Relative metabolic efficiency of concentric and eccentric exercise determined by 31P magnetic resonance spectroscopy

To determine the relative metabolic efficiency (metabolic energy used per unit of mechanical energy output) of negative to positive muscular power, we used 31P magnetic resonance spectroscopy to monitor the cellular energy metabolism of limb muscles in eight healthy subjects during a nonfatiguing, mixed concentric-eccentric activity and during its concentric and eccentric components. We also studied isometric contractions. We found that in terms of the flow of metabolic energy through the muscle cells, the cost of concentric exercise at this intensity was proportional to the mechanical power generated, but the cost of eccentric and isometric exercise did not increase significantly as the apparent intensity of the exercise increased over the range studied. Although the pattern was similar in all subjects, the quantitative relationship between metabolic cost and mechanical output was different in subjects with different muscular strength. The qualitative results can be explained in the context of the known biochemistry and biophysics of the cellular contractile apparatus (sliding filament theory, with independent force generators).