Figure 2. MHC II fibre size, absolute Po, and specific Po from younger and older adults.
Individual study data and collective means for (A) cross-sectional area (CSA), (B) absolute peak force (Po), and (C) specific tension (Po /CSA) of MHC II fibres. Symbols represent the means from men (blue), women (red), and when the sexes were combined (green). The collective means (shaded grey regions) for men, women, and all studies combined (black symbols) were calculated by pooling the results from all studies. Unpaired t-tests were performed on the collective means to test for age differences (young vs. old) using the pooled data. If assumptions of normality or homogeneity of variance were violated, then a non-parametric Mann-Whitney U test was used. To evaluate whether differences in the sample sizes between studies influenced the pooled results, the collective means were also compared between age groups using a weighted linear regression, with the caveat that not all studies could be included in this calculation because the sample sizes were not reported. None of the conclusions differed between the analyses, and thus, we report the collective means and p-values based on the unweighted analyses to allow inclusion of all the studies in the pooled results. For ease of identifying the data from each study both within and between figures, all studies were arranged in chronological order and assigned a numerical value that is located beneath the x-axes. In instances where the numerical data were unavailable in the tables or text, values were extracted from figures or obtained from the authors. Data shown from individual studies were restricted to MHC IIa fibres if the MHC IIx and hybrid IIa/IIx were also reported; otherwise, the data represent the MHC II fibres which may include a mixture of MHC IIa, hybrid IIa/IIx, and IIx isoforms. For studies with interventions, the pre-intervention values are presented, and when data from several cohorts or muscle groups were available, the data are reported from the vastus lateralis of the mobile, independently living cohort. If absolute Po was not provided, it was calculated from CSA and Po/CSA. Statistical significance (P < 0.05) was extracted from the results of the individual studies. Y = Young > Old. O = Old > Young. N/A = no value was reported or was unable to be determined. ? = statistical comparisons were not provided. Values are mean ± SE. In instances where SE was not provided, it was calculated based on the SD and fibre sample size, if available.