Effects of Fatigue on Muscle Stiffness and Intermittent Sprinting during Cycling

DITROILO, M., M. WATSFORD, E. FERNANDEZ-PENA, G. D'AMEN, F. LUCERTINI, and G. DE VITO. Effects of Fatigue on Muscle Stiffness and Intermittent Sprinting during Cycling. Med. Sci. Sports Exerc., Vol. 43, No. 5, pp. 837-845, 2011. Purpose: It was recently demonstrated that musculoarticular (MA) stiffness is related to sprint cycling performance in nonfatigued conditions. This study examined whether relatively stiffer cyclists were more effective at sprinting under fatigued conditions, as occurs during endurance cycling competitions. Methods: MA stiffness of the quadriceps was assessed in 21 trained male cyclists (28.7 +/- 9.5 yr, 1.74 +/- 0.08 m, 67.5 +/- 7.2 kg). Participants also performed a maximal 6-s sprint on a cycle ergometer to assess peak power output (POpeak), peak crank torque (CTpeak), and peak rate of crank torque development (RCTDpeak). A cycling fatigue protocol then required cyclists to pedal at 30%, 35%, and 40% of POpeak and sprint at the end of each stage. Surface EMG was recorded from vastus lateralis during each sprint and analyzed in the time domain as integrated EMG (iEMG) and in the frequency domain as instantaneous median frequency (MDF) adopting a continuous wavelet transform. Participants were then retested for MA stiffness. Results: MA stiffness (-12%) was significantly reduced after the cycling protocol. Further, POpeak, CTpeak, RCTDpeak, and iEMG were reduced by 20%, 15%, 13%, and 20%, respectively, after the fatigue protocol (P < 0.05). When the cyclists were divided into relatively stiff (SG) and relatively compliant groups (CG), only SG exhibited significant decreases in MA stiffness, CT(pea)k, RCTDpeak (P < 0.05), and instantaneous MDF (R-2 = 0.705). Conclusions: Whereas neuromechanical parameters were generally reduced under conditions of fatigue, stiff and compliant cyclists were affected differently, with the sprint abilities of SG decreased to the level of CG. It seems important for endurance cyclists to incorporate training strategies to maintain MA stiffness during competition to offset declines in sprint performance.