The purpose of this investigation was to compare the myosin heavy chain (MHC) isoform expression of the triceps brachii muscle and isoinertial, isometric and isokinetic strength indices in bodybuilders (B; n=5), competitive rowers (R; n=5) and untrained control (C; n=5) subjects. Muscle tissue samples were analysed for MHC isoform content using electrophoresis. The B possessed significantly smaller (p<0.05) percentage of MHC type IIb proteins (12.9± 7.1%) than R (31.2± 2.7%) and C (38.2± 3.0%) groups (i.e., B<R<C). C (55.8± 5.4%) presented also significantly more type IIa MHC isoforms than R (42.8± 3.0%) and C (34.7± 1.6%) subjects. The MHC type I protein content did not differ significantly among different groups (B: 31.3± 2.7%; R: 25.4± 1.7%; C: 27.1± 1.8%). However, when the results of MHC type proteins were converted to effect sizes (ES), it appeared that low statistical power rather than the absence of an effect accounted for the non-significant differences between B and other groups (i.e., B>R» C). Significant differences existed in isoinertial strength among B and R subjects (i.e., B>R» C), while isometric and isokinetic strength were not significantly different among trained groups. However, the ES transformation of data demonstrated that large differences existed between B and other groups for isometric and isokinetic strength (i.e., B>R» C). A statistically significant negative correlation (p<0.05) was found between MHC type IIb isoforms and isoinertial strength index (r= -0.68). The MHC type IIa proteins were positively related to all the strength measures considered (r= 0.51-0.61; p<0.05). These data demonstrated different patterns of MHC isoform expression among the different groups of athletes and it is suggested that these differences on occasion may affect the expression of strength.
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