Section 7.2 – EFFECTS OF AGE ON OPERATIONAL PHYSICAL PERFORMANCE

Age is widely regarded as the most important determinant of the loss of an individual’s physical performance capability over time. Said loss is of concern for employers of personnel involved in physically demanding occupations, particularly military personnel. A minimum level of physical performance is a prerequisite for military personnel because the absolute physical job demands are often dictated by extrinsic factors that cannot be altered. A reduced work capacity may lead to inadequate performance, increase the risk of overuse injury, and, ultimately, compromise effectiveness of military operations. However, appropriate training can counter the effects of aging on strength and endurance and enable the individual to maintain adequate levels of performance late into life. 7.2.1 BACKGROUND Increasing age leads to inadvertent changes in maximal performance capacity (Samson et al., 2000; Booth et al., 1994; Bemben, 2003). Physiological functional capacity (PFC), the ability to perform the tasks of daily life in an effortless and successful manner (Donato et al., 2003) also decreases. Muscular strength and endurance are the most relevant of all physiological measures limiting physical performance (Frontera et al., 2000; Fitzgerald et al., 1997; Janssen et al., 2005). Deficits in strength and endurance have a negative impact on performance in common tasks like locomotion, or lifting, handling, or carrying loads (Bemben, 2003; Leyk et al., 2006a; Leyk et al., 2006b). With the changing demographic structures in western societies, this will become a major concern not only in everyday life, but especially in physically demanding occupations like the automotive and shipping industries or the military services where exogenous factors often cannot be altered, thus dictating the physical demands of occupations. 7.2.2 AGE-RELATED CHANGES IN MUSCULAR STRENGTH The human musculoskeletal system produces the strength and power to move and interact with the physical world. Its capacity determines the performance in primarily load-related tasks. For example, handgrip INTRINSIC AND EXTRINSIC FACTORS AFFECTING OPERATIONAL PHYSICAL PERFORMANCE RTO-TR-HFM-080 7 3 strength can play a crucial role in load-bearing or transportation tasks such as stretcher carriage (Bilzon et al., 2002; Leyk et al., 2006a; Leyk et al., 2006b). Various studies have described a peak in muscular strength and muscle mass around the age of 20 – 30 years (Janssen et al., 2000; Brooks et al., 1994; Booth et al., 1994; Reeves et al., 2006). A 10 – 15% loss of whole body muscle mass can be observed from age 20 to 80 years, with a concomitant increase in relative and absolute body fat (Janssen et al., 2000). Also, a two-staged decrease in muscular performance can be observed from age 30, with a moderate decline in muscular parameters up to age 55 – 65 years and an accelerated reduction thereafter (Booth et al., 1994). Extents as well as rates of decline appear to be dependent on individual factors, such as genetics, lifestyle, and habitual physical activity (Wilmore et al., 2004; Roubenoff et al., 2000). Age-related changes occur in: • Muscle mass; • Muscle cross-sectional area (MCSA); • Muscle fiber type composition; • Reduced number of capillaries; • Changes in enervation and neural drive; • Myofibril protein content; and • Muscle force production and power output. The most noticeable and critical effect of the aging process is the loss of muscle mass, termed sarcopenia. Sarcopenia occurs around age 55 (Roubenoff, 2003; Booth et al., 1994), and is evidenced by a reduction in MCSA. This reduction is a function of cellular, nutritional, and hormonal changes (Brooks, 2003; Reeves et al., 2006; McArdle et al., 2001; Booth et al., 1994). Reduction in contractile tissue, thickening of Type I fibers, clustering of muscle fibers, and changes in muscle fiber composition concomitant to the reduction in MCSA are also observed (Brooks et al., 1994; Brooks, 2003; Reeves et al., 2006; Roos et al., 1999; Lexell et al., 1986). The loss of MCSA, combined with an increase of non-contractile tissue, results in lowered capacity for force production and, consequently, power output (Maharam et al., 1999; Doherty et al., 1993; Brooks, 2003; Reeves et al., 2006; Lanza et al., 2003). However, force production capability of the contractile tissue itself appears to be largely unaffected by aging (Maharam et al., 1999). A promising hypothesis for the mechanisms underlying sarcopenia has been formulated. Sarcopenia can be explained as the combined effects of progressive neuromotor deterioration and chronic under-loading of the musculoskeletal system on the single motor unit level (Brooks et al., 1994; McArdle et al., 2001). Lack of use and suspected morphological aspects specific to Type II motor units, together with deterioration in motor unit remodeling capabilities, appear to cause the preferred denervation of Type II fibers (Brooks et al., 1994). The results are muscle fiber atrophy and ultimately the irreversible degeneration of end plate structures and muscle fibers (McArdle et al., 2001). However, denervated Type II fibers may be re-innervated by adjacent Type I motor units by the sprouting of motor nerves and end plates. This hypothesis can account for the observed loss of muscle mass, the change in muscle fiber composition, and the increase in non-contractile tissue. Sprouting of motor nerves to re-innervate previously denervated muscle fibers may provide an explanation for the increase in size of slow motor units as well as for the clustering of fiber types that was observed in both animals and humans with increasing age (Brooks et al., 1994). 7.2.3 AGE-RELATED CHANGES IN ENDURANCE The relocation of units to remote areas where supplies and equipment must be carried over prolonged periods of time is not solely dependant on strength capabilities. Endurance plays a crucial role in sustained INTRINSIC AND EXTRINSIC FACTORS AFFECTING OPERATIONAL PHYSICAL PERFORMANCE 7 4 RTO-TR-HFM-080 performance. Endurance capacity is typically predicted in a technical approach by determining the maximal rate of oxygen consumption (VO2max) from respiratory gas exchange. Available data from numerous cross-sectional and a few longitudinal studies predict various rates of decline from approximately 5% to 20% per decade, beginning at the age of 25 years. Differences in rates as well as in timing and extent are probably due to differences in occupational or leisure time physical activity, lifestyle, genetic profile, disease, and other factors (Leyk et al., 2006c; Dehn et al., 1972; Maharam et al., 1999). However, even though VO2max is the most frequently used criterion to evaluate endurance capacity, it is only one of the determinants for successful endurance performance. For example, it is not uncommon to see master athletes with lower VO2max performing as good as, or even better than younger individuals with higher VO2max values (Allen et al., 1985; Daniels, 1985; Sjödin et al., 1985; Sleivert et al., 1996). Thus, the age-related decline in VO2max may not inevitably mirror changes in endurance performance. Running times of endurance events such as marathon or half-marathon races provide an excellent opportunity to study age-associated changes in endurance performance. Leyk et al., (2006a) examined more than 400,000 results from marathon and half-marathon events and found endurance performance, as assessed by running times, remained virtually unchanged within an age group from 20 to 49 years with only minor decreases thereafter. The authors concluded that lifestyle factors have a considerably stronger influence on endurance capacity than age per se. 7.2.4 PREVENTIVE MEASURES TO COUNTER AGE-RELATED CHANGES Both endurance and muscular capabilities deteriorate with age. This results in a decline in both maximal attainable performances as well as in decrements in PFC. However, it is often difficult to determine whether the observed reduction is a result of biological aging, or of lifestyle, physical inactivity, genetics, or disuse (Rittweger et al., 2004). Age-related changes recorded in physiological measures of highly trained, competitive master athletes can be regarded as a performance “ceiling”, reflecting mainly the results of primary aging with negligible additional margins for improvement through training (Leyk et al., 2006c; Fitzgerald et al., 1997; Rittweger et al., 2004). While the strictly age-elicited changes in maximal performance capabilities seem to be inevitable, physical exercise may still slow, delay, or reverse decrements in PFC. Physically active elderly individuals, such as master athletes, show a slower decline in VO2max over time compared with their inactive peers, and it has been shown that adequate strength levels can also be maintained (Tanaka et al., 2003; Trappe et al., 1996; Wiswell et al., 2000; Krivickas et al., 2006). Numerous studies have found that both the cardiovascular and the musculoskeletal system show remarkable plasticity late into life (Pollock et al., 1997; Wiswell et al., 2000; Reeves et al., 2006). As a result, the effects of age on PFC may be slowed, halted, or even be reversed. As seen in Leyk et al’s. (2006a) study, physically active elderly are able to maintain levels of performance equal to those of 20-year-old peers. This implies that endurance training can counter the age-related decrements in endurance that can be observed in less-active peers. Strength training also yields similar results as shown by (Pearson et al., 2002). While maximal lifting performance decreased with age, athletes consistently showed results that were significantly better than those of their age-matched controls. Isometric strength of 80to 89-year-old weightlifters was higher than that of the less-active control group (ages 40 – 49 years). The capacity for performance improvements for untrained subjects remains almost constant with increasing age. A roughly 10 to 20% increase in relative performance after a 3-month training regimen can be derived for both strength (Baum et al., 2003) and endurance training as measured in VO2max from age 20 to 80 years (Hagberg et al., 1989; Meredith et al., 1989). Given the development of age structures in the western world, and the related impending impact on personnel in the workplace, who will have to work until an older age, physical exercise and activity may be the key to maintaining appropriate levels of fitness required both for work and everyday life. Appropriate strengthand endurance training at any age INTRINSIC AND EXTRINSIC FACTORS AFFECTING OPERATIONAL PHYSICAL PERFORMANCE RTO-TR-HFM-080 7 5 is the easiest and most effective way to counter the effects of age-related loss of performance (Mazzeo et al., 1998).