Author(s): EIHARA, Y., ARAI, H., ITO, A., MAEO, S., KANEHISA, H., ISAKA, T., Institution: RITSUMEIKAN UNIVERSITY, Country: JAPAN, Abstract-ID: 1001

INTRODUCTION:
Increasing step frequency typically improves running economy [1] and is associated with increased leg stiffness [2], which is often linked to shorter ground contact time [3]. Accordingly, reducing ground contact time may be considered an effective adjustment pattern when step frequency is increased. However, shortening ground contact time may increase the demand for the lower-limb stretch-shortening cycle (SSC) function, because runners must generate the same impulse in a shorter time window [4]. Thus, this study examined whether individual temporal adjustment patterns (stance vs. flight phase) in response to altered step frequency interact with SSC ability to determine the magnitude of running economy improvements.
METHODS:
Fifty-five endurance runners (35 males) completed measurement tests, including isokinetic ankle plantarflexion torques with and without countermovement, as well as squat, countermovement, and rebound jumps. These SSC-related functions were summarized using principal component analysis (PCA) into PC1 (jump performance), PC2 (simple isokinetic and slow countermovement plantarflexion torque), and PC3 (fast countermovement plantarflexion torque). Running kinematics and economy were measured at 11 km/h in females and 12 km/h in males under five step-frequency conditions ranging from -5% to +15% relative to baseline (5% increments). Linear mixed-effects models tested (1) associations of changes in duty factor (ground contact time relative to stride time) and leg stiffness with changes in running economy, and (2) interactions between duty factor and SSC-related functions (PC1–3) on running economy responses to altered step frequency.
RESULTS:
Running economy was optimized by increasing step frequency by 4.6% on average. A temporal adjustment pattern characterized by a relatively shorter flight phase (i.e., a higher duty factor) was associated with a reduced energy cost of running (β = -0.213, P = 0.005). Moreover, this effect depended on an interaction between duty factor and PC1 (β = 0.071, P = 0.023), indicating that runners with lower jump performance benefited more from shortening flight times than those with higher jump performance. In contrast, changes in leg stiffness showed a positive but non-significant association with those in energy cost (β = 0.396, P = 0.052), suggesting that increases in leg stiffness were likely detrimental to running economy.
CONCLUSION:
A temporal adjustment pattern that relatively shortens flight time rather than ground contact time was more effective for improving running economy during step-frequency increases. Furthermore, runners with lower jump performance gained greater benefits from shortening flight time. Collectively, temporal adjustment patterns in response to increased step-frequency are key determinants of running economy improvements, and their effectiveness may vary according to individual SSC ability.
REFERENCES:
[1] Connick et al. (2014) [2] Farley et al. (1996) [3] Morin et al. (2007) [4] Douglas et al. (2019)