Type IIa Muscle Fibers Influence The Force-velocity Profile Of Healthy Adults During Maximal Cycling

During maximal cycling, vastii muscles contract during the extension phase of the pedaling cycle to generate a large portion of pedal forces. The contractile properties of the vastii muscles vary between individuals, depending on myosin heavy chain (MHC) composition. A positive association was reported between the cross-sectional area of type II muscle fibers in vastus lateralis (VL) and optimal cadence (Copt) during maximal cycling derived from power-cadence (P-C) relationships computed over the full pedal cycle. However, the contribution of vastii muscles to power produced over the full cycle may also vary across individuals due to potential differences in the relative contribution of the flexor and extensor muscles of the left and right lower limbs. PURPOSE: To investigate if MHC composition in VL influences variables derived from P-C relationships computed over the extension phase during maximal cycling. METHODS: Thirteen healthy adults (9 males, 4 females) performed a Force-Velocity test on a stationary cycle ergometer. We modelled individual P-C relationships using third order polynomial regressions to extract maximal power (Pmax), Copt and maximal cadence (C0) for the extension phase of the right leg only. From a muscle biopsy of the right vastus lateralis, we used electrophoresis to determine the MHC isoform composition (I, IIa and IIx). Pearson correlations were determined between % of each MHC type (I, IIa, IIx) and the variables derived from the P-C relationships (Pmax, Copt and C0). RESULTS: All individual P-C relationships were well fitted (r2 = 0.92 ± 0.01), allowing extraction of Pmax (11.9 ± 2.6 W/kg), Copt (130 ± 11 rpm) and C0 (235 ± 25 rpm). MHC isoform composition comprised 44 ± 10% type I, 47 ± 12% type IIa and 9 ± 5% IIx. Type I % MHC was negatively associated with Pmax (r = -0.553, p = 0.05) and Copt (r = -0.737, p < 0.05). Type IIa % MHC was positively associated with Pmax (r = 0.623, p < 0.05), Copt (r = 0.849, p < 0.001) and C0 (r = 0.805, p < 0.01). No significant associations were found between type IIx % MHC and Pmax, Copt or C0 (all P > 0.05). CONCLUSIONS: Healthy adults with higher type IIa % MHC in the VL muscle produced greater power at higher optimal cadences and reached higher maximal cadences during maximal cycling, when P-C relationships were computed over the extension phase of the ipsilateral limb.