Energy cost and metabolic regulation during intermittent and continuous tetanic contractions in human skeletal muscle

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Abstract
Muscle ATP turnover, glycogenolytic, and glycolytic rates were estimated to compare the energy cost and glycolytic regulation of 102.4 s of continuous and intermittent stimulation. Quadriceps femoris muscles of male subjects were stimulated at 20 Hz for one continuous contraction (n = 6) or a series of 64 contractions (1.6 s on, 1.6 s off; n = 6). Leg blood flow was occluded and muscle biopsies were obtained at rest and following 51.2 and 102.4 s of contraction time in both conditions. Isometric force production by the activated knee extensors decreased to 55% of initial contraction force with intermittent and 80% of initial contraction force with continuous stimulation following 51.2 s of contraction time. Corresponding ATP turnover rates were 4.49 ± 0.39 and 3.80 ± 0.44 mmol∙kg dry muscle−1∙s−1.When normalized for tension production the respective energy costs of intermittent and continuous contractions were 3.66 ± 0.47 and 2.64 ± 0.36 mmol ATP∙kg−1∙100 N−1.Glycogenolytic rates were identical during the first 51.2 s of stimulation but glycolysis was higher in the intermittent group (1.05 ± 0.10 vs. 0.86 ± 0.11 mmol∙kg−1∙s−1). We suggest that the increased ATP utilization of intermittent contractions is associated with enhanced Ca2+-transport ATPase activity during relaxation and enhanced actomyosin ATPase activity during the early portion of each contraction. Glycolytic rate is dependent on ATP demand and regulated by allosteric modulators of phosphofructokinase and pyruvate kinase which are released or consumed in the reactions associated with contraction.