Cross-Sectional Areas of Various Muscle Fibers in Skeletal Muscles of Athletes Training for Endurance, Strength, or Speed-Strength

Based on published data, the mean cross-sectional areas (CSAs) of various muscle fibers (MFs) were compared in athletes training for endurance (EAs), strength (SAs), or speed-strength (SSAs). Articles that described MF CSA data for male athletes over 18 years old were searched for among articles published from 1972 to 2021. A total of 50 articles with results from 807 athletes were selected. The data were divided into four groups. The first group included the mean MF CSAs in EAs. CSA was analyzed separately for the muscles taxed most or taxed less during main exercise in the group. The second and third groups included the data on the muscles taxed most in SAs and SSAs, respectively. The fourth group included the data on the same muscles in untrained controls from the same studies. In EAs, the absolute values of type I and type II MF CSAs in the muscles taxed most (6438 ± 1074 and 7296 ± 1242 µm 2 ) were higher ( p < 0.05) than in the muscles taxed less (5310 ± 1200 and 5800 ± 1100 µm 2 ). Type I MF CSA of the muscles taxed most in EAs was higher than in SAs and SSAs (6438 ± 1074, 5715 ± 1090, and 5690 ± 1010 µm 2 ; p < 0.05). Type II MF CSA of the muscles taxed most in EAs was lower than in SAs (7296 ± 1242 and 8712 ± 1520 µm 2 ; p < 0.05) and much the same as in SSAs (6820 ± 1200 µm 2 ; p < 0.05). When type II MF CSA of the muscles taxed most was normalized to body weight, the value in EAs was higher than in SSAs (103.1 ± 20 and 84.0 ± 18 µm 2 /kg, p < 0.05) and much the same as in SAs (97.3 ± 18 µm 2 /kg, p > 0.05). Both an increase in oxidative capacity and MF hypertrophy were assumed to play a role in muscle adaptation to long-term training in EAs. The functional load on the muscles in the main competitive exercise is necessary to consider when comparing CSA in athletes of different specializations.