Neither total muscle activation nor co-activation explains the youthful walking economy of older runners.
BACKGROUND: Older adults (≥65 years) exhibit greater metabolic rates during walking (worse walking economy) compared to young adults. Yet, previous research suggests that habitual running, but not habitual walking, exercise mitigates the age-related deterioration of walking economy. RESEARCH QUESTION: Does total leg muscle activation and/or agonist-antagonist co-activation explain the superior walking economy of older runners versus older walkers? METHODS: We quantified metabolic power, leg muscle activation, and co-activation in older walkers and older runners during walking at 0.75, 1.25, and 1.75 m/s. RESULTS: While accounting for multiple comparisons, neither total stride (each speed p ≥ 0.024), stance- (each speed p ≥ 0.217), nor swing- (each speed p ≥ 0.170) phase EMG amplitude differed between older walkers and older runners at 0.75, 1.25, or 1.75 m/s. Stride averaged medial gastrocnemius and biceps femoris activation was lower in older runners than older walkers at 1.25 and 1.75 m/s (all p ≤ 0.025). We also calculated shank, thigh, and overall (shank and thigh) agonist-antagonist leg muscle co-activation over each stride, and the only difference between groups was a greater shank co-activation in older runners at 0.75 m/s (p = 0.024). Across groups, stride, stance-, and swing-phase total muscle activation positively correlated with gross metabolic power (R2 = 0.58-0.66; all p < 0.001). Paradoxically, across groups, stride, stance-, and swing-phase muscle co-activation indices were negatively correlated with gross metabolic power (R2 = 0.08-0.29; all p ≤ 0.007). SIGNIFICANCE: Neither total leg muscle activation nor co-activation explains the superior walking economy of older runners versus older walkers.