Coactivation pattern in human quadriceps during isokinetic knee-extension by muscle functional MRI.

We tested activation patterns of individual muscles and neuromuscular compartments (NMCs) of the quadriceps femoris (QF) during knee-extension exercises (KEE). Eight healthy men performed repetitive isokinetic KEE at 120 degrees s(-1). Before and after KEE, muscle functional magnetic resonance images of the thigh were taken to calculate transverse relaxation times (T2) for the individual muscles of the QF. In terms of exercised T2, a moderate correlation was observed between the rectus femoris (RF) and vastus medialis (VM) ( r=0.493, P<0.001), and between the vastus lateralis (VL) and VM ( r=0.347, P<0.01). Furthermore, a strong correlation was also observed between the vastus intermedius (VI) and VM ( r=0.719, P<0.0001), and between the VL and VI ( r=0.783, P<0.0001). There was a significant difference in T2 between the proximal and distal NMCs of the RF. We conclude that at least two strategies are applied for force production in the human QF: (1) coactivation of specific pairs of individual muscles and (2) regional-specific activation in NMCs in the RF.