In the analysis of relative fatigue, there was a significant group by set interaction (p < 0.001) (Figure 3). Post hoc analyses revealed that relative fatigue was significantly greater in young participants during sets 2 through 5 (old vs. young: set 2: 17.1 vs. 26.6%; set 3: 25.5 vs. 39.7%; set 4: 28.1 vs. 45.1%; set 5: 29.3 vs. 46.4%; overall relative fatigue: old 22.2%; young 38.1%).
There are several factors to consider when determining why some people report age-related enhanced fatigue resistance, whereas others do not. First, the duty (contract/relax) cycle in studies of muscle fatigue can vary considerably. The exercise protocol in the current study consisted of 30 seconds of intermittent contractions interspersed with 60 seconds of rest or approximately 150 seconds of total work. Lanza and colleagues (14) noted that studies using fatigue protocols with a duty cycle less than 50% show increased fatigue resistance with age (3,10), whereas those using a test with a duty cycle greater than 50% show no effect of age on muscle fatigue (4,28). The exercise protocol used in the current study could be viewed as 30 seconds of exercise to 60 seconds of rest or approximately 0.40 seconds per contraction to approximately 0.70 seconds rest between repetitions. Either perspective characterizes the exercise test as low duty cycle, which supports the finding of enhanced fatigue resistance in older men in the current study. Potentially, using the same number of sets and repetitions with a different duty cycle could influence the results. However, Russ et al. (26) used a dorsiflexion exercise challenge with a 70% duty cycle to test the hypothesis that a high duty cycle would eliminate age-related differences in fatigue resistance. Contrary with their hypothesis, older men and women (n = 16; 73 yr) demonstrated enhanced fatigue resistance compared with young men and women (n = 16; 25 yr). Thus, it is currently unclear what the role of the work to rest ratio is when examining age-related changes in muscle function.
The type of contraction and the number of sets may also influence the outcomes of studies of the effects of age on muscle fatigue. For instance, Petrella et al. (25) assessed muscle fatigue in older (n = 24; ≈64 yr) and younger (n = 28; ≈27 yr) adults after 10 repetitions of knee extensions at 40% of 1 repetition maximum. Older adults experienced a 24% decrease in maximum concentric velocity, whereas the young adults only tended to decrease (not significant). These data indicate that younger adults are more resistant to fatigue during a single set of isotonic contractions. Interestingly, there was no effect of age on performance of a fatiguing sit to stand test. Baudry and colleagues (2) also reported that older men and women (n = 16; ≈77 yrs) are more fatigable than young men and women (n = 16; ≈30 yrs), but their tests were during 5 sets of 30 concentric contractions of the dorsiflexor muscles at 50° sec−1. Although the exercise protocol used by Baudry et al. (2) is similar to the current study, data from dorsiflexion fatigue may not approximate knee extensor fatigue. Force production and contractile properties may not follow a similar pattern of change between muscle groups with increasing age (8,9). In older individuals, the proportion of type II fibers in the dorsiflexors and knee extensors are 16% and 45%, respectively. In terms of function, isometric strength of the knee extensors may decrease 40% in older adults up to 70 years, whereas dorsiflexor strength may decline more slowly (21,22). Evidence for age-associated differences between muscle groups was also provided by Lanza et al. (15), who showed 16% less concentric torque of the dorsiflexors and 36% less torque in the knee extensors in older, relative to younger, men. Similarly, peak power of the knee extensors was 41% less, and peak power of the dorsiflexors was 28% less in older relative to young men. Although it is informative to study aging in several muscle groups, collectively, these data indicate that it may not be appropriate to compare the dorsiflexors and the knee extensors between studies. For instance, young and old dorsiflexors may have similar dorsiflexor characteristics but dissimilar knee extensor properties, which could explain the divergent findings of Baudry et al. (2) with the current study. If it is the case that the knee extensors undergo changes that make this muscle group more oxidative in nature, then perhaps the dorsiflexors need several more years to undergo significant age-associated transformation.
Although the current study used a 2-group (old vs. young) design and found age-associated differences in fatigue resistance, a potentially influencing factor is the age of the older subjects. In two recent studies, a 3-group study design was used with young (26 yr), old (65 yr), and very old (84 yr) categories (22,23), because muscle properties in some muscle groups may change more rapidly than others (22,23). Older adults were more fatigable relative to young adults, but this only became statistically significant in the very old group (84 yr) (22). Thus, older adults in some studies may have been too young for any differences in fatigability to be detected.
Finally, neural drive could be reduced in older individuals, and this may also explain why some report an effect of aging on fatigue resistance, whereas others do not. However, there is no consensus on an inability of older adults to maximally activate their muscles (11). Differences in the methodology used to assess voluntary muscle activation (i.e., interpolated torque (IT) ratio vs. central activation ratio), muscle groups studied (e.g., quadriceps vs. dorsiflexors), and habitual physical activity levels all influence comparisons of voluntary activation between young and older adults. As an example, Yoon et al. (30) recently reported that aging increases central fatigue immediately after a low-force (20% MVC) fatiguing contraction but not after an 80% MVC. It is known that older adults are able to reach target velocity during isokinetic exercise ranging from 60 to 270° sec−1 (12), so it is not surprising that subjects in the current study were able to reach the target velocity of 180° sec−1. In the current study, an extra evoked force was not used and electromyography measurements were not performed, so any discussion of age-associated differences in neural drive is speculative.
The current study indicates that the knee extensors of older men are less fatigable than younger men during repeated sets of intermittent contractions, possibly as a result of the more oxidative nature of older muscle. The isokinetic exercise mode used in the current study builds upon previous investigations with the addition of repeated sets. The results of the current study, however, are specific to the isokinetic mode of exercise, the speed (180° sec−1), duty cycle, and the exercise protocol (5 sets of 30 repetitions). Therefore, conclusions about enhanced fatigue resistance in older muscle may not extend to other exercise modes, protocols, or contraction types. It could be argued that isotonic contractions better approximate activities of daily living, so the results of the current study should be verified using concentric/eccentric actions in a controlled laboratory setting as well as practical field tests (e.g., sit to stand, stair climb, get up and go tests). Although the exercise challenge used in the current study is not commonly performed in everyday life, these data indicate that exercise prescription in older adults need not focus primarily on resistance to fatigue. Resistance to muscle fatigue is only one component of healthy aging muscle, and enhanced fatigue resistance in the knee extensors of older men should be incorporated into resistance training program prescriptions. Perhaps exercise interventions targeted toward prevention of falls should focus on balance or power training, which may be more closely associated with performance of ADLs, rather than fatigability/sustainability of contractions.
The author wishes to thank the dedicated subjects who participated in this research study, Ian Lanza, PhD, for his data management expertise, and Priscilla Clarkson, PhD, for her support of this article. No external financial support is declared. The results of the present study do not constitute endorsement by the authors or the NSCA.
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