Isometric exercise

Anatomy

Operating system

Physics

Engineering

Biomedical engineering

Torque

Artificial intelligence

Biomechanics

Actuator

Physical therapy

Work (physics)

Tendon

Computer science

Simulation

Medicine

Mechanical engineering

Accelerometer

Thermodynamics

Physical medicine and rehabilitation

Nature Communications

Muscles are the actuators that drive human movement. However, despite many decades of work, we still cannot readily assess the forces that muscles transmit during human movement. Direct measurements of muscle–tendon loads are invasive and modeling approaches require many assumptions. Here, we introduce a non-invasive approach to assess tendon loads by tracking vibrational behavior. We first show that the speed of shear wave propagation in tendon increases with the square root of axial stress. We then introduce a remarkably simple shear wave tensiometer that uses micron-scale taps and skin-mounted accelerometers to track tendon wave speeds in vivo. Tendon wave speeds are shown to modulate in phase with active joint torques during isometric exertions, walking, and running. The capacity to non-invasively assess muscle–tendon loading can provide new insights into the motor control and biomechanics underlying movement, and could lead to enhanced clinical treatment of musculoskeletal injuries and diseases.

Gauging force by tapping tendons