Force-generating capacity of MHC I and II fibers from well-trained arm and leg muscles of cross-country skiers

Background: Little is known about potential differences in contractile properties of muscle fibers of the same type in arms and legs. Accordingly, the present study compared the force-generating capacity and Ca²⁺-sensitivity (pCa₅₀) of fibers from arm and leg muscles of highly trained cross-country skiers.

Methods: Chemically skinned single fibers of m. vastus lateralis and m. triceps brachii of 8 highly trained cross-country skiers were analyzed with respect to maximal Ca²⁺-activated force, specific force and pCa₅₀.

Results: The maximal Ca²⁺-activated force was greater for MHC II than MHC I fibers in both the arm (+62 %, P<0.001) and leg muscle (+77 %, P<0.001), with no differences between limbs for each MHC isoform. Also, the specific force of MHC II fibers was higher than that of MHC I fibers in both arms (+41 %, P=0.002) and legs (+95 %, P<0.001). The specific force of MHC II fibers was similar in both limbs, whereas MHC I fibers from m. triceps brachii were, on average, 39% stronger than MHC I fibers from m. vastus lateralis (P=0.003). pCa₅₀ was not different between MHC I and II fibers in neither arms nor legs, but the MHC I fibers of m. triceps brachii demonstrated a higher pCa₅₀ than MHC I fibers from m. vastus lateralis (+20%, P=0.007).

Conclusion: Comparison of muscles from well-trained limbs revealed that MHC I fibers in the arm muscle exhibited a greater specific force-generating capacity and Ca₂₊-sensitivity than MHC I fibers in the leg, with no such differences in MHC II fibers.