Effects of muscle length and age on neuromuscular responses to maximal eccentric contractions of the plantar flexors

INTRODUCTION: Eccentric contractions induce greater neuromuscular impairment and muscle damage than concentric contractions. Muscle length is a key modulator of these responses, yet the combined effects of eccentric contractions and muscle length on neuromuscular function in older adults remain unclear. We investigated acute and delayed neuromuscular responses to eccentric plantar flexor contractions performed at different muscle lengths in young and older adults. METHODS: Twenty-four young and 26 older adults performed 10 sets of 10 maximal eccentric plantar flexor contractions at either short (30°-0° plantar flexion) or long (0°-30° dorsiflexion) muscle lengths (12 young and 13 older adults per condition). Maximal voluntary isometric contraction, voluntary activation, and evoked responses (100 Hz and 10 Hz doublets) were measured before, immediately after, and 1, 24, and 48 h post-exercise. Changes were compared between groups and conditions. RESULTS: Despite similar torque-time integrals between young and older adults within the same muscle length condition, a muscle length effect was only found in young adults, with a greater (p = 0.021) MVIC decrease at long (-33.6 ± 13.2%) than short lengths (-8.4 ± 15.7%). Older adults showed similar reductions across conditions (≈ - 14-21%). Voluntary activation decreased only at long lengths, with no age difference. Evoked responses showed an age effect (p = 0.032), and the 10/100 Hz ratio decreased only in young after long muscle lengths. Neuromuscular function returned to baseline within 24 h indicating no evidence of severe alteration, although muscle soreness remained elevated at 48 h. CONCLUSION: Long muscle length eccentric contractions induced greater force loss in young but not older adults, with no evidence of prolonged functional impairment in healthy, physically active older adults.