Lifelong Strength Versus Endurance Training: Differential Adaptations In Efferent Neural Drive And Spinal α-motoneuron Excitability

Abstract

PURPOSE: Neural factors play a critical role in the age-related decline in maximal strength and rate of force development (RFD). However, it is uncertain how the neural attenuation may be mitigated by strength or endurance training, respectively. METHODS: In this study we applied evoked spinal motoneuron recordings to examine efferent neural drive (V/Msup during maximal voluntary contraction; MVC) and α-motoneuron excitability (Hmax/Mmax at 10% MVC) in a total of 69 older (>65 yrs) and younger (<35 yrs) strength athletes (n = 21), endurance athletes (n = 17) and control subjects (n = 31). RESULTS: Leg press strength (one repetition maximum: 1RM) but not maximal oxygen uptake (V̇O2max), were positively associated with V/Msup (r = 0.60, p < 0.001). Leg press 1RM and RFD were higher (all p < 0.05) in older strength athletes (1RM: 164 ± 18 kg; RFD (0-200 ms); 4262 ± 1022 N·s-1) than endurance athletes (1RM: 132 ± 25 kg; RFD: 3080 ± 669 N·s-1) and older controls (1RM: 109 ± 15 kg; RFD: 2202 ± 915 N·s-1), but lower compared to training-matched young groups (p < 0.05). V̇O2max was positively associated with Hmax/Mmax (r = 0.47, p < 0.001) while no correlation was observed for 1RM. V̇O2max was higher (all p < 0.05) in older endurance athletes (47.6 ± 7.6 ml·kg-1·min-1) than in older strength athletes (30.9 ± 6.0 ml·kg-1·min-1) and older controls (34.2 ± 4.4 ml·kg-1·min-1), but lower than all young groups (p < 0.001). CONCLUSIONS: Across the life span the plasticity in skeletal muscle strength and aerobic capacity, respectively, seems to be associated with differential neural factors. Specifically, chronic strength training seems to result in a higher efferent neural drive during maximal muscle contractions (elevated V-wave responses), largely dependent on corticospinal factors, whereas chronic endurance training appears to be related to a high α-motoneuron excitability, at least during low-level muscle contractions. Since high efferent neural drive is a key component for strong muscle contractions with age, strength training should be emphasized for the ability to carry out force-dependent tasks at older age.