Copyright @ 2006 by the American College of Sports Medicine. Unauthorized reproduction of this article is prohibited. Fatigue Induced by a Cross-Country Skiing KO Sprint RAPHAE ¨ L ZORY 1 , GUILLAUME MILLET 2 , FEDERICO SCHENA 3 , LORENZO BORTOLAN 3 , and ANNIE ROUARD 1 1 Laboratoire de Mode´lisation des Activite´s Sportives, Ba ˆtiment Beaufortain, Universite´ de Savoie, Savoie, FRANCE; 2 Unite´PPEH, Me´decine du Sport–Myologie, Ho ˆpital Bellevue, Bellevue, FRANCE; and 3 Laboratorio di Ricerca, Centro Inter-Universitario di Bioingegneria e Scienze Motorie Universita` di Brescia, ITALY ABSTRACT ZORY, R., G. MILLET, F. SCHENA, L. BORTOLAN, and A. ROUARD. Fatigue Induced by a Cross-Country Skiing KO Sprint. Med. Sci. Sports Exerc., Vol. 38, No. 12, pp. 1–7, 2006. Purpose: The aims of the present study were 1) to analyze whether the KO sprint simulation induced a phenomenon of fatigue of upper and lower limbs and 2) if there was any fatigue, to determine its origin. Methods: Seven elite male skiers were tested before and after a simulation of KO sprints consisting of three 1200-m laps separated by 12 min of recovery. Surface electromyographic activity and force obtained under voluntary and electrically evoked contractions (single twitch) on knee-extensor muscles were analyzed to distinguish neural adaptations from contractile changes. A maximal power output test of the upper limbs was also performed. Results: During the last lap, the final sprint velocity was significantly lower than during the first lap. After the KO sprint, knee-extensor voluntary (j9.8 T 9.5%) and evoked (j16.2 T 11.9%) isometric force and upper-limb power output (j11.0 T 9.3%) and force (j11.3 T 8.7%) significantly decreased, whereas the blood lactate concentration increased to 11.6 mM. On the other hand, no changes were seen in RMS measurement during maximal voluntary contractions, RMS normalized by M-wave amplitude, or M-wave characteristics. Conclusion: Changes in performance, lactate concentration, knee- extensor strength, and upper-limb power indicated that the KO sprint test led the skiers to a state of fatigue. On lower-limb muscles, the decrease of knee-extensor strength was exclusively caused by peripheral fatigue, which was at least in part attributable to a failure of the excitation–contraction coupling. Key Words: CROSS-COUNTRY SKIING, CLASSICAL STYLE, NEUROMUSCULAR, CENTRAL, PERIPHERAL F or many years, the official distances for individual cross-country races have ranged from 5 to 50 km. In the last decade, new, very short races (KO sprints, G 2 km) have been designed to make cross- country skiing more entertaining. During a KO (knockout; meaning the notion of direct opposition) sprint, each skier must run the qualification lap again the clock, and the best 30 racers qualify for the quarterfinal. Quarterfinal, semifinal, and final races are run in direct opposition where the two first are qualified for the next bout. As a consequence, the four best skiers perform four high- intensity bouts, separated by short rest (less than 15 min between the semifinal and the final). The KO sprint represents short, repeated races that are 2–4 min in duration; this is in contrast to conventional long-distance races, which range from 20 min up to several hours. Because fatigue is known to be task dependent, it is important to investigate the presence and extent of fatigue in this new kind of race, to better understand factors of performance in this discipline. Fatigue has been defined as an acute impairment of performance (13), an inability to maintain a level of strength (9) or a temporary decrease in work capacity (1). Fatigue in elementary movements has often been studied, whereas fatigue of complex movements in a natural environment has been investigated to a much lesser extent (22). Fatigue could be measured by several indices as a significant decrease in strength or power output, a decrease in displacement velocity, a shift in electromyographic (EMG) signals at a given force (4), or a modification of the intracellular environment (12). Several authors have investigated the origins (central vs peripheral) of the loss of force-generating capacity (neuromuscular fatigue), which may be developed at various levels of the neuraxis, from the activation of the primary motor cortex to the muscle machinery (17). Central fatigue is associated with neural- drive reductions, whereas peripheral fatigue is localized between the neuromuscular junction and the production of force by the actin–myosin cross-bridges (23). A few studies have analyzed the fatigue induced by cross-country skiing races. In the classical technique, only Forsberg et al. (15) and Viitasalo et al. (29) have investigated fatigue phenomena, highlighting a significant loss of strength in the knee-extensor muscles after a 85-km race. Viitasalo et al. (29) observed a significant decrease in maximal EMG. In skating technique, Millet et al. (22) investigated neuromuscular fatigue in knee-extensor muscles after a marathon skiing race, observing a Address for correspondence: Raphae ¨l Zory, Laboratoire de Mode ´lisation des Activite ´s Sportives, Ba ˆtiment Beaufortain, Universite ´ de Savoie, 73376 Le Bourget du Lac, France; E-mail: raphael.zory@univ-savoie.fr. Submitted for publication March 2006. Accepted for publication June 2006. 0195-9131/06/3812-0001/0 MEDICINE & SCIENCE IN SPORTS & EXERCISE Ò Copyright Ó 2006 by the American College of Sports Medicine DOI: 10.1249/01.mss.0000235354.86189.7e 1