Spring-Mass Behavior during Exhaustive Run at Constant Velocity in Elite Triathletes

Abstract : Purpose: The aims of this study were i) to evaluate changes in leg-spring behavior during an exhaustive run in elite triathletes and ii) to determine whether these modifications were related to an increase in the energy cost of running (C r). Methods: Nine elite triathletes ran to exhaustion on an indoor track at a constant velocity corresponding to 95% of the velocity associated with the maximal oxygen uptake (mean T SD = 5.1 T 0.3 mIs j1 , time to exhaustion = 10.7 T 2.6 min). Vertical and horizontal ground reaction forces were measured every lap (200 m) by a 5-m-long force platform system. C r was measured from pulmonary gas exchange using a breath-by-breath portable gas analyzer. Results: Leg stiffness (j13.1%, P G 0.05) and peak vertical (j9.2%, P G 0.05) and propulsive (j7.5%, P G 0.001) forces decreased significantly with fatigue, whereas vertical stiffness did not change significantly. Leg and vertical stiffness changes were positively related with modifications of aerial time (R 2 = 0.66, P G 0.01 and R 2 = 0.72, P G 0.01, respectively) and negatively with contact time (R 2 = 0.71, P G 0.01 and R 2 = 0.74, P G 0.01, respectively). Alterations of vertical forces were related with the decrease of the angle of velocity vector at toe off (R 2 = 0.73, P G 0.01). When considering mean values of oxygen uptake, no change was observed from 33% to 100% of the time to exhaustion. However, between one-third and two-thirds of the fatiguing run, negative correlations were observed between oxygen consumption and leg stiffness (R 2 = 0.83, P G 0.001) or vertical stiffness (R 2 = 0.50, P G 0.03). Conclusions: During a constant run to exhaustion, the fatigue induces a stiffness adaptation that modifies the stride mechanical parameters and especially decreases the maximal vertical force. This response to fatigue involves greater energy consumption. Key Words: MUSCULOSKELETAL STIFFNESS, RUNNING, FATIGUE, ENERGY COST, TRIATHLON M any studies have attempted to identify factors involved in performance of elite triathletes by analyzing biomechanical parameters with exercise duration during triathlon (3,18). Although swimming and cycling have both energetic (11,26) and mechanical (9,12) influences on the subsequent mode(s) of locomotion, tri-athlon overall finishing position is strongly determined by running performance (38). Consequently, a better understanding of the effects of running fatigue as it influences mechanical modification and metabolic energy costs in tri-athletes continues to be a challenge to improve training programs. In humans, many studies have been carried out to characterize running using the spring–mass model (SMM) (4,6,27). This model consists of a point mass bouncing on a massless spring. SMM provides a well-accepted theoretical basis from which leg stiffness (k leg , ratio of the vertical ground reaction force to the leg-spring compression at middle of the stance phase) and vertical stiffness (k vert , ratio of the maximal force to the vertical downward maximal displacement of the center of mass (COM)) can be determined. These types of mechanical stiffness have been frequently used for characterization of running mechanics because they influence the regulation of temporal and kine-matic variables (14,30). Numerous studies specifically analyzed the changes in biomechanical parameters during running in fatigue conditions (2,7,25,35), although only few explored the evolution of leg stiffness and its relationships with step temporal variables. Furthermore, the results have been quite contradictory. For constant and moderate running intensities (under the lactic threshold), Dutto and Smith (13) showed that k leg and k vert both decreased with fatigue, whereas Hunter and Smith (22) reported no significant change. Both studies, however, reported that stride frequency was more strongly related to k vert than k leg. For severe running intensities (above the lactic threshold), Slawinski et al. (35) did not detect any variation of k leg or k vert before or after a 2000-m maximal run. In contrast, Girard et al. (16) found a decrease of k vert while k leg remained constant during a self-paced 5000-m run. For sprint running intensities, the
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Giuseppe Rabita, Jean Slawinski, Olivier Girard, Frank Bignet, Christophe Hausswirth. Spring-Mass Behavior during Exhaustive Run at Constant Velocity in Elite Triathletes. Medicine and Science in Sports and Exercise, American College of Sports Medicine (ACSM), 2011, 43, pp.685 - 692. ⟨10.1249/MSS.0b013e3181fb3793⟩. ⟨hal-01561309⟩

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