The purpose of this study was to estimate the efficiency of ATP utilization for concentric, eccentric, and isometric muscle action in the human tibialis anterior and extensor digitorum longus in vivo. A dynamometer was used to quantitate muscle work, or tension, while simultaneous 31P-nuclear magnetic resonance data were collected to monitor ATP, phosphocreatine, inorganic phosphate, and pH. The relative efficiency of the actions was estimated in two ways: steady-state effects on high-energy phosphates and a direct comparison of ATP synthesis rates with work. In the steady state, the cytosolic free energy dropped to the lowest value with concentric activity, followed by eccentric and isometric action for comparative muscle tensions. Estimates of ATP synthesis rates revealed a mechanochemical efficiency [i.e., ATP production rate/work (both in J/s)] of 15.0 +/- 1.3% in concentric and 34.7 +/- 6.1% in eccentric activity. The estimated maximum ATP production rate was highest in concentric action, suggesting an activation of energy metabolism under these conditions. By using direct measures of metabolic strain and ATP turnover, these data demonstrate a decreasing metabolic efficiency in human muscle action from isometric, to eccentric, to concentric action.

The implementation of muscle ergometry during magnetic resonance spectroscopy and imaging is complicated by the restrictive dimensions of the magnet bore and the presence of a strong static magnetic field. We have developed a dynamometer that is compatible with these constraints. This device can provide resistance to voluntary muscle contraction during isometric, isokinetic concentric, and isokinetic eccentric muscle contractions. While controlling muscle contraction speed, the dynamometer simultaneously records muscle torque production at a 10-Hz sampling frequency to allow assessment of compliance and retrospective normalization of power output for the mass of active muscle. All parameters relevant to muscle contraction are selectable, including percentage of maximal voluntary contraction, velocity of muscle contraction, duty cycle, and range of motion for the contraction. This paper provides examples of 31P-magnetic resonance spectroscopic measurements during isokinetic concentric contractions of the ankle dorsiflexors, isokinetic eccentric contractions of the soleus, and isometric contractions of the soleus. Operation of the dynamometer has no adverse effects on the integrity of the 31P-magnetic resonance spectra at 4 T, permitting temporal resolution of the phosphocreatine resynthesis rate of approximately 1 spectrum/s. The unique capabilities of this dynamometer will facilitate studies into the metabolic response of working muscle in healthy and diseased populations.

A critical requirement of submaximal exercise tests is the comparability of workload and associated metabolic stress between subjects. In this study, 31P-magnetic resonance spectroscopy was used to estimate metabolic strain in the soleus muscle during dynamic, submaximal plantar flexion in which target torque was 10 and 15% of a maximal voluntary contraction (MVC). In 10 healthy, normally active adults, (PCr + Pi)/PCr, where PCr is phosphocreatine, was highly correlated with power output normalized to the volume of muscle in the plantar flexor compartment (r = 0.89, P < 0.001). The same variable was also correlated, although less strongly (r = 0.78, P < 0.001), with power normalized to plantar flexor cross-sectional area. These findings suggest that comparable levels of metabolic strain can be obtained in subjects of different size when the power output, or stress, for dynamic plantar flexion is selected as a function of plantar flexor muscle volume. In contrast, selecting power output as a function of MVC resulted in a positive linear relationship between (PCr + Pi)/PCr and the torque produced, indicating that metabolic strain was increasing rather than achieving constancy as a function of MVC. These findings provide new insight into the design of dynamic muscle contraction protocols aimed at detecting metabolic differences between subjects of different body size but having similar blood flow capacity and mitochondrial volume per unit of muscle.