Can muscle shortening alone, explain the energy cost of muscle contraction in vivo?

Abstract

Decreased whole-body energy cost of running has been associated with an increased Achilles tendon stiffness. It is usually assumed that this lower energy cost can be attributed to less muscle fascicle shortening with a stiffer tendon. Increased fiber shortening is an important determinant of muscle energetics in vitro. However, other factors, like increased muscle activation may be important when considering whole muscle energetics in vivo. To determine the effects of a small additional muscle shortening on skeletal muscle energy requirement, 19 subjects performed 30 plantarflexions on two separate occasions: isometric (ISO) and isokinetic (KIN, 6.98 rad s(-1)), each with a target of 50 % of maximum isometric torque. Medial gastrocnemius muscle fascicle length (FL) was measured by ultrasound and rate of oxyhemoglobin (HbO2) desaturation was measured during blood flow occlusion using near-infrared spectroscopy. KIN resulted in significantly greater muscle shortening (23.8 ± 1.3 mm) than ISO (18.3 ± 1.0 mm, p < 0.001, mean ± SEM), and greater shortening velocity (KIN = 2.5 ± 0.3 FL s(-1), ISO = 1.1 ± 0.1 FL s(-1), p < 0.001). Rate of HbO2 desaturation was 19 ± 7 %, greater in KIN than ISO (p < 0.01), despite 19 ± 2 % lower mean torque (p < 0.001) and 9.8 ± 1.6 Nm s lower mean impulse per contraction (p < 0.001) in KIN compared to ISO. Root mean square for EMG was significantly greater (p < 0.05) during KIN (73 ± 3 %) than during ISO (63 ± 2 %). These results illustrate that muscle energy requirement is greater when muscle fascicle shortening and/or velocity of shortening is increased, and suggest that greater activation contributes to that increased energy requirement.