Neuromuscular contributions to the age-related reduction in muscle power: Mechanisms and potential role of high velocity power training.

Although much of the literature on neuromuscular changes with aging has focused on loss of muscle mass and isometric strength, deficits in muscle power are more pronounced with aging and may be a more sensitive measure of neuromuscular degeneration. This review aims to identify the adaptations to the neuromuscular system with aging, with specific emphasis on changes that result in decreased muscle power. We discuss how these changes in neuromuscular performance can affect mobility, and ultimately contribute to an increased risk for falls in older adults. Finally, we evaluate the literature regarding high-velocity muscle power training (PT), and its potential advantages over conventional strength training for improving functional performance and mitigating fall risk in older adults.

Motor unit loss is accompanied by decreased peak muscle power in the lower limb of older adults.

This study investigated the relationship between motor unit (MU) properties and the isometric strength and power of two lower limb muscles in healthy young and older adults. Twelve older adults (6 men, mean age, 77 ± 5 years) and twelve young adults (6 men, mean age, 24 ± 3 years) were studied. MU properties of the tibialis anterior (TA) and vastus medialis (VM) muscles were determined electrophysiologically using decomposition-enhanced spike-triggered averaging (DE-STA). Motor unit number estimates (MUNEs) of the TA were significantly reduced (p<0.05) in older adults (102 ± 76) compared to young adults (234 ± 109), primarily as a result of significantly larger surface-detected motor unit potentials (S-MUPs) in older adults (63 ± 29 µV) compared to young adults (27 ± 14 µV). Although VM S-MUP values were larger in older adults (60 ± 31 µV) compared to young (48 ± 42 µV), the difference was not significant. Maximal isometric strength was significantly larger in both the TA and knee extensors of young adults (TA: 0.56 Nm/kg, KE: 2.2 Nm/kg) compared to old (TA: 0.4 Nm/kg, KE: 1.3 Nm/kg). Similar reductions in peak muscle power were observed between young (TA: 33 W, KE: 35 7 W) and old adults (TA: 26 W, KE: 224 W). The greatest deficit between young and old subjects in peak power output occurred at 20% MVC for the TA and 40% MVC for the knee extensors. Results from this study indicate that there are changes in MU properties with age, and that this effect may be greater in the more distal TA muscle. Further, this study demonstrates that muscle power may be a sensitive marker of changes in neuromuscular function with aging.

Increased motor unit potential shape variability across consecutive motor unit discharges in the tibialis anterior and vastus medialis muscles of healthy older subjects.

OBJECTIVE: To study the potential utility of using near fiber (NF) jiggle as an assessment of neuromuscular transmission stability in healthy older subjects using decomposition-based quantitative electromyography (DQEMG). METHODS: The tibialis anterior (TA) and vastus medialis (VM) muscles were tested in 9 older men (77 ± 5 years) and 9 young male control subjects (23 ± 0.3 years). Simultaneous surface and needle-detected electromyographic (EMG) signals were collected during voluntary contractions, and then analyzed using DQEMG. Motor unit potential (MUP) and NF MUP parameters were analyzed. RESULTS: NF jiggle was significantly increased for both the TA and VM in the old age group relative to the younger controls (P<0.05). NF jiggle was significantly higher in the TA compared to VM (P<0.05). For TA, NF jiggle was negatively correlated with MUNE, and positively correlated with S-MUP amplitude, NF count, MUP duration, MUP peak-to-peak voltage, and MUP area (P<0.05). For VM, NF jiggle was positively correlated with NF count and MUP area (P<0.05), and no significant correlations were found between NF jiggle and S-MUP amplitude, MUP duration, or MUP peak-to-peak voltage (MUNE was not calculated for VM, so no correlation could be made). CONCLUSIONS: Healthy aging is associated with neuromuscular transmission instability (increased NF jiggle) and MU remodeling, which can be measured using DQEMG. SIGNIFICANCE: NF jiggle derived from DQEMG can be a useful method of identifying neuromuscular dysfunction at various stages of MU remodeling and aging.

Effect of creatine supplementation on muscle damage and repair following eccentrically-induced damage to the elbow flexor muscles.

We investigated effects of creatine (Cr) supplementation (CrS) on exercise-induced muscle damage. Untrained males and females (N = 27) ages 18-25, with no CrS history in the past 4 months, were randomly assigned to CrS (creatine and carbohydrate) (n = 9), placebo (P) (carbohydrate only) (n = 9), or control (C) (no supplements) groups (n = 9). Participants followed a 5-day Cr loading protocol of 40 g·day(-1), divided for 5 days prior to exercise, reduced to 10 g g·day(-1) for 5 days following exercise. Testing consisted of 5 maximal isometric contractions at 90 arm flexion with the preferred arm on a CYBEX NORM dynamometer, assessed prior to, immediately following, and 24, 48, 72, and 96 hours post muscle-damaging procedures. Damage was induced to the elbow flexor muscles using 6 sets of 10 eccentric contractions at 75 °/sec, 90 °/sec and 120 °/sec. Participants were asked to rate their muscle soreness on a scale of 1-10. Data was analyzed using repeated-measures ANOVA, with an alpha of 0.05. No significant differences were found between muscle force loss and rate of recovery or muscle soreness between groups over the 96 hr recovery period (p > 0.05). Across all 3 experimental groups an initial decrease in force was observed, followed by a gradual recovery. Significant differences were found between baseline and all others times (p = 0.031,0 .022, 0.012, 0.001 respectively), and between the 48 hour and 96 hour time periods (p = 0.034). A weak negative correlation between subjectively rated muscle soreness and mean peak isometric force loss (R(2) = 0.0374 at 96 hours), suggested that muscle soreness and muscle force loss may not be directly related. In conclusion, 5 days of Cr loading, followed by a Cr maintenance protocol did not reduce indices of muscle damage or speed recovery of upper body muscles following eccentrically induced muscle damage.