Large Hypertrophy but Unmodified Specific Tension of Single Fibers of Body Builders

Introduction Muscle hypertrophy is the main outcome of training for body building. Previous studies showed a discrepancy between the functional (force/tension) and structural (cross-sectional area, CSA) proprieties of leg extensor muscles measured in vivo and the corresponding parameters measured in single muscle fibers [1] [2]. In particular, a puzzling finding from these studies was that of a lower specific tension of skinned fibers of body builders (BB) compared to a control (C) population. In this study we aimed to investigate the possible reasons behind the alleged lower specific tension in single fibers of a strictly diet-and-training-controlled BB population. Methods Five male body builders (26±4.7 yr; 87.2±9.7 kg; 178.4±7.5 cm) with a training history of at least 5 years were recruited for this study. Five age-matched recreationally active males (24.6±3.6 yr; 84.4±14.9 kg; 186.4±4.4 cm) were recruited as controls. After ethical approval, biopsies from the vastus lateralis (VL) muscle were collected together with in-vivo quadriceps maximum isometric voluntary contraction (MVC) and architecture and CSA. Data analysis included (i) single skinned fiber mechanics (n=200 fibers; maximum force, CSA, specific tension and calcium diffusion), (ii) single fiber myosin quantification (n=100 fibers), (iii) slow and fast fiber CSA via histological section analysis. In addition, swelling ratio (skinned fibers CSA/histological CSA) was calculated for each fiber type and for each subject. A general linear mixed model was used to assess statistically significant differences. Results Quadriceps MVC and CSA were 64% and 33% higher in BB than C (p<0.05), VL pennation angle (PA) and thickness (MT) were 16% and 15% higher in BB than C (p<0.05). Quadriceps specific force was 25% higher in BB than C (n.s.). Single skinned type 2 CSA and force were 48% and 28% higher in BB than C (p<0.05), while specific tension was 19% lower (p<0.05). Type 1 fibers specific tension was 32% higher in BB than C (p<0.05). Myosin content and calcium diffusion were similar among populations. CSA determined in histological sections was 10% and 27% higher in BB than C (p<0.05) for type1 and 2 fibers, respectively. When type 1 and type 2 fiber tensions were corrected for individual swelling ratio, the difference between BB and C disappeared. Conclusions Although at first sight these findings seem to confirm that in type 2 skinned fibers of BB specific tension is lower than in C, once accounting for fiber swelling, this difference disappears. This is further confirmed by the lack of difference in myosin concentration and calcium diffusion kinetics. In contrast, the results point to the existence of a differential swelling response to the skinning process by BB and C fibers, possibly due to adaptations of the fiber cytoskeleton proteins or specific permeability of the membrane of slow and fast fibers.