Influence of contractile force on the architecture and morphology of the quadriceps femoris

The primary purpose of this study was to document the influence of muscle contraction on quadriceps femoris (QF) muscle architecture [fascicle length (L_f) and pennation angle (θ_p)] and effective physiological cross-sectional area (_effPCSA). Secondarily, we aimed to determine whether _effPCSA measured during maximal voluntary contraction (MVC) had a stronger relationship to maximal strength than _effPCSA at rest. Fifteen young men performed a series of voluntary knee-extension isometric ramp contractions. Isometric maximal voluntary torque (MVT) was recorded during separate MVCs. Measurements of architecture and _effPCSA of each constituent muscle of the QF and, subsequently, the whole QF were made at rest, during 20% increments of maximal voluntary torque and during an MVC. The QF muscle architecture and morphology changed in a curvilinear manner with relative torque (%MVT), with significant differences being observed between incremental torque levels for L_f, θ_p and _effPCSA. Specifically, from rest to MVC, QF L_f decreased (-23.5 ± 3.3%), whereas θ_p increased (+39.7 ± 6.6%). The QF _effPCSA was +26.5 ± 5.7% greater during MVC than at rest. Similar moderate correlations existed for MVT and _effPCSA at rest (r = 0.519, P = 0.047) and for MVT and _effPCSA during MVC (r = 0.530, P = 0.042). Substantial changes in QF architecture (L_f, θ_p) and _effPCSA occur in a curvilinear manner with relative torque production. The _effPCSA during MVC was no more strongly associated with MVT than _effPCSA measured at rest, which implies that resting measurements of muscle size are suitable for characterizing the muscle size-strength relationship.