Effects of Heavy‐Load Strength Training During (Neo‐)Adjuvant Chemotherapy on Cardiorespiratory Fitness, Aerobic Enzymes, Capillaries, Fatigue and Quality of Life in Women With Breast Cancer

To investigate the effects of heavy-load strength training during (neo-)adjuvant chemotherapy on cardiorespiratory fitness and muscle content of aerobic enzymes and capillaries, as well as to explore its effects on cancer-related fatigue and health-related quality of life (HR-QoL) in women with breast cancer. Women diagnosed with stage I-III breast cancer were randomized to a strength training group (ST, n = 23), performing supervised heavy-load training twice weekly during chemotherapy, or a usual care control group (CON, n = 17). Peak oxygen consumption (VO_2peak), hemoglobin concentration ([Hb]), cancer-related fatigue, and HR-QoL were assessed and muscle biopsies of the vastus lateralis were collected before chemotherapy (T0) and after chemotherapy and training (T1). VO_2peak and [Hb] decreased in both groups (VO_2peak: ST: -5.7 [-11.4 to -0.1]%, p = 0.014; CON: -11.0 [-16.4 to -1.3]%, p = 0.0005), [Hb]: (ST: -6.3 [-11.2 to -1.3]%, p = 0.011; CON: -7.1 [-11.9 to -2.3]%, p = 0.007). Aerobic enzyme levels (COX4, CS, HADH combined) remained stable in ST but decreased in CON (-11.4 [-19.5 to -3.2]%, p = 0.003). The effect size (ES) (0.79) was moderate in favor of the strength training group. Capillaries were unchanged in ST but reduced in CON for type 2 fibers (capillaries around each fiber: -16 [-31 to -2]%, p = 0.04; capillaries around each fiber divided by fiber area: -19 [-34 to -5]%, p = 0.0138) with moderate ESs in favor of ST. Physical fatigue increased in CON but remained stable in ST. Strength training during chemotherapy mitigated reductions in capillary density and aerobic enzyme levels, as well as the rise in physical fatigue observed with usual care.