Estimation of power transfer by biarticular leg extensors on the acceleration phase of the sprint

Introduction: In sport movements, such as sprinting and jumping, the ability to produce explosive leg extension movements is extremely important. Values for maximal knee extension moment of force, calculated by inverse dynamics, are higher than the maximal isometric and isokinetic moments of force values obtained with tests performed on Isokinetic machines. Values of maximal knee extensor moments of force could reach 310 Nm on sub maximal running (Jacobs, et all, 1993) and around 500 Nm in drop jump exercises (Bobbert, et all, 1987a). These values exceed the values for maximal isometric knee extension, around 200 Nm obtained on the classical studies of Wickiewicz, et all (1984). The same discrepant behaviour was found for plantar flexion (Bobbert, 1988). One of the mechanisms that might explain these discrepancies is the fact that the biarticular muscles perform differently during actual sports movements than they do on Isokinetic testing. The major difference concerns the length variations of the biarticular muscles. These length changes are dependent on the simultaneous movement of the two joints crossed by the muscle. This means that the force-length and the force-velocity relationships of biarticular leg extensors are different from those obtained during isometric testing, where the adjacent limb is in a fixed position. Another potential influence of biarticular muscles is the transfer of mechanical power from one joint to the adjacent one, considering where the two joints they cross (Prilutsky et all, 1994). The purpose of this study was to estimate the transport of mechanical power from hip to knee and from knee to ankle, performed by rectus femoris (RF) and gastrocnemius (GAS) muscles, during the acceleration phase of sprinting. This power transport action partially explained the difference found on the moment of force between maximal isometric testing and the value obtained by inverse dynamic on this explosive leg extension movement.