Age-related differences in agility are related to both muscle strength and corticospinal tract function

Agility is essential for "healthy" aging, but neuromuscular contributions to age-related differences in agility are not entirely understood. We recruited healthy (n = 32) non-athletes (30-84 years) to determine: (1) if aging is associated with agility and (2) whether muscle strength or corticospinal tract function predicts agility. We assessed muscle strength via a validated knee extension test, corticospinal tract function via transcranial magnetic stimulation, and agility via spatiotemporal values (i.e., leg length-adjusted hop length and hop length variability) collected during a novel propulsive bipedal hopping (agility) task on an electronic walkway. Pearson correlation revealed aging is associated with leg length-adjusted hop length (r = -0.671, p < 0.001) and hop length variability (r = 0.423, p = 0.016). Further, leg length-adjusted hop length and hop length variability correlated with quadriceps strength (r = 0.581, p < 0.001; r = -0.364, p = 0.048) and corticospinal tract function (r = -0.384, p = 0.039; r = 0.478, p = 0.007). However, hierarchical regressions indicated that, when controlling for sex, muscle strength only predicts leg length-adjusted hop length (R2 = 0.345, p = 0.002), whereas corticospinal tract function only predicts hop length variability (R2 = 0.239, p = 0.014). Therefore, weaker quadriceps decrease the distance hopped, and deteriorating corticospinal tract function increases variability in hop length.