Energetically optimal running requires torques about the centre of mass

Area of Science:

• Biomechanics
• Locomotion Dynamics
• Human and Animal Physiology

Background:

• Bipedal locomotion involves ground reaction forces (GRFs) interacting with the center of mass (CoM).
• These forces cause fluctuations in kinetic energy, necessitating muscular work for deceleration and re-acceleration during stance.
• Previous models often assumed GRF alignment through the CoM, potentially overlooking energetic costs.

Purpose of the Study:

• To analytically and numerically investigate the energetic cost of GRF-CoM alignment during bipedal locomotion.
• To determine if deviations from GRF alignment through the CoM are energetically favorable under realistic conditions.
• To explore the implications of these findings for human and animal locomotion, particularly concerning body inertia and speed.

Main Methods:

• Analytical modeling to assess mechanical work requirements for GRF-CoM alignment versus vertical forces.
• Numerical simulations to evaluate energetically favorable GRF pathways under realistic biomechanical conditions.
• Comparison of simulated CoM-torque profiles with empirical data from human running.

Main Results:

• Extreme cases with high body pitch moment of inertia show that GRF alignment through CoM demands greater mechanical work than a maintained vertical force.
• Numerical results indicate that GRFs passing between the CoM and vertical are energetically favorable in realistic running scenarios.
• Human running CoM-torque profiles are broadly consistent with mechanical work minimization, especially with appropriate pitch moment of inertia.

Conclusions:

• Shifting GRFs vertically, rather than aligning them through the CoM, offers energetic advantages in bipedal locomotion.
• These energetic savings, though small in typical human running, become significant at higher speeds and with greater body pitch moment of inertia.
• The findings suggest a potential biomechanical advantage for animals like kangaroos with high pitch moment of inertia due to their unique body structure.