The normal human gait cycle is divided into two phases, namely, stance and swing. The objective of stance is to provide support, stability and propulsion and that of swing is to provide ground clearance and limb advancement. Knee flexion is essential during swing to lift the foot off the ground for limb advancement. The complex mechanisms involved in producing limb advancement can produce excessive knee flexion at faster walking speeds. Under these circumstances the shank needs to be decelerated to reduce the amount of knee flexion. It is assumed that rectus femoris (RF) is active for a very short period at the beginning of the swing phase (Perry J. Gait Analysis—Normal and Pathological Gait. Slack Incorporated, USA, 1992; Scott L, Ringwelsky D, Carroll N. Transfer of rectus femoris: effects of transfer site on moment arms about the knee and hip. J Biomech 27;1994:1201–1211) and the amount of this activation is proportional to the walking speed and thus to the generated knee moment and the angular acceleration of the lower limb segments. However, there is very little evidence to support these assumptions. The objective of this study was to study this relationship. Quantified electromyogram of RF and vastus lateralis (VL), using surface electrodes, were examined, body mounted kinematic sensors such as seismic accelerometers and gyroscopes were used to measure segments' angular accelerations and the net muscular knee torque calculated from the kinematics of the segments at various speeds. The results showed that RF and VL work independent of each other during the initial swing phase. The amount of RF activity is clearly related to walking speed. The muscle activity increases with increasing walking speed. The relationship between the angular acceleration of the shank and the amount of RF activity is linear. The active knee moment, as a function of the shank's angular acceleration, shows the same high correlation to the EMG signal of RF.
Nene, A. V., Mayagoitia, R. E., Mayagoitia, R., & Veltink, P. H. (1999). Assessment of rectus femoris function during initial swing phase. Gait & posture, 9(9), 1-9. https://doi.org/10.1016/S0966-6362(98)00042-3