Spatial and temporal heterogeneity of superficial muscle strain during in situ fixed-end contractions

Numerical models of contracting muscle offer a powerful tool to study local mechanical load. For validation of these models, the spatial and temporal distribution of strain was quantified in fixed-end contracting rat tibialis anterior muscle in situ at optimal muscle length (Lo) and at 120° plantar flexion as well as at 125 and 33 Hz stimulation frequency. We studied the hypothesis that after termination of stimulation in situ muscle segments near the motor endplates elongate while segments away from the endplates shorten. We show that both spatial and temporal inhomogeneities in muscle deformation occurred during contraction. Muscle plateau shortening strain equalled 4.1%. Maximal plateau shortening of a muscle segment was much larger (9.6%) and occurred distally (at 0.26 of the scaled length of the muscle). Manipulating torque levels by decreasing the stimulation frequency at the same muscle length induced a decrease in torque ( 20%) with a smaller effect on the level and no effect on the pattern of muscle deformation. During relaxation, distal segments actively shortened at the expense of proximal muscle segments, which elongated. The segments undergoing lengthening were nearer to motor endplates than segments undergoing shortening. In conclusion, the present study provides experimental data on magnitude of contraction-induced deformation needed for validation of numerical models. Local muscle deformation is heterogeneous both temporally and spatially and may be related to proximity to the motor endplates.