Knee angle-specific EMG normalization: the use of polynomial based EMG-angle relationships

Area of Science:

• Biomechanics
• Human Movement Science
• Exercise Physiology

Background:

• Electromyography (EMG) signal normalization to maximal voluntary contraction is standard for comparing muscle activity.
• Dynamic movements involve muscle length dependence and architectural changes, necessitating joint angle-dependent normalization.

Purpose of the Study:

• Quantify muscle activity variations across a large range of motion (ROM).
• Assess the accuracy of EMG-joint angle curves in characterizing these variations.
• Compare EMG normalization using absolute peak versus joint angle-specific methods.

Main Methods:

• Fifteen subjects performed isokinetic leg extensions (110° ROM, 30°s⁻¹) to derive EMG-joint angle relationships via polynomial fitting.
• Ten subjects performed loaded countermovement leg extensions for normalization comparisons.
• EMG signals were normalized to both peak activity and derived EMG-angle curves.

Main Results:

• Significant variations in EMG amplitude were observed across the ROM.
• Normalization using EMG-angle curves yielded significantly higher peak (EMG-P) and average (EMG-A) amplitudes compared to absolute peak EMG.
• Higher-order polynomial fits demonstrated better accuracy in matching filtered EMG amplitudes.

Conclusions:

• EMG amplitude is significantly dependent on joint angle during large ROM movements.
• EMG-angle polynomial fits offer a more accurate and robust method for normalizing EMG signals in dynamic, large ROM activities.
• This approach enhances the validity of relative muscle activity comparisons in biomechanical studies.