Age-related changes in motor unit function

Abstract

This review focuses on the functional relationship between age‐related morphological and physiological changes at the level of the motor unit (MU). It is well established that older humans are weaker than younger people, exhibit reduced force control, and have slower neuromuscular contractile properties. Older people may also exhibit a decrease in MU discharge rate, and an increase in variability of MU discharge at high force levels. The matching of MU discharge and contractile properties may be an age‐related neurophysiological strategy adopted to optimize motor control, similar to that observed in acute conditions such as fatigue. Because muscle force output is modulated partially by MU discharge behavior, the study of these properties may offer insights into the physiology of muscular weakness and motor function in older people. In turn, this will allow the implementation of optimal exercise and rehabilitation programs to reduce the degree of dependence associated with aging. © 1997 John Wiley & Sons, Inc. Muscle Nerve, 20, 679–690, 1997.

An exploration of the association between frailty and muscle fatigue

“…Factors associated with muscle fatigue include aging, disease, gender, nutrition, physical activity, subjective fatigue, inflammation, muscle strength, size, structure, metabolism, and blood flow, and neural and hormonal factors (Abbiss and Laursen 2005;Allman and Rice 2001;Hunter et al 2004;Roos et al 1997;Vandervoort 2002;Vandervoort and Symons 2001). To our knowledge, no one has examined the association between frailty and muscle fatigue.…”

“…Investigators have examined the differences in fatigability between various muscles in young and older adults (Allman and Rice 2001;Roos et al 1997), yet findings vary among studies (Chan et al 2000). Some studies have shown that older adults experienced less fatigue than younger adults during isometric, isokinetic, and isotonic contractions (Hakkinen 1995;Hunter et al 2004Hunter et al , 2005Yassierli and Nussbaum 2007), whereas other researchers reported that older adults exhibit greater fatigue during isotonic and electrical stimulation contractions (Cupido et al 1992;McNeil and Rice 2007;Petrella et al 2005), and still others report the existence of no age-related differences during isometric, isokinetic, and electrical stimulation contractions (Klein et al 1988;Lanza et al 2005;Smolander et al 1998; (Table 2).…”

“…The major components of the neuromuscular function and structure that are associated with muscle fatigue are muscle strength, muscle size, muscle structure, and neural factors (Roos et al 1997;Vandervoort 2002;Vandervoort and Symons 2001) (Fig. 2).…”

“…The reduction of muscle size is most likely due to the de- cline in the number and size of muscle fibers (Roos et al 1997). Human muscle fibers are usually divided into three groups; type I (slow twitch, oxidative), type IIa (fast twitch, oxidative/glycolytic), and type IIx (fast twitch, glycolytic) (Sargeant 1994).…”

“…Neuromuscular function is closely associated with ADL performance in both healthy and ill older adults (Katsiaras et al 2005;Petrella et al 2005). Age-related changes in neuromuscular morphology and function have been extensively examined (Porter et al 1995;Roos et al 1997;Short and Nair 1999) and agreement exists among researchers that muscle strength and quality decline with advancing age (Allaire et al 2004;Hunter et al 2004); hence the link to frailty. In addition to muscle strength, muscle fatigue is an important component of the neuromuscular function that may be linked to frailty.…”