Quantifying thigh muscle co-activation during isometric knee extension contractions: within- and between-session...

Area of Science:

• Biomechanics
• Human Movement Science
• Kinesiology

Background:

• Muscle co-activation around the knee is vital for ambulation and balance.
• Inconsistent methods for quantifying co-activation index (CI) hinder cross-study comparisons.

Purpose of the Study:

• To assess the within- and between-session reliability of different methodological approaches for quantifying knee extensor and flexor muscle CI.
• To identify optimal analytical methods for reliable CI measurement.

Main Methods:

• Eight healthy volunteers underwent two testing sessions performing maximum voluntary isometric contractions (MVICs) of knee extensors.
• Electromyographic (EMG) and torque data were recorded, and CI was calculated using various analytical methods.
• Reliability was assessed using Intraclass Correlation Coefficient (ICC) and Coefficient of Variation (CV).

Main Results:

• Within-session reliability was high (ICC > 0.861, CV < 21.8%) compared to between-session reliability (ICC < 0.645, CV > 24.2%).
• Using a 500ms or larger Root Mean Square (RMS) EMG window around the peak torque (PT) yielded more reliable and less variable CI results.
• Specific analytical approaches significantly impacted CI measurement reliability.

Conclusions:

• The co-activation index (CI) can be a reliable measure for within-session comparisons of knee muscle activity.
• Methodological choices, particularly EMG window selection, critically influence CI reliability.
• Standardizing CI quantification methods is recommended for future research, favoring within-session designs.