MUSCULAR FUNCTION, METABOLISM AND ELECTROLYTE SHIFTS DURING PROLONGEDREPETITIVE EXERCISE IN HUMANS

Marked functional changes occur in human skeletal muscle during prolon ged repetitive exercise. The maximum voluntary contraction force (MVC) falls gradually and may reach 50% of control within 30-60 min. The tw itch tension declines faster and to a larger extent. During repetitive submaximal isometric contractions. the rate of relaxation increases p rogressively. in parallel with an increased energy cost of contraction . These functional changes are all slowly reversed in the post-exercis e period, as indicated by only minor changes over the first 30 min of recovery. Minor changes in substrates and metabolites. together with t he slow rate of recovery. indicate that the alterations in contractile properties and energetics are independent of these metabolic factors. Alternative explanations may be related to electrolyte shifts over th e sarcolemma or between cellular compartments. The total loss of K+ is small, and could not be detected by analysis of muscle biopsies. Only a slight initial rise in muscle content of calcium was found. The ava ilable data indicate that the increased energy cost of contraction is not connected to mitochondrial dysfunction, which might be caused by c alcium accumulation. Rather, it seems that the ratio of AIP utilizatio n to force is increased, and this could possibly be connected to the f aster relaxation rate. Considering the low excitation rates during sub maximal voluntary contractions, each motor unit generates an oscillati ng force closely associated with Ca2+ fluctuations between SR and cyto sol. Increased relaxation rate might be caused by faster reuptake of C a2+ into the SR. and this could contribute to the faster ATP turnover.