Skeletal muscle adaptation is a noteworthy and fascinating feature. Muscle properties can adapt by several factors such as: activity pattern, age, and muscle fiber composition. The modifications of the skeletal muscle in the adult population is well described. In contrast, the adaptation of skeletal muscle in the elderly population is less known. Particularly, in the area of physical inactivity-induced modifications. Physical Inactivity may influence the magnitude of the changes of skeletal muscle modification with aging (Thompson L.V. 2002). On average, by the age of 65 years the skeletal muscle mass in cross-sectional area (CSA) is reduced by approximately 25-30% compared to the peak values at the age of 25 to 30 years. (Booth et al. 1994; Frontera et al. 1991). The loss of muscle mass with aging, named sarcopenia, is mostly like due to the loss of muscle fibers. In particular, there is a loss in the fast-twitch type II muscle fibers (Lexell et al. 1988; Thompson L.V. 1994). Beside the loss of muscle fibers, the muscle fibers size of the fast-twitch type II muscle fibers will also decline. The slow-twitch type I muscle fibers are not likely to decline in size (Grimby et al. 1984; Lexell et al. 1988). A reduction in muscle mass is associated with an increase in morbidity and mortality (Newman et al., 2001; Morley, 2003; Morley et al., 2006).
The loss of muscle strength with aging is well described (Bortz W.M. 1982). But the rate of strength decline appears not to be linear over the life span. There is an acceleration in loss of strength in the late-life (Thompson L.V. 1994). There are strong associations between muscular strength and performance of necessary activities of daily living in elderly (Brach et al 2002; Davis et al 1998; Skelton et al 1994). To perform frequent tasks or tasks for a extended period, the muscle requires force and fatigue resistance (Christie et al. 2011). However, the effect of aging on fatigability has only been investigated in few studies, and the results have not been found consistent (Allman et al. 2002). Some variance in the outcomes can be explained by variances in fatigue task performed, the variability may arise due performance of diverse exercise forms (voluntary vs electrical stimulated, isometric vs dynamic, sustained vs intermittent, high vs low forces), or performance of the same exercise paradigm by muscles of differing contractile properties (Bigland-Ritchie et al. 1995).
According to Morley et al. 2001, a lack of physical activity is the crucial factor to muscle weakening in elderly. Physical activity has direct influences on muscle quality and quantity (Breen et al. 2013). Physical inactivity or muscle wasting is associated with selective loss and atrophy of specific fiber types. Slow-twitch type I muscle fibers show greater atrophy with muscle wasting then fast-twitch type II muscle fibers (Alley, et al. 1997; Fitts et al. 1986; Thompson L.V. 19991; Thompson L.V. 19992). After a period of a week of inactivity, the muscle fibers will adapt into a decrease of slow-twitch type I muscle fibers and an increase in fast-twitch type II muscle fibers (Martin et al. 1989). The fatigability of the muscle will increase following inactivity, the underlying mechanisms are a higher rate of glycogen depletion and lactate production (McDonald et al. 1992; Grichko et al. 2000).
The main research question to investigate the influences of physical activity on fatigue resistance and the underlying mechanisms is; What are the difference in fatigue resistance during repeated electrically evoked isometric knee extensor contractions between frequently active elderly persons and sedentary elderly aged between 65 and 70 years old?
Besides physical activity and aging, nutritional status also influences skeletal muscle adaptations. Malnutrition and undernutrition have some nameable contributing factors to sarcopenia (Beck et al. 2013; Beasley et al. 2013; Cerri et al. 2014; Agarwal et al. 2013; Velazquez et al. 2013). Considerable evidence shows a positive association between protein intake and lean muscle mass, strength and vitality among elderly (Borsheim et al. 2008; Houston et al. 2008; Lord et al. 2007; Beasley et al. 2010). A lack of protein intake to balance daily requirements leads to negative protein balance and results in skeletal muscle atrophy, impaired muscle growth, and functional decline. The elderly population are particularly vulnerable to insufficient protein intake (Deer et al. 2015; Paddon-Jones and Rasmussen 2009). Skeletal muscle contractile proteins are the largest protein reservoir in the human body that respond in an anabolic way to feeding and can be rapidly utilized to supply amino acids to the entire organism during fasting or stress. The Recommended Dietary Allowance (RDA) is set to provide a sufficient quantity of a particular nutrient to prevent deficiency in the majority of the population. At the moment, the RDA for protein (0.8 g protein/kg of body weight/day) is the same for all adults, regardless of age or sex (National Research Council. 1989). The RDA may be insufficient to adequately meet the metabolic and physiological needs of the elderly (Campbell et al. 1994; Campbell et al. 1994; Gersovitz et al. 1982). Therefore, studies suggest a higher dietary protein and/or supplementation offer effective ways of preventing, delaying or slowing the progression of sarcopenia in older adults (Baier et al. 2009; Pannemans et al. 1998; Symons et al. 2007; Fujita et al. 2007).
It is well documented that protein intake is positively associated with preservation of lean mass is older adults, and that a higher protein intake is associated with a reduction in the loss of muscle strength and function (Houston et al. 2008; Beasley et al. 2010; Beasley et al. 2013; Gray-Donald et al. 2014; Gregorio et al. 2014). To investigate if the level of protein intake is also related to the fatigue resistance there is a second research question; Is the level of protein intake related to fatigue resistance during repeated electrically evoked isometric knee extensor contractions within the frequently active elderly and sedentary elderly aged between 60 and 70 years old?