PURPOSE: The objective of this study is to determine whether muscle water content (H2Omuscle) expands with training in deconditioned middle-age men and the effects of this expansion in other muscle metabolites.

METHODS: Eighteen obese (BMI = 33 ± 3 kg⁻¹·m⁻²) untrained (V˙O2peak = 29 ± 7 mL⁻¹·kg⁻¹·min⁻¹) metabolic syndrome men completed a 4-month aerobic cycling training program. Vastus lateralis muscle biopsies were collected before and 72 h after the completion of the last training bout. Water content, total protein, glycogen concentration, and citrate synthase activity were measured in biopsy tissue. Body composition was assessed using dual-energy X-ray absorptiometry, and cardiometabolic fitness was measured during an incremental cycling test.

RESULTS: Body weight and fat mass were reduced -1.9% and -5.4%, respectively (P < 0.05), whereas leg fat free mass increased with training (1.8%, P = 0.023). Cardiorespiratory fitness (i.e., V˙O2peak), exercise maximal fat oxidation (i.e., FOmax), and maximal cycling power (i.e., Wmax) improved with training (11%, 33%, and 10%, respectively; P < 0.05). After 4 months of training, H2Omuscle increased from 783 ± 18 to 799 ± 24 g·kg⁻¹ wet weight (ww) (2%, P = 0.011), whereas muscle protein concentration decreased 11% (145 ± 15 to 129 ± 13 g·kg⁻¹ ww, P = 0.007). Citrate synthase activity (proxy for mitochondrial density) increased by 31% (17 ± 5 to 22 ± 5 mmol·min⁻¹·kg⁻¹ ww, P = 0.024). Muscle glycogen concentration increased by 14% (22 ± 7 to 25 ± 7 g·kg⁻¹ ww) although without reaching statistical significance when expressed as per kilogram of wet weight (P = 0.15).

CONCLUSIONS: Our findings suggest that aerobic cycling training increases quadriceps muscle water although reduces muscle protein concentration in obese metabolic syndrome men. Reduced protein concentration coexists with increased leg lean mass suggestive of a water dilution effect that however does not impair increased cycling leg power with training.