Surface electromyography does not accurately reflect rectus femoris activity during gait: Impact of speed and crouch on vasti-to-rectus crosstalk

Surface EMG (sEMG) is routinely used in gait analysis laboratories to record rectus femoris (RF) activity during gait. This is despite the fact that the literature has suggested sEMG may be inaccurate for RF monitoring secondary to its susceptibility to crosstalk from the neighboring vasti. If true, one might expect this error to be exacerbated by crouch gait when vasti demand is elevated. The purpose of this study was to investigate the impact of vasti crosstalk on RF sEMG across a variety of speed (four) and crouch (four) combinations. A group of 20 able-bodied adults were studied walking in 16 different combinations of speed and crouch while computerized gait analysis and EMG data were collected. RF EMG activity was monitored using both surface and fine wire electrodes simultaneously. Results showed that sEMG is affected by vasti crosstalk during substantial portions of both stance and swing. At the two slowest speeds tested, RF was not active at any point during the gait cycle, though sEMG suggested RF activity during some to all of stance phase. Despite sEMG indicating otherwise, true RF did not occur during loading response in any of the 16 conditions tested. During crouch at the two fastest speeds, a burst of true RF activity occurred during a short period of single limb stance, though sEMG incorrectly reflected RF activity throughout stance. Vasti-to-rectus crosstalk also occurred regularly during terminal swing. Surface EMG incorrectly suggests RF activity during portions of both stance and swing secondary to vasti crosstalk. This may lead to interpretation errors which could affect clinical recommendations.