Where should the black (active) electrode be placed on the distal dorsal forearm and the orange (reference) electrode on the dorsal wrist to record from the extensor carpi radialis?

Electrode Placement for Recording Extensor Carpi Radialis

Place the black (active) electrode at the distal 1/4 point of the forearm length on the dorsal surface, and the orange (reference) electrode at the extensor carpi radialis tendon on the dorsal wrist. This configuration maximizes signal amplitude and recording accuracy 1.

Optimal Active Electrode (Black) Placement

• Position the active electrode at the distal quarter of the forearm length measured from the elbow to the wrist on the dorsal surface 1.
• This location corresponds to the largest portion of the extensor carpi radialis muscle belly, where motor unit density is highest and signal amplitude is maximized 1, 2.
• The electrode should be aligned parallel to the muscle fiber direction to optimize signal quality and reduce noise 3.

Optimal Reference Electrode (Orange) Placement

• Place the reference electrode at the extensor carpi radialis tendon point on the dorsal wrist 1.
• This tendon location provides an electrically neutral reference point that minimizes cross-talk from adjacent active muscles 3.
• Positioning the reference electrode on the tendon rather than another muscle belly prevents contamination of the recorded signal 3.

Critical Technical Considerations

Skin Preparation

• Shave the area, cleanse with alcohol-saturated gauze, and perform gentle abrasion with fine sandpaper to reduce skin impedance below 5 kΩ 4, 3.
• Proper skin preparation significantly improves the signal-to-noise ratio and recording quality 4.

Electrode Configuration

• Use a bipolar configuration with both electrodes over the extensor carpi radialis to isolate this muscle and limit interference from the adjacent extensor digitorum communis 3.
• The stimulation site (if performing nerve conduction studies) should be 8 cm proximal to the active electrode 1.

Common Pitfalls to Avoid

• Do not place the active electrode over the motor end-plate region (approximately the middle third of the muscle belly), as this creates unstable recordings with excessive baseline noise 3.
• Avoid placing the reference electrode on another active muscle, which introduces cross-talk and degrades signal quality 3.
• Do not position electrodes without proper anatomical landmarks, as this leads to inconsistent placement and poor reproducibility 4.
• Electrode placement that is too proximal on the forearm may record activity from the extensor carpi radialis longus rather than brevis, or vice versa, depending on your target muscle 2, 5.

Anatomical Distinction Between ECR Longus and Brevis

• If you need to distinguish between extensor carpi radialis longus (ECRL) and brevis (ECRB), note that ECRB is located more distally along the forearm compared to ECRL 2, 5.
• The muscle bellies show significant spatial separation along the forearm length, with ECRB positioned approximately at the distal 1/4 point and ECRL more proximally 2.
• High-density EMG mapping confirms that these two heads occupy distinct spatial positions that can be differentiated with proper electrode placement 5.