de Jager, K; (2012) Knee extension with less hip flexion: biomechanical and evoked EMG analysis during selective surface stimulation of the quadriceps. Doctoral thesis , UCL (University College London).

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Abstract

During FES cycling when the Quadriceps muscles are activated both knee extension and hip flexion moments occur simultaneously, decreasing the total power output. Of the three superficial muscles, Rectus femoris is a biarticular muscle that produces both a knee extension and a hip flexion, while Vastus lateralis and Vastus medialis are only knee extensors. This thesis is an investigation whether, using surface stimulation, selective stimulation of the Vastii can produce knee extension moment with less hip flexion. A system was developed for measurement of the joint moments and evoked myoelectric response in these three muscles, while seated subjects were stimulated. The dynamometer measures the magnitude and position of two forces that restrain the leg, from which joint moments are calculated. The design and construction of the hardware, electronics and software is presented. Validation of the dynamometer against known moments produced with a spring-loaded dummy leg showed good correlation. The influence of random and systematic errors on the estimated joint moments indicate that the dynamometer should be used for comparing the responses for different electrode configurations within single sessions. The close proximity of the EMG recording electrodes to the stimulating electrodes causes artefact that obscures the M-wave. This was partly overcome by amplifying with a current conveyor circuit, and by a novel biphasic stimulator with pulse width ratio adjustment. The design and construction of both stimulator and amplifier are discussed, also the mechanisms causing the artefact (voltage gradient, skin-electrode interface and common mode voltage). A study with ten able-bodied subjects was performed. EMG analysis showed that it is possible to selectively stimulate the Vastii and this does reduce hip flexion moment, however beside the inevitable reduction of knee extension moment, the reduction of the hip flexion is less than expected, perhaps due to stimulation of other muscles of the anterior thigh.

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