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

NEW FINDINGS: What is the central question of this study? Does contraction influence the fascicle length, pennation angle and effective physiological cross-sectional area (eff PCSA) of the quadriceps femoris muscle? Is there a stronger relationship between eff PCSA and maximal strength if eff PCSA is measured during maximal contraction rather than at rest? What is the main finding and its importance? Fascicle length decreased, pennation angle increased and eff PCSA increased in a non-linear manner with isometric torque. The eff PCSA during maximal contraction and rest were correlated in a similar manner to maximal strength. The eff PCSA at rest is sufficient to characterize the muscle size-strength relationship. The primary purpose of this study was to document the influence of muscle contraction on quadriceps femoris (QF) muscle architecture [fascicle length (Lf ) and pennation angle (θp )] and effective physiological cross-sectional area (eff PCSA). Secondarily, we aimed to determine whether eff PCSA measured during maximal voluntary contraction (MVC) had a stronger relationship to maximal strength than eff PCSA 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 eff PCSA 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 Lf , θp and eff PCSA. Specifically, from rest to MVC, QF Lf decreased (-23.5 ± 3.3%), whereas θp increased (+39.7 ± 6.6%). The QF eff PCSA was +26.5 ± 5.7% greater during MVC than at rest. Similar moderate correlations existed for MVT and eff PCSA at rest (r = 0.519, P = 0.047) and for MVT and eff PCSA during MVC (r = 0.530, P = 0.042). Substantial changes in QF architecture (Lf , θp ) and eff PCSA occur in a curvilinear manner with relative torque production. The eff PCSA during MVC was no more strongly associated with MVT than eff PCSA measured at rest, which implies that resting measurements of muscle size are suitable for characterizing the muscle size-strength relationship.