Muscle contributions to center of mass acceleration adapt to asymmetric walking in healthy subjects.

Symmetrical limb movement requires complex muscle coordination patterns. Consequently, coordination impairments lead to asymmetric gait patterns, as often seen in stroke subjects. Split-belt walking has previously been used to induce limping-like walking in able-bodied adults. The goal of this study is to analyze how muscle coordination patterns that control the centre of mass are modulated during an asymmetric gait pattern imposed on healthy subjects. These modulations can be uniquely related to the biomechanics of limping as no pathology is present. Forward simulations of limping-like walking (split-belt) and corresponding symmetric conditions (tied-belt) were generated for twelve healthy subjects. Our results show that the differences between 'fast' and 'slow' leg contributions during split-belt walking are not attributable to simple differences in speed between the belts, because most split-belt muscle contributions differ from tied-belt walking. Different types of modulations, inducing increased, decreased or even reversed asymmetry (e.g. plantarflexors, biceps femoris short head, and quadriceps respectively), underlie limping-like walking in healthy subjects. In general, these patterns present large similarities with adaptations previously described in hemiplegic subjects. However, differences were found with gluteus medius and biceps femoris short head contributions in hemiplegic subjects, suggesting that the latter are not just related to limping, but to concomitant deficits.