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Neuromuscular determinants of contraction specific weakness in the elderly

Neuromuscular plasticity and ageing:
Ageing of the motor function is characterized by the decrease in muscle mass and strength. Thus, we evaluated neuromuscular function in older individuals. Maximal torques of lower limbs are particularly affected under isometric and concentric conditions. The alteration is more important at high velocities. Motoneuronal excitability was estimated by using Hmax/Mmax ratio (Scaglioni et al 2003). This excitability seems to decrease in ageing. However, torques, under eccentric conditions, appear relatively preserved (Valour et al 2003, Pousson et al 2001). The twitch interpolation technique (Merton, 1954) allowed us to assess maximal voluntary activation capacities. The validity of this technique under isometric, concentric and eccentric conditions was evaluated and proved (Babault et al, 2001, 2002, 2003). Maximal voluntary activation capacities seem to be preserved in ageing. The effect of ageing on the mechanical and electromyographic characteristics of the motor units activated by the maximal Hoffmann reflex and by the direct muscle compound action potential was also assessed. Results suggest an on-going neuromuscular remodelling, resulting in an increased innervation ratio. The neural rearrangement may be viewed as a compensatory adaptation of the motor system to preserve the mechanical efficiency of the surviving motor units, despite the age-related impairment of the segmental reflex system (Scaglioni et al 2003).

Muscular activities, Activation, Co-activation:
The progressive decline in neuromuscular function and performance with aging can be estimated by the measure of the maximal voluntary contraction torque (MVC). However, this impairment of strength is muscle group dependent. Thus we made an original observation, whose functional implications in terms of stability in standing posture are crucial: at the level of the ankle joint, plantar-flexion (PF) MVC torques in elderly men were, on average, 40 % lower than in young adults, whereas there was no significant difference between the two age groups in dorsi-flexion (DF) MVC torques (Simoneau et al 2005). The decline observed in PF maximal performance would be partly explained by changes intervening at the peripheral level, i.e. at the muscle level. Hence, we investigated the effects of a 6-month strength training program of the plantar-flexors on muscular and neural factors of the major muscle groups (agonists and antagonists) around the ankle, in older participants. We have thus shown that older adults developed PF and DF MVC torques greater than those initially produced (Simoneau et al in press). A 6-month strength training program of the plantar-flexors in old age is very effective in recovering strength in PF, and thus reduced the imbalance in the DF-to-PF MVC torque ratio. However, this strength training does not seem to improve the reflex activity of the plantar-flexor muscles, which declines in the older adults, suggesting that this alteration could be linked to degenerative phenomenon (Scaglioni et al 2002).

Contractility and elasticity:
The main purpose of this part was to evaluate the plasticity of muscle elastic properties with aging and following a strength training program in older individuals. The first study (Ochala et al, 2004a) was designed to determine musculotendinous (in situ Quick-release) and musculoarticular stiffness (Sinusoidal perturbations) during plantar flexion in young and older people. The results showed a bigger musculotendinous stiffness in older individuals, what may be due to changes in the muscle tissue. Musculoarticular stiffness was similar between the age groups, suggesting an adaptive mechanism in articular structures to avoid the continuous integration of the musculoarticular stiffness by the central nervous system (simplifying most daily motor tasks). The second study (Ochala et al, 2004b) was devoted to evaluate differences between older women and men. The bigger musculotendinous and musculoarticular stiffness in women compared to men of the same age may be due to disparities in the muscle tissue and explain in part their greater frequency of falling. The third study (Ochala et al, 2005) was devoted to evaluate musculotendinous stiffness during plantar flexion in older individuals following a 6-month strength training program (48 sessions, twice a week, 3 sets of 10 concentric-repetitions at 75% of 3 RM). The smaller musculotendinous stiffness following training may be due to structural and neural adaptations, counterbalancing some effects of the neuromuscular aging.

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