Leg stiffness can be maintained during reactive hopping despite modified acceleration conditions

Area of Science:

• Biomechanics
• Human Physiology
• Sports Science

Background:

• Reactive hopping requires precise neuromuscular control to maintain performance under varying conditions.
• Leg stiffness and preactivity are crucial for efficient force transmission during jumping.
• Understanding neuromuscular adaptation to altered gravity is vital for sports and space exploration.

Purpose of the Study:

• To investigate the effects of modified acceleration levels on reactive hopping performance.
• To test the hypothesis that leg extensor preactivity and phase-specific muscle activation compensate for altered acceleration.
• To determine if high leg stiffness and short ground contact times can be maintained under different gravitational loads.

Main Methods:

• Twenty healthy subjects performed reactive hops in a sledge jump system.
• Data collection included ground reaction forces (GRF), kinematics, and electromyography.
• Seven acceleration levels were tested, ranging from 0.7g to 1.3g.

Main Results:

• No significant changes in leg extensor preactivity, leg stiffness, or rate of force development were observed.
• Peak ground reaction forces increased by 15% with higher acceleration.
• Ground contact time increased by 10%, and ankle/knee joint angular excursion increased by 3°.

Conclusions:

• The neuromuscular system successfully maintained high leg stiffness and reactive hopping performance across a wide acceleration range.
• Constant high preactivity and adjusted later-phase muscle activation enabled adaptation to altered gravitational conditions.
• The study demonstrates the robustness of the neuromuscular system in handling diverse acceleration environments.