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The Effect of Flywheel Inertia on Peak Power and Its Inter-session Reliability During Two Unilateral Hamstring Exercises: Leg Curl and Hip Extension

Introduction

The physical demands of soccer require adequate strength and conditioning programs to optimize the long-term development of physical capacities and reduce the likelihood of injury.

The physical preparation and training "load" management of elite soccer players is a complex challenge during busy fixture periods throughout the competitive season. This challenge is amplified by a high incidence and recurrence of muscular injuries, particularly hamstring

strain injuries. In an attempt to reduce injury likelihood and improve soccer players' performance, various training methods have been implemented with a particular focus on eccentric-based exercises.

This study investigated the effect of flywheel moment inertia (0.029, 0.061, and 0.089 kg.m2) on the concentric and eccentric peak power and eccentric: concentric peak power ratio during unilateral flywheel leg curl and hip extension exercises.

What They Did

Twenty healthy male university soccer athletes voluntarily participated in this investigation. The study utilized a cross-sectional design to determine the impact of different flywheel moments of inertia during unilateral flywheel leg curl and hip extension on concentric and eccentric peak power and eccentric:concentric peak power ratio.

Each participant attended the laboratory five times over 3 weeks. An initial familiarization session was performed, with both limbs performing the unilateral leg curl and unilateral hip extension exercises. In the subsequent sessions, participants were randomized to perform one exercise (leg curl or hip extension) per session with their self-selected kicking leg, using all moments

of inertia in each session. The randomization of inertia and a 2min rest period between sets were prescribed to minimize the effects of accumulated fatigue on

performance. The testing procedures were repeated with the same methodology to analyze the inter-session reliability for both exercises (performed in the subsequent week).

What They Found

Researchers found that leg curl reliability scores ranged from acceptable to good, while all hip extension reliability scores for concentric and eccentric peak power were rated as excellent. The eccentric:concentric peak power ratio was rated from acceptable to good and unacceptable to questionable for leg curl and hip extension. In disagreement with

a hypothesis, greater peak power values were obtained with higher rather than lower moments of inertia during the hip extension exercise. In contrast, no differences in peak power were seen between moments of inertia during the leg curl exercise.

Greater eccentric:concentric ratio was reported with the highest moment of inertia during the hip extension exercise. This supports the hypothesis that higher moments of inertia would obtain greater eccentric overload, while all moments of inertia achieved similar eccentric overload during unilateral leg curl.

This study reports that leg curl reliability ranged from acceptable to good. In contrast, all hip extension reliability scores for concentric and eccentric peak power were rated excellent, highlighting that practitioners can use such parameters.

The eccentric:concentric ratio should not be utilized for monitoring training outcomes. This study is the first to determine that moments of inertia impact training parameters differently (concentric and eccentric peak power) between two unilateral flywheel hamstring exercises with the same population.

Practical Application

Based on this study's findings, various moments of inertia (0.029, 0.061, and 0.089 kg.m2) can be prescribed during unilateral flywheel leg curl to achieve high eccentric knee flexor demands. It is recommended to utilize higher moments of inertia (e.g., 0.089 kg.m2) with unilateral hip extensions to obtain greater eccentric overload and higher individual concentric and eccentric peak power outputs.

Although this study demonstrates that peak power can be helpful in daily practice, practitioners may also wish to measure other parameters (e.g., velocity) to further characterize training demands and outcomes. If you want to improve your fitness, whether for athletic performance or physique, book a free consultation with our team, and we can discuss which Exerfly device fits your outcomes best.

References

de Keijzer KL, McErlain-Naylor SA and Beato M (2022) The Effect of Flywheel Inertia on Peak Power and Its Inter-session Reliability During Two Unilateral Hamstring Exercises: Leg Curl and Hip Extension. Front. Sports Act. Living 4:898649. doi: 10.3389/fspor.2022.898649

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