Programmed electromyographic activity and negative incremental muscle stiffness in monkeys jumping downward.

Area of Science:

• Biomechanics
• Neuroscience
• Motor Control

Background:

• Understanding the neural control of rapid, complex movements like landing from a jump is crucial for explaining motor adaptation.
• Previous research has focused on reflex mechanisms, but the role of pre-programmed motor commands in such dynamic actions remains less understood.

Purpose of the Study:

• To investigate the neural control mechanisms underlying arm muscle activation during self-initiated jumps from varying heights in a primate model.
• To differentiate between reflex-driven and pre-programmed muscle responses during the landing phase of jumping.

Main Methods:

• Monkeys were trained to jump from different heights, with simultaneous recording of electromyograms (e.m.g.) from arm muscles and ground reaction forces.
• High-speed cinematography captured landing kinematics, while experiments with collapsible platforms and light extinction tested reflex and pre-programmed responses.

Main Results:

• Triceps muscle electromyograms showed pre-landing activation, time-locked to the expected landing, not the actual impact, indicating pre-programmed control.
• Muscle activation amplitude scaled with jump height, demonstrating adaptive motor control.
• Calculated elbow joint torque and stiffness revealed dynamic changes during landing, including a period of negative stiffness, differing from static muscle properties.

Conclusions:

• The observed muscle activation patterns are pre-programmed before takeoff, not solely reflex-driven, allowing for adaptive control based on jump height.
• The dynamic mechanical properties of the arm, including negative stiffness, are exploited to manage impact forces during landing.
• Segmented electromyogram patterns may enable submaximal contractions in muscle fibers despite high neural drive, optimizing force regulation.