Metabolic cost of lengthening, isometric and shortening contractions in maximally stimulated rat skeletal muscle

Aim: The present study investigated the energy cost of lengthening, isometric and shortening contractions in rat muscle (n=19). Methods: With electrical stimulation the rat medial gastrocnemius muscle was maximally stimulated to perform 10 lengthening, isometric and shortening contractions (velocity 25 mm s(-1)) under experimental conditions (e.g. temperature, movement velocity) that resemble conditions in human movement. Results: Mean+/-SD force-time-integral of the first contraction was significantly different between the three protocols, 2.4+/-0.2, 1.7+/-0.2 and 1.0+/-0.2 N s, respectively (P<0.05). High-energy phosphate consumption was not significantly different between the three modes of exercise but a trend could be observed from lengthening (7.7+/-2.7 mumol similar toP muscle(-1)) to isometric (8.9+/- 2.2 mumol similar toP muscle(-1)) to shortening contractions (10.4+/-1.6 mumol similar toP muscle(-1)). The ratio of high-energy phosphate consumption to force-time-integral was significantly lower for lengthening [0.3+/-0.1 mumol similar toP (N s)(-1)] and isometric [0.6+/-0.2 mumol similar toP (N s)(-1)] contractions compared with shortening [1.2+/-0.2 mumol similar toP (N s)(-1)] contractions (P<0.05). Conclusions: The present results of maximally stimulated muscles are comparable with data in the literature for voluntary human exercise showing that the energy cost of force production during lengthening exercise is similar to30% of that in shortening exercise. The present study suggests that this finding in humans probably does reflect intrinsic muscle properties rather than effects of differential recruitment and/or coactivation.