Effect Of Inertial Loading On Muscle Activity In Cycling

Abstract

The inertial and gravitational components of the pedal force applied by a cyclist may be quantified using several methods (Hull & Jorge, 1985; Kautz & .Hull, 1993). This paper presents a theoretical model of different components of the hip joint muscle torque and examines the activation of the hip muscles in relation to the inenial component of thigh motion. In the model, the hip joint torque has three components, associated with: 1) inertia [TI =0oI(21t/T)2COS(21t/T)), 2) external torque [TE =ToSIN(21t/T)] and 3) gravity [Tm = mgO/2)COS(0oCOS(21t/T))]. Our purpose was to examine the effect of changing pedalling cadence on muscular activity as measured by EMG, and to relate these changes to model predictions. Our hypothesis was that the EMG pattern of the hip joint muscles would shift to accommodate the increasing importance of the inertial torque component with the higher pedalling rate. One elite and one recreational cyclist pedaled at 70 and 150 RPM against a constant work load of 150 W. EMG data from the rectus femoris (RF) and harnstring (HAMS) muscles were collected by A/D conversion at 500 Hz, rectified and low pass filtered at 20 Hz with a recursive 4th order Butterworth filter. Each model torque component has a unique phasing pattern during the crank cycle. Figure 1 shows the time history of the model inertia torque component, which shows a peak in the flexion direction at mid-cycle. Figure 2 shows the patterns of EMG activity of RP for the elite cyclist in both cadence conditions. It can be seen that the EMG peak at 70 RPM occurs at 0% of the cycle, coincident with the extension inertia torque peak. As the cadence was increased to 150 RPM the pe ak EMG shifted (to 60%) toward the peak of flexion inertia torque (50%). The activity pattern of HAMS (not shown) displayed a similar trend with regards to the extension inertia peak. The recrea tional cyclist showed a same tendencies, but with less dramatic shifts in muscle activity patterns. Differences between the elite and recreational subjects may be related to their skill level and their ability to alter muscular performance to match task demands. These initial results appear to support our hypothesis, but must be verified by more data from other muscles and subjects before the usefulness of the model may be assessed.