Eccentric exercise ≠ eccentric contraction.

Area of Science:

• Biomechanics
• Exercise Physiology
• Muscle Physiology

Background:

• Eccentric exercise is commonly prescribed, but its effect on muscle fascicles is debated due to muscle-tendon unit (MTU) compliance.
• The precise fascicle kinematics during apparent eccentric contractions remain unclear, particularly the influence of muscle activation timing and level.

Purpose of the Study:

• To investigate how varying preactivation levels affect muscle fascicle kinematics and kinetics during maximal voluntary MTU-stretch-hold contractions.
• To determine the impact of preactivation on residual force enhancement (rFE) following MTU stretch.

Main Methods:

• Maximal voluntary MTU-stretch-hold contractions of the tibialis anterior muscle were performed using a dynamometer.
• Muscle fascicle kinematics were assessed using B-mode ultrasound.
• Muscle activity levels were measured using surface electromyography.

Main Results:

• Increasing preactivation significantly increased fascicle shortening and stretch amplitudes, leading to greater positive and negative fascicle work.
• Peak fascicle forces during stretch occurred at similar fascicle lengths and joint angles, irrespective of preactivation level.
• Residual force enhancement (rFE) was not significantly affected by preactivation but was strongly correlated with peak fascicle force during stretch.

Conclusions:

• Apparent eccentric exercise elicits shortening-stretch contractions at the fascicle level, not isolated eccentric contractions.
• Fascicle kinematics are dependent on preactivation and cannot be solely inferred from MTU or joint kinematics.
• MTU series compliance and muscle preactivation are critical factors to consider when designing exercise protocols to understand mechanical drivers of adaptation.