Electrically evoked eccentric and concentric torque-velocity relationships in human knee extensor muscles

Area of Science:

• Human physiology
• Biomechanics
• Muscle physiology

Background:

• The torque-velocity relationship in human knee extensors shows eccentric torque plateaus at isometric levels.
• In contrast, animal muscle preparations exhibit a sharp eccentric force increase beyond isometric maximum.
• A protective mechanism is hypothesized to regulate high-tension eccentric muscle actions in humans.

Purpose of the Study:

• To investigate the presence and nature of a force-regulating mechanism during maximal voluntary and submaximal, electrically evoked knee extensor actions.
• To compare the torque-velocity relationships under voluntary and electrically stimulated conditions.
• To determine if eccentric maximal voluntary contractions are limited by a protective mechanism.

Main Methods:

• 10 healthy subjects performed isometric, isokinetic eccentric, and concentric knee extensor actions.
• Torques were measured during maximal voluntary contractions and three levels of submaximal, electrically evoked contractions.
• Low velocities (10, 20, 30 degrees s-1) were used for isokinetic actions.

Main Results:

• Eccentric torque exceeded isometric torque during electrically evoked actions, but not during maximal voluntary actions.
• Concentric torque was lower than isometric torque for both maximal voluntary and submaximal, electrically evoked conditions.
• Normalized data indicated maximal voluntary eccentric torque required a 20% increase to match electrically stimulated curves, supporting a tension-regulating mechanism.

Conclusions:

• A tension-regulating mechanism is primarily active during maximal voluntary eccentric knee extensor actions.
• This mechanism appears to limit eccentric torque output during voluntary contractions, distinguishing them from electrically evoked contractions.
• Findings support the concept of neural or mechanical safeguards during high-intensity eccentric muscle actions.