Single Limb Fatigue Compromises The Ability To Generate Force In The Opposite Limb

Abstract

Performing strong voluntary contractions with a single limb produces unintended muscle activity in the homologous muscle of the opposite resting limb. The mechanisms underlying these crossed-effects are likely to involve supraspinal centres, as neural activity in the active motor cortex is known to spread within and between motor cortices during repeated muscle contractions. Although strong contractions result in unintended activity in the resting opposite limb, it is unknown how brief contractions or fatigue inducing contractions affect the maximum force generating ability of the opposite limb. PURPOSE: To determine how intermittent maximal contractions of a single limb influences the ability to produce force in the opposite limb. METHODS: Twelve healthy participant (age: 23 ± 3.4 yr) were recruited into the study. Motor evoked potentials (MEP), EMG of the first dorsal interosseous (FDI), and index finger abduction forces were obtained before, during, and after a single limb intermittent maximal contraction protocol. The contraction sets consisted of four 4 s contractions separated by 2 s of rest for the dominant limb- followed a 10 s period where MEP, FDI EMG and force data were collected for the non-dominant limb. Contractions were continued until MVC reduced by 50%. RESULTS: In unfatigued muscles, only small increases in unintended activity occurred when the opposite limb was maximally contracting (∼5% increase in force with no changes in MEP amplitude). After one contraction set was performed by the dominant limb, the capacity to generate force in the non-dominant limb was significantly reduced by 15% which corresponded to a 20% reduction in MEP amplitude. Maximal force output for both limbs significantly reduced across the duration of contraction sets, where maximum force output declined by 52% for the exercising limb and 36% for the contralateral limb when the criteria for termination was achieved. CONCLUSIONS: Crossed-effects were evident across all conditions; however fatiguing contractions were most effective in eliciting responses in the contralateral limb. Given that repeated single limb contractions reduced the maximal force generating capacity in the opposite limb, it appears that inhibitory mechanisms were activated on the opposite side of the body, presumably at the level of the motor cortex.