Differences between young and older women in maximal force, force fluctuations and SEMG during isometric knee extension and elbow flexion

The loss of muscle strength with aging appears to be greater in the lower than upper limbs, but strength and its neural control have never been compared in the same population of individuals in both upper and lower limbs. The aim of this study was to investigate differences between eight young (20–31 years) and eight older (68–76 years) healthy women in maximal voluntary contraction (MVC), force fluctuations, median frequency (MDF) of the surface electromyogram (sEMG), and muscle fiber conduction velocity (MFCV) during sustained isometric elbow flexion (EF) and knee extension (KE), performed at moderate to high force intensity. Older women showed larger fluctuations of force with endurance and changes in sEMG pointing to less fatigue, especially at high level of force, with no differences between upper and lower extremities. This may have significant implications in the design of rehabilitation programs directed to this population. Muscle Nerve, 2004. Strength decline in older people is greater in the lower than upper extremities.17, 21, 32 This has been ascribed to a decreasing use of lower- than upper-limb muscles,17, 32 which is supported by the fact that age-related morphological changes are more pronounced in the quadriceps than biceps brachii muscle.3 Older people, in addition, show a reduced ability to exert a precise force during submaximal contractions, which involves measures of force fluctuations during both isometric and anisometric contractions and has been referred to as steadiness.7, 8, 13, 14, 19, 27, 54 Surface electromyography (sEMG) can provide important information on the peripheral properties and central strategies of the neuromuscular system, especially during constant-force sustained isometric contraction.38, 39 A decline in median frequency (MDF) or other parameters of the power spectrum and muscle fiber conduction velocity (MFCV), which can be attributed to muscle fatigue, has been observed in both young and older individuals, with the rate of decrease of spectral parameters being lower in older people. This phenomenon can be ascribed to selective atrophy of type II fibers, slowing in the contractile properties, and lower motor unit (MU) firing rates of the older muscle.40, 42. To our knowledge, the maximal voluntary force (MVC), steadiness, and sEMG of muscles of upper and lower limbs have never been compared in the same population of individuals, regardless of age. It could be of importance to verify whether muscle strength and neuromuscular control of the upper and lower limbs are affected by aging to a similar extent, since this information is relevant to direct rehabilitation and training programs for older individuals. As women not only may reach levels of strength below the thresholds for tasks important for an independent life, but also do so before men, it is reasonable to consider them as the first target of any study on aging.49 The aim of this study was therefore to investigate any differences between young and older healthy women in MVC, steadiness, MDF, and MFCV during knee extension (KE) and elbow flexion (EF), by performing constant-force sustained isometric contractions from moderate to high intensity. We expected that the lower levels of muscle function in the older women would be more pronounced for KE than EF