Isometric preactivation before active lengthening increases residual force enhancement.

Area of Science:

• Biomechanics
• Human Physiology
• Muscle Mechanics

Background:

• Residual force enhancement (RFE) is a phenomenon where active stretch results in greater isometric force than purely isometric contraction.
• Understanding factors influencing RFE is crucial for optimizing muscle performance and rehabilitation.

Purpose of the Study:

• To investigate the effect of isometric preactivation immediately preceding active stretch on the magnitude of RFE in plantar flexors.
• To determine if preactivation influences the force output following an active stretch protocol.

Main Methods:

• Three conditions were tested: control (isometric contraction), no preactivation (eccentric contraction followed by isometric contraction), and isometric preactivation (isometric contraction, then eccentric, then isometric contraction).
• Isometric torque was measured at the end of the final isometric contraction at 15° dorsiflexion.
• Medial gastrocnemius fascicle behavior was monitored using ultrasonography.

Main Results:

• Isometric torque was significantly greater in the preactivation condition compared to the control condition (P=0.017).
• No significant difference in torque was found between the no preactivation and control conditions (P=0.744).
• Fascicle elongation during active stretch was significantly greater in the preactivation condition than in the no preactivation condition (P=0.002).

Conclusions:

• Substantial RFE was induced only when isometric preactivation was applied before the active stretch.
• The increased isometric torque in the preactivation condition is likely due to a lesser degree of fascicle elongation during the active stretch phase.