Non-local muscle fatigue effects on muscle strength, power, and endurance in healthy individuals: A systematic review and meta- analysis

Objective: To examine whether non-local muscle fatigue occurs following performance of a fatiguing bout of exercise of a different muscle(s). Design: Systematic review and meta-analysis. Search and Inclusion: A systematic literature search using a Boolean search strategy was conducted with PubMed, SPORTDiscus, Web of Science, and Google Scholar in April 2020 and was supplemented with additional ‘snowballing’ searches up to September 2020. To be included in our analysis, studies had to include at least one intentional performance measure (i.e., strength, endurance, or power), which if reduced could be considered evidence of muscle fatigue, and also had to include the implementation of a fatiguing protocol to a location (i.e., limb or limbs) that differed to those for which performance was measured. We excluded studies that measured only mechanistic variables such as electromyographic, or spinal/supraspinal excitability. After search and screening, 52 studies were eligible for inclusion including 57 groups of participants (median sample = 11) and a total of 303 participants. Results: The main multilevel meta-analysis model including all effects sizes (278 across 50 clusters [median = 4, range = 1 to 18 effects per cluster) revealed a trivial point estimate with high precision for the interval estimate (-0.02 [95%CIs = -0.14 to 0.09]), yet with substantial heterogeneity (Q(277) = 642.3, p < 0.01), I2 = 67.4%). Subgroup and meta-regression analyses showed that NLMF effects were not moderated by study design (between vs. within-participant), homologous vs. heterologous effects, upper or lower body effects, participant training status, sex, age, the time of post-fatigue protocol measurement, or the severity of the fatigue protocol. However, there did appear to be an effect of type of outcome measure where both strength (0.11 [95%CIs = 0.01 to 0.21]) and power outcomes had trivial effects (-0.01 [95%CIs = -0.24 to 0.22]), whereas endurance outcomes showed moderate albeit imprecise effects (-0.54 [95%CIs = -0.95 to -0.14]). Conclusions: Overall, the findings do not support the existence of a general NLMF effect; however, when examining specific types of performance outcomes there may be an effect specifically upon endurance-based outcomes (i.e., time to task failure). However, there are relatively fewer studies that have examined endurance effects or mechanisms explaining this possible effect, in addition to fewer studies including women or younger and older participants, and considering causal effects of prior training history through the use of longitudinal intervention study designs. Thus, it seems pertinent that future research on NLMF effects should be redirected towards these still relatively unexplored areas.