Systematic review and meta-analysis of skeletal muscle fatigue in old age

Abstract

Despite intense interest in understanding how old age may alter skeletal muscle fatigability, a quantitative examination of the impact of study design on age-related differences in muscle fatigue does not exist.

Purpose: The purpose of this study was to conduct a systematic review of the differences in muscle fatigue between young and older adults, with specific examination of moderator variables suggested to contribute to discrepancies across studies: contraction intensity, contraction mode, duty cycle, fatigue index, sex, muscle group, and contraction type.

Methods: The standardized effect of age on muscle fatigue was computed for 37 studies (60 standardized effects). Standardized effects were coded as positive when less fatigue was reported in older individuals compared with young individuals.

Results: The overall standardized effect of age on muscle fatigue was positive (0.56). In studies using dynamic contractions or using muscle power as the index of fatigue, the standardized effect was negative (-0.12 and -2.5, respectively). The standardized effect for all other moderator categories was positive (range = 0.09-0.90), indicating less fatigue in older adults under all other methodological conditions.

Conclusion: This review provides the first quantitative analysis of the effect of study design on age-related differences in muscle fatigue. The results indicate that older individuals develop less muscle fatigue than young individuals, particularly during isometric contractions of the elbow flexor and knee extensor muscles. However, the results also suggest that older adults develop greater fatigue during dynamic contractions, particularly when the decline in power is assessed. Studies that verify this latter outcome are needed, as are studies designed to elucidate the mechanisms of fatigue.

FIGURE 1

Funnel plot of SE and standardized effect for each study. Each of the 60 standardized effects and their associated SE (from Table 2) are represented by open circles. The angled lines define the area in which 95% of the standardized effects are expected to fall, in the absence of publication bias. The vertical line defines the middle of the funnel. Symmetry within the funnel on either side of the midline indicates a lack of publication bias.

FIGURE 2

Standardized effect for each moderator variable (mean ± SE). The standardized effects are the weighted average across all categories of each moderator. All standardized effects were positive, indicating less fatigue in older individuals compared with young individuals.

FIGURE 3

Standardized effect for each moderator variable (mean ± SE) for each fatigue index. A, Standardized effects when a drop in force or a drop in power was used (n = 38 standardized effects) as the fatigue index. B, Standardized effects when endurance time (n = 22 standardized effects) was used as the fatigue index.