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Little is known about potential differences in contractile properties of muscle fibers of the same type in arms and legs. Accordingly, the present study was designed to compare the force-generating capacity and Ca 2+  sensitivity of fibers from arm and leg muscles of highly trained cross-country skiers.    Method  Single muscle fibers of  m. vastus lateralis  and  m. triceps brachii  of eight highly trained cross-country skiers were analyzed with respect to maximal Ca 2+ -activated force, specific force and Ca 2+  sensitivity.    Result  The maximal Ca 2+ -activated force was greater for myosin heavy chain (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. In addition, 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 the same in both limbs, whereas MHC I fibers from the  m. triceps brachii  were, on average, 39% stronger than fibers of the same type from the  m. vastus lateralis  ( P  = 0.003). pCa 50  was not different between MHC I and II fibers in neither arms nor legs, but the MHC I fibers of  m. triceps brachii  demonstrated higher Ca 2+  sensitivity than fibers of the same type from  m. vastus lateralis  ( P  = 0.007).    Conclusion  Comparison of muscles in limbs equally well trained revealed that MHC I fibers in the arm muscle exhibited a higher specific force-generating capacity and greater Ca 2+  sensitivity than the same type of fiber in the leg, with no such difference in the case of MHC II fibers. These distinct differences in the properties of fibers of the same type in equally well-trained muscles open new perspectives in muscle physiology.

Contractile Properties of MHC I and II Fibers From Highly Trained Arm and Leg Muscles of Cross-Country Skiers