Repeatability of training-induced skeletal muscle adaptations in active young males.

OBJECTIVES: Measurements of protein content, enzymatic activity, and/or capillarization are frequently utilized as markers of skeletal muscle adaptation following exercise training. Whether changes in these markers of muscle adaptation are repeatable when individuals are repeatedly exposed to the same training stimulus is unknown. The purpose of this study was to test the repeatability of skeletal muscle adaptations to two identical training periods. METHODS: Ten active young males (age: 22 ±â€¯2 years; VO2max: 57 ±â€¯7 ml/kg/min) were exposed to two identical four-week periods of supervised high-intensity interval running (4 × 4 min at 90-95% of HRmax interspersed with 3-min at 70-75% HRmax) separated by a 3-month wash-out period. Vastus lateralis biopsies were obtained before and after each training period for the measurement of protein content, enzyme activity, and capillary density. RESULTS: Training-induced changes in citrate synthase (CS) maximal activity, protein content (PGC-1α, OXPHOS, and LDH-A), and capillary density were not repeatable within individuals (r = -0.52-0.15; ICCs: -0.42-0.04; CVs: 11-67%). Several OXPHOS complex subunits also demonstrated dissimilar group-level adaptations (period × time interaction effects, p < 0.05) with large differences (ηp2 > 0.4) between training periods. A large (ηp2 = 0.65) increase in capillary density was apparent irrespective of training period (main effect of time, p = 0.05). CONCLUSIONS: An individual (or a group of individuals) may exhibit dissimilar skeletal muscle adaptations when re-exposed to the same training stimulus. Our findings challenge the utility of classifying of individuals as high/low responders using measurements of mitochondrial protein content, CS activity and/or capillary density following a single training period.

Investigating the reproducibility of maximal oxygen uptake responses to high-intensity interval training.

OBJECTIVES: To test the hypothesis that observed maximal oxygen uptake (VO2max) and time to fatigue (TTF) responses to two identical periods of standardized high-intensity interval training are reproducible. DESIGN: Fourteen recreationally active and healthy young males completed two identical four-week periods of high-intensity interval training (4×4-min intervals at 90-95% maximum heart rate [HRmax] separated by 3-min periods of active recovery at 70-75% HRmax). Training periods were separated by a three-month washout period. METHODS: VO2max and TTF were assessed via incremental tests with supramaximal verification before and after each training period. Pearson correlation coefficients (r), intraclass correlation coefficients (ICC), and within-subjects coefficients of variation (CV) were used to assess reproducibility of observed VO2max and TTF responses. RESULTS: VO2max and TTF values before the second training period were not significantly higher than baseline values and there were no significant (p>0.05) interaction effects (period 1: VO2max: +4.04±2.29mL/kg/min, TTF: +70.75±35.87s; period 2: VO2max: +2.83±2.74mL/kg/min, TTF: +83.46±34.55s). We found very weak-to-moderate correlations and poor reproducibility for observed VO2max (mL/kg/min: r=0.40, ICC=0.369, CV=74.4) and TTF (r=0.11. ICC=0.048, CV=45.6) responses to training periods 1 and 2. CONCLUSIONS: Our ANOVA results confirmed that the three-month washout period returned VO2max and TTF levels to baseline and prevented carryover effects. Contrary to our hypothesis, our results suggest that individual observed VO2max and TTF responses to identical training stimuli are not reproducible.