Torque-producing capacity is affected by moment arm in the human knee extensors

Abstract

Objective: The torque-producing capacity can be assessed as maximal isometric torque per muscle size. Nevertheless, the factors contributing to this capacity remain poorly understood. In general, the magnitude of joint torque production is determined not only by muscle size but also by joint moment arm (MA). Based on this background, we hypothesized that longer MA would be related to higher torque-producing capacity despite a given muscle size. To test this hypothesis, we examined the relationship between MA and toque-producing capacity in the knee extensors. The quadriceps femoris muscle volume (MV) and knee extensor MA in 30 healthy young men were measured using magnetic resonance imaging. The knee extensor isometric torque was measured using a dynamometer. The knee extensor torque-producing capacity was calculated as the knee extensor isometric torque per quadriceps femoris MV.

Results: The quadriceps femoris MV and knee extensor MA correlated significantly with the knee extensor isometric torque (r = 0.785 and 0.790, respectively, both Ps < 0.001). Furthermore, the knee extensor MA correlated significantly with the knee extensor torque-producing capacity (r = 0.635, P < 0.001). These findings suggest that longer MA is an important factor for achieving higher torque-producing capacity in the human knee extensors.

Keywords: Isometric torque; Knee extension; Magnetic resonance imaging; Muscle volume; Quadriceps femoris.

Fig. 1

Representative magnetic resonance imaging scans of mid-thigh quadriceps femoris cross-sectional area and knee extensor moment arm. The quadriceps femoris cross-sectional area included the rectus femoris (RF), vastus intermedius (VI), vastus lateralis (VL), and vastus medialis (VM). The quadriceps femoris muscle volume was calculated by multiplying the sum of the cross-sectional areas along their length at intervals of 10 mm. The knee extensor moment arm was calculated as the distance between the tibio-femoral contact point and the mid-line of the patellar tendon

Fig. 2

Relationships of the quadriceps femoris muscle volume (right) and knee extensor moment arm (left) with knee extensor isometric toque

Fig. 3

Relationship between knee extensor moment arm and torque-producing capacity. The knee extensor torque-producing capacity was calculated as the knee extensor isometric torque per quadriceps femoris muscle volume