Individual physiological and mitochondrial responses during 12 weeks of intensified exercise

Aim Observed effects of exercise are highly variable between individuals, and subject‐by‐training interaction (i.e., individual response variability) is often not estimated. Here, we measured mitochondrial (citrate synthetase, cytochrome‐c oxidase, succinate dehydrogenase, and mitochondrial copy‐number), performance markers (W peak, lactate threshold [LT], and VO2peak), and fiber type proportions/expression (type I, type IIa, and type IIx) in multiple time points during 12‐week of high‐intensity interval training (HIIT) to investigate effects of exercise at the individual level. Methods Sixteen young (age: 33.1 ± 9.0 years), healthy men (VO2peak 35–60 ml/min/kg and BMI: 26.4 ± 4.2) from the Gene SMART study completed 12‐week of progressive HIIT. Performance markers and muscle biopsies were collected every 4 weeks. We used mixed‐models and bivariate growth models to quantify individual response and to estimate correlations between variables. Results All performance markers exhibited significant (W peak 0.56 ± 0.33 p = 0.003, LT 0.37 ± 0.35 p = 0.007, VO2peak 3.81 ± 6.13 p = 0.02) increases overtime, with subject‐by‐training interaction being present (95% CI: W peak 0.09–0.24, LT 0.06–0.18, VO2peak 0.27–2.32). All other measurements did not exhibit significant changes. Fiber type IIa proportions at baseline was significantly associated with all physiological variables (p
Exercise training leads to whole body and tissue‐specific adaptations. However, the observed effects of exercise are highly variable between individuals, and subject‐by‐training interaction (i.e., individual response variability) is often not estimated. Here, we measured mitochondrial (citrate synthetase, cytochrome‐c oxidase, succinate dehydrogenase and mitochondrial copy‐number), performance markers (W peak, lactate threshold, and VO2peak) and fiber type proportions/expression (type I, type IIa, and type IIx) in multiple time points during 12‐week of high‐intensity interval training to investigate effects of exercise at the individual level, and if measurement changes in different domains covary each other as exercise goes by.