Relationship between biomechanics and energy cost in graded treadmill running (original) (raw)

The objective of this study was to determine whether the relationships between energy cost of running (Cr) and running mechanics during downhill (DR), level (LR) and uphill (UR) running could be related to fitness level. Nineteen athletes performed four experimental tests on an instrumented treadmill: one maximal incremental test in LR, and three randomized running bouts at constant speed (10 km h −1) in LR, UR and DR (± 10% slope). Gas exchange, heart rate and ground reaction forces were collected during steady-state. Subjects were split into two groups using the median Cr for all participants. Contact time, duty factor, and positive external work correlated with Cr during UR (all, p < 0.05), while none of the mechanical variables correlated with Cr during LR and DR. Mechanical differences between the two groups were observed in UR only: contact time and step length were higher in the economical than in the non-economical group (both p < 0.031). This study shows that longer stance duration during UR contributes to lower energy expenditure and Cr (i.e., running economy improvement), which opens the way to optimize specific running training programs. Although complex physiological and biomechanical factors play important roles in level running (LR), the energy cost of running (Cr) appears as one of the three main predictive factors determining performance 1. The Cr represents the amount of energy required per unit of kilometer at a given submaximal running velocity allowing to maintain a physiological steady state 2. Though, the influence of LR Cr on graded running performance remains unclear 3-6. It has been reported that LR cost of locomotion is a poor indictor of performance in short distance trail races 3,4 but the importance of energy cost on ultramarathon remains debated 5,6. A relationship has been observed between the oxygen cost (amount of oxygen consumed per distance unit) in uphill running (UR) and LR in elite ultra-trail runners 7. However, additional specific parameters such as knee extensor muscle endurance and UR Cr may play a role on inclined running performance 4. It has been suggested that changes in the running pattern from negative to positive slopes explain the positive linear increase in Cr with positive slope 8-11 , but not in downhill running (DR), where the relationship between the slopes and Cr has an U-shape with the lowest Cr value at approximately − 10 to − 20% slope 9,12. While each individual naturally develops their optimal running pattern (i.e., spatiotemporal parameters of stride, running gait) according to their personal characteristics in order to lower their Cr 13,14 , it is well known that changing this self-selected running pattern may alter the Cr 15-18. Therefore, one may suggest that the most economical runners efficiently adapt their running mechanics to the slope condition. Though, which biomechanical adaptations are associated with a lower Cr remains an open question. According to experimental data, DR involves braking muscle actions of the lower limbs and is considered a predominantly eccentric exercise modality 19. In contrast, UR predominantly involves concentric propulsive muscle contractions 19. While the physiological adaptations to hilly terrain are currently being widely investigated, the main performance determinants for LR, UR and DR may differ, with a greater contribution of biomechanical parameters in DR performance 20. Compared to LR, UR induces a decrease of aerial time and step length, whereas DR increases the aerial time and the step length. However, the contact phase is less affected by slope, leading to an increase of step frequency during UR 9,10,21,22. Furthermore, the ratio of positive to negative work is another important biomechanical factor that may explain the slope-dependent variations in Cr 9,23. The mechanical positive and negative external works (W ext + and W ext − , respectively) represent the work performed at each step to support the upward and downward