Structure and function of the ankle dorsiflexor muscles in young healthy men and women

The overall aim of this thesis was to increase our knowledge of the relationship between structure and function of human skeletal muscle, and about factors affecting strength. The ankle dorsiflexor muscles (DF) were chosen as a representative model of human skeletal muscle, and because these muscles have important roles in gait and maintenance of balance. Specific aims in this thesis were: i) to develop reliable methods to assess DF structure and function, ii) to examine several statistical measures for the interpretation of test-retest reliability, iii) to investigate determinants, and iv) potential sex-related differences, of DF strength and size. Young, healthy men and women between 20 and 32 years of age, with comparable physical activity levels, participated in the five studies on which this thesis is based. Protocols to examine DF strength and fatigue using an isokinetic dynamometer (Biodex) were developed in studies I-III. The subjects performed three maximal concentric (CON) DF contractions at five angular velocities (study I), three maximal eccentric (ECC) DF contractions at two angular velocities (study II), and 50 consecutive maximal CON DF contractions at 60°/s (study III), on two different occasions separated by 7-10 days. Intra-rater test-retest reliability of muscle strength and fatigue measurements was highly reliable. In study IV, measurements of DF contractile and noncontractile components were obtained using magnetic resonance imaging (MRI), and intra- and inter-rater test-retest reliability for repeated measurements from the same MR-image was examined and found to be highly reliable. Several statistical indices of test-retest reliability were used in studies I-IV. In study V, determinants and potential sex-related differences of DF strength and size were assessed in 30 men and women with comparable physical activity levels. Men had higher isokinetic DF CON and ECC strength (≈24% and ≈27%; p<0.01), and larger DF contractile cross-sectional area (cCSA: 19%; p<0.001), than women. Biopsies from the tibialis anterior muscle, the main DF, were prepared enzyme histochemically to determine areas (AREA) and proportions (PROP) of types I and II fibres. Men had larger AREA I and AREA II (21% and 31%; p<0.01) than women, whereas PROP I was similar between the sexes. Multivariate linear regression revealed that CON DF strength was up to 72% determined by cCSA and PROP I, and ECC DF strength was up to 81% determined by cCSA, PROP I and sex. For both CON and ECC DF strength, variables other than cCSA explained at most 9% of the variation. Body weight and fibre areas explained more than 50% of the variation in cCSA. In conclusion, test-retest reliability of DF strength and fatigue measurements, and assessments of DF tissue components, were highly reliable. Test-retest reliability based on continuous data is best analysed with several statistical indices – a specified ICC, method error statistics and analyses of systematic change in the mean. The structure and function of the DF in young, healthy individuals were closely related. The greater strength in men was almost exclusively explained by the greater muscle size, which, in turn, was determined by differences in body weight and fibre areas. Sex-specific difference existed in the ECC mode – when muscle size and physical activity levels were accounted for men produced more ECC torque for the same amount of muscle. This sex-specific difference is unclear and requires further study.