Effects of fatigue on muscle stiffness and intermittent sprinting during cycling

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 (PO peak ), peak crank torque (CT peak ), and peak rate of crank torque development (RCTD peak ). A cycling fatigue protocol then required cyclists to pedal at 30%, 35%, and 40% of PO peak 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, PO peak , CT peak , RCTD peak , 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, C T peak , RCTD peak (P < 0.05), and instantaneous MDF (R = 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. © 2011 by the American College of Sports Medicine

Search for dark matter produced in association with bottom or top quarks in √s = 13 TeV pp collisions with the ATLAS detector

A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb−1 of proton–proton collision data recorded by the ATLAS experiment at √s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements

Muscle stiffness and rate of torque development during sprint cycling

PURPOSE Crank torque (CT) application and rate of CT development (RCTD) are important considerations in sprint cycling. The stiffness of the musculotendinous unit is related to the isometric rate of torque development (RTD); however, this relationship has yet to be examined in sprint cycling. METHODS Maximal isometric torque (MIT) and isometric RTD of the quadriceps were assessed in 21 trained male cyclists (28.7 +/- 9.5 yr, 1.74 +/- 0.08 m, and 67.5 +/- 7.2 kg). Unilateral musculoarticular (MA) stiffness of the quadriceps was quantified using an oscillation test. Further, the participants performed a maximal 6-s sprint to assess peak power output (POpeak), peak CT (CTpeak), peak RCTD (RCTDpeak), and the crank angles associated with CTpeak and RCTDpeak. Participants were ranked on MA stiffness properties and were divided into a relatively stiff group (SG) and a relatively compliant group (CG). RESULTS The SG displayed a significantly higher MA stiffness than the CG (P &lt; 0.05). Furthermore, the SG reported significantly elevated MIT (27%), RTD (26%), and RCTDpeak (16%) when compared with the CG (P &lt; 0.05), along with trends for increased POpeak (7%) and CTpeak (8%). The angles at CTpeak and RCTDpeak were 7% and 12% lower for the SG, respectively (P &lt; 0.05). MA stiffness was significantly correlated with RCTDpeak, MIT, RTD, and POpeak. CONCLUSIONS Higher stiffness is related to superior RCTDpeak in trained cyclists during a single sprint. A significant proportion of the variance in RCTDpeak was attributed to MA stiffness (37%), which was of greater magnitude than the relationship between RCTDpeak and MIT. Furthermore, the lower CTpeak angle and RCTDpeak angle may contribute to a more rapid development of CT. Accordingly, MA stiffness seems to be an important consideration for sprint cycling