Effects of age on force steadiness: A literature review and meta-analysis.

The variability of force is indicative of the biological variability inherent in the human motor system. Previous literature showed inconsistent findings of the effect of age on the variability of force and hence a systematic review was performed. Twenty studies were included in this systematic review, of which twelve provided sufficient data to determine effect sizes for the effect of age. After determining the pooled effect size, the effect of sample size on dichotomized effect sizes (significant vs. non-significant) was determined. Also, the effect of possible determinants, age difference between age groups, dominance of investigated limb, muscle group, muscle location (proximal vs. distal and upper vs. lower extremity) and target force level on effect size (categorized as small, medium, or large) were investigated. A large pooled effect size of age was found (rtotal=0.67, 95% CI [0.61; 0.72]). No relation between sample size and effect size significance was found, indicative of no lack of power in the studies reviewed. No relations were found of associations between age difference, upper vs. lower extremity muscle location, and dominance and effect size. Significant relations of effect size with muscle group, proximal vs. distal muscle location and target force level were found. Also, an interaction effect of muscle group and target force level was suggested. The meta-analysis results are in line with motor unit loss as the main cause of the effect of ageing on force steadiness and this effect can partially explain decreased motor performance associated with ageing.