# An Electromyographic Study Comparing Muscle Function During Supination and Pronation of the Forearm

## TL;DR

This study compares muscle activity during forearm rotation movements to understand how different muscles contribute under varying loads.

## Contribution

The study provides new insights into load-dependent recruitment patterns of forearm muscles during supination and pronation.

## Key findings

- The supinator muscle is more active at low torques during supination, while the biceps brachii becomes more active at higher torques.
- During pronation, the pronator quadratus is more active at low torques, and the pronator teres becomes more active at higher torques.
- Multi-joint muscles are progressively recruited as torque demands increase in both movements.

## Abstract

Background: Forearm pronation and supination are fundamental movements essential for daily activities and clinical applications. While supinator and biceps brachii contribute to supination, and pronator quadratus and pronator teres facilitate pronation, the precise activation patterns and torque-dependent recruitment strategies of these muscles remain incompletely understood. Conflicting evidence exists regarding muscle contribution at varying load conditions, with implications for rehabilitation protocols, surgical planning, and prosthetic design.

Purpose: This study aimed to quantify and compare the electromyographic (EMG) activity of forearm rotator muscles during isometric contraction under progressively increasing torque loads during both supination and pronation movements.

Methods: Four healthy right-handed subjects (3 males, 1 female; mean age 32.5 years) underwent simultaneous EMG and motion capture recording. Surface electrodes captured activity from biceps brachii, triceps, pronator teres, and pronator quadratus, while fine-wire electrodes measured supinator muscle activity. Participants maintained a neutral forearm position against applied loads of 1 kilogram positioned at increasing distances (10-35 cm) from the supination axis, creating progressively higher torques. EMG signals were filtered, rectified, normalized, and analyzed using root mean square values across three trials per loading condition.

Results: During supination resistance, the supinator demonstrated higher activation at lower torques compared to the biceps brachii. As applied torque increased, the biceps brachii activity increased proportionally more than the supinator, indicating load-dependent recruitment. Triceps showed increased co-activation for joint stability. During pronation resistance, the pronator quadratus exhibited greater activity at lower torques relative to the pronator teres. With increasing torque, the pronator teres demonstrated relatively greater activation increases than the pronator quadratus. Both movement patterns demonstrated progressive recruitment of multi-joint muscles as torque demands increased.

Conclusions: This pilot EMG study (n=4) provides preliminary descriptive evidence suggesting that supinator and pronator quadratus primarily govern low-torque forearm rotation, while biceps brachii and pronator teres become increasingly dominant during high-torque demands. These observed patterns are consistent with a hierarchical muscle recruitment strategy optimized for mechanical efficiency and joint stability. Understanding these activation patterns may have potential clinical applications for rehabilitation protocol design following nerve injury or tendon rupture, surgical planning for nerve transfer procedures, and development of myoelectric prosthetic control algorithms.