.18 +/- 0.04 units, whilst ATP levels r emained unchanged throughout the exercise. PCr, P-i and pH recovered t o near control values within 5 min of exhaustion. Force, however, was not fully restored after 30 min recovery. The results support the hypo thesis that fatigue from submaximal RIE is unrelated to changes in P-i and H+ levels. The decline in 20:50 Hz force ratio implies that fatig ue may be associated with excitation-contraction coupling impairment. No sudden changes were observed in mechanical or metabolic factors at exhaustion. Exhaustion was probably not caused by lack of substrates f or ATP resynthesis, since pH had decreased only marginally.