Changes in Muscle Activity and Kinematics of Highly Trained Cyclists During Fatigue

Muscle fatigue may alter kinematics and contribute to repetitive strain injuries. This study quantified how both localized muscle fatigue and movement kinematics change over time during exhaustive cycling. Seven highly trained cyclists rode a stationary bicycle ergometer at 100% of their maximum oxygen consumption ( $\hbox{VO}_{2}$ max) until voluntary exhaustion. Cycling kinematics and electromyography (EMG) activity from select lower extremity muscles were recorded. Cross-correlations were computed to quantify how EMG median frequencies (MDFs) changed with changes in movement kinematics. All athletes maintained both cadence and power output for $\sim$90% of the trial duration. Significant sustained muscle fatigue occurred in 18 of 28 muscles tested, most prominently in the biceps femoris ($p= 0.020$) and gastrocnemius ( $p= 0.018$). Kinematics and MDF both fluctuated nonmonotonically as subjects fatigued. Changes in MDF significantly preceded changes in mean trunk lean ( $p= 0.009$) and hip angles ($p= 0.025$), and trunk lean range of motion ($p= 0.029$). Fluctuations in MDF were positively correlated with fluctuations in mean trunk lean ($p= 0.009$ ) and knee splay angles ($p= 0.011$), and with trunk lean ($p= 0.002$) and ankle ($p= 0.001$) range of motion. These results therefore establish a direct link between changes in muscle fatigue state and subsequent changes in movement kinematics during cycling.