Runners adjust leg stiffness for their first step on a new running surface

Area of Science:

• Biomechanics
• Human locomotion
• Sports science

Background:

• Human runners dynamically adjust leg stiffness to match surface stiffness during steady-state running.
• This adaptation maintains consistent running mechanics, such as ground contact time and stride frequency, across different surfaces.
• Abrupt surface changes pose a challenge, requiring rapid adjustments to prevent disruption of running gait.

Purpose of the Study:

• To investigate the speed at which human runners adjust their leg stiffness in response to an abrupt, anticipated change in running surface stiffness.
• To quantify the immediate mechanical adaptations made by runners when transitioning between surfaces of differing stiffness.

Main Methods:

• Six participants ran at a constant speed (3 m/s) on a rubber track.
• The track featured two distinct surfaces: a compliant soft surface (ksurf = 21.3 kN m(-1)) and a non-compliant hard surface (ksurf = 533 kN m(-1)).
• Leg stiffness and center of mass displacement were measured during transitions between these surfaces.

Main Results:

• Runners fully adjusted their leg stiffness within the first step after encountering a new surface.
• Leg stiffness decreased by 29% when transitioning from the soft to the hard surface (10.7 to 7.6 kN m(-1)).
• Vertical center of mass displacement remained consistent (~7 cm) despite significant changes in surface compression.

Conclusions:

• Human runners demonstrate a rapid and effective ability to adjust leg stiffness upon encountering expected changes in surface stiffness.
• This rapid adjustment mechanism ensures smooth transitions between surfaces, preserving the stability of the center of mass trajectory.
• The findings highlight the adaptability of human running mechanics to environmental variations.