Moderators of fatigue: the complexity of interactions

Fatigue is a difficult phenomenon to study because the response can vary based upon task-specific (i.e. contraction type, intensity, position– vs. load-matching and muscle group/joint region) and subject-specific (i.e. sex and age) variables. Although numerous investigations have provided insight into muscle fatigue, further efforts were needed to better characterize the influence of age, sex, joint/muscle group, contraction type, and task complexity have upon fatigue. The primary purpose of this series of three studies was to identify and characterize the influences of potential moderating variables (i.e., sex, joint, age, contraction type, and task complexity) upon fatigue resistance during voluntary muscle contraction fatigue tasks through both empirical (systematic review and meta-analysis) and experimental methods. In general, women demonstrated either the same or better fatigue resistance than men (men never better), the sex advantage was joint specific not systematic, old men were more fatigue resistant than young men, task complexity was not an influential factor and fatigue differences were more readily apparent under isometric conditions. The inclusion of empirical and experimental methods helped clarify the driving factors of localized muscle fatigue. This in turn will better direct future study design and power for mechanistic, training and performance response studies.