Muscle Matters: Understanding how Muscle Function Contributes to Reactive Stepping Performance

Muscle weakness and balance control are two important predictors of fall risk. The main objective of this work was to determine how leg muscle function contributes to reactive stepping performance – a strategy used to recover balance. Studies 1-3 were conducted in young, healthy adults and study 4 was performed in individuals with chronic stroke. The first study demonstrated that force plate measures used to estimate foot-off and swing times of the stepping limb following a loss of balance had good to excellent between-session reliability. The second study highlighted unique function-specific increases in muscle activity following larger balance perturbations. Namely, the timing of muscle activation suggested that the modulation of medial gastrocnemius and biceps femoris was likely important for reducing forward momentum, propelling the swing limb forward, and applying braking forces at foot contact. In addition, increased muscle activity of the vastus lateralis and tibialis anterior likely contributed to earlier foot-off times and restabilization of the centre of mass following foot contact. The third study found that while most measures of muscle strength and explosive force were not related to reactive step timing, knee extensor explosive force was correlated with swing time. The last study revealed strong correlations between the average number of steps required to recover balance and knee flexor and extensor concentric peak torque in chronic stroke. Overall, the findings of this work suggest that muscle function contributes to reactive stepping performance, with the knee extensors having an important role particularly during restabilization. Variability in strength and balance control due to aging or neurological injury may accentuate these correlations to a greater extent and warrant further investigation.Ph.D