Larger strength losses and muscle activation deficits in plantar flexors induced by backward downhill in reference to distance-matched forward uphill treadmill walk

We tested the hypothesis that backward downhill walking (eccentric component) impairs both voluntary activation and muscle contractile properties in the plantar flexors and delays recovery as compared to a gradient and distance-matched uphill walk. Fourteen males performed two 30-min walking exercises (velocity: 1 m/ s; grade: 25%; load: 12% of body weight), one downhill (DW) and one uphill (UP), in a counterbalanced order, separated by 6 weeks. Neuromuscular test sessions were performed before, after, 24-, 48- and 72-h post-exercise, including motor nerve stimulations during brief (5 s) and sustained (1 min) maximal isometric voluntary contractions of the plantar flexors. DW (−18.1 ± 11.1%, P <.001), but not UP (−6.0 ± 7.7%, P =.15), decreased torque production during brief contractions for at least three days post-exercise (P <.05). Voluntary activation during brief contractions decreased after DW (P <.05), but not UP, and recovered by 24 h. Both UP (−9.3 ± 9.0%, P =.024) and DW (−25.6 ± 10.3%, P <.001) decreased torque production during sustained contractions but voluntary activation (P =.001) was lower in DW than UP. Peak twitch torque and maximum rates of torque development and relaxation were equally reduced after UP and DW (P <.05), and recovered by 24 h. DW induced an increase in muscle soreness with peak values observed 48 h post-walking (P <.001), whereas post-UP exercise changes were non-significant (all P >.05). Using a direct comparison, the capacity to drive the plantar flexors during sustained contractions remains sub-optimal during the three-day recovery period in response to non-exhaustive, downhill backward walking in reference to an uphill exercise matched for distance covered.