Thumb Muscle Activation During First Dorsal Interosseous Exercise in Thumb Base Osteoarthritis

Trapeziometacarpal osteoarthritis (OA TMC) of the thumb is a chronic and progressive degenerative disease characterized by loss of articular cartilage, subchondral sclerosis, capsuloligamentous laxity, and osteophyte formation in the first metacarpal. Patients typically present with pain and muscle weakness, which impair hand function and limit activities of daily living (ADLs), resulting in a considerable personal and social burden, including reduced quality of life and work disability. Previous studies have highlighted the clinical importance of thumb TMC joint stabilizers. Research has demonstrated that the first dorsal interosseous (FDI) functions as the main stabilizer of the TMC joint, whereas the abductor pollicis longus (APL) acts as a primary destabilizer. These findings suggest that rehabilitation programs should focus on strengthening the FDI to improve joint stability. However, despite the clinical importance of targeting these muscles, the literature reports varied exercise protocols for FDI activation and limited information on how the APL behaves during the same exercise.

The aim of this study is to investigate whether there are differences in muscle activation of the FDI and APL between individuals with and without thumb OA TMC and to identify which exercises are most effective or activating these muscles in order to inform rehabilitation strategies. Various activities of daily living will also be analyzed to assess FDI and APL activation during functional tasks. Adequate rehabilitation is essential for maintaining muscle function, slowing the progression of joint degeneration, and improving hand performance, yet it remains unclear which exercises and tasks optimally engage the FDI and APL muscles.

This is a cross-sectional observational comparative study with both within-subject and between-group analyses. The study includes two groups: adults aged 40 years and older, both men and women, with a clinical diagnosis of OA TMC and healthy controls without musculoskeletal or neurological disorders affecting the hand. Exclusion criteria include sensory disturbances such as numbness or tingling, cardiac pacemaker implantation, upper limb entrapment neuropathies or cervical radiculopathy, neuromuscular diseases, systemic conditions including diabetes or hypothyroidism, history of cancer, rheumatoid arthritis, anticoagulation therapy, chemotherapy, exposure to neurotoxins, or recent upper limb trauma or surgery.

Surface electromyography (sEMG) is used to record muscle activity of the FDI and APL. Skin is prepared by shaving if necessary and cleaning with alcohol to reduce impedance and ensure optimal adherence of surface electrodes. Bipolar surface electrodes with a diameter of 20 millimeters and an inter-electrode distance of 20 millimeters are placed according to SENIAM guidelines. For the FDI, electrodes are positioned over the dorsal belly of the muscle aligned with fiber direction, with one electrode in the distal ulnar region between the first and second metacarpals. For the APL, electrodes are placed approximately five centimeters distal to the wrist, medial to the radius, longitudinally aligned and transversely away from adjacent muscles. Electrode placement is verified by palpation and selective activation tasks.

Participants perform a series of twelve standardized exercises and functional tasks commonly used in conservative rehabilitation of OA TMC. The first six tasks are exercises designed to activate the FDI and include variations of index finger abduction with ulnar support, elastic resistance with ulnar support, elastic resistance with palmar support using a ball, palmar support without resistance, elastic resistance with palmar support, and elastic resistance with maximal palmar abduction of the thumb. The remaining six tasks simulate ADLs and include turning a key toward the ulnar side, turning a key toward the radial side, picking up a coin, writing a word with a pen, squeezing a full tube of toothpaste, and holding a glass of water while performing a drinking gesture. Each task is performed in three consecutive three-second isometric repetitions with a thirty-second rest between repetitions to minimize fatigue.

It is hypothesized that individuals with thumb TMC osteoarthritis will demonstrate altered activation patterns of the FDI and APL compared with healthy controls, and that specific exercises and functional tasks will elicit greater FDI activation, supporting targeted interventions to enhance joint stability and hand function in patients with thumb base osteoarthritis.

Inclusion criteria

for the TMC OA group:

• Clinical diagnosis of thumb TMC osteoarthritis.

Inclusion criteria

for the healthy group:

• Participants with no history of musculoskeletal or neurological disorders affecting the dominant hand were included.

Exclusion criteria

• Presence of numbness, tingling, or other sensory disturbances
• Cardiac pacemaker implantation
• Upper limb entrapment neuropathy or cervical radiculopathy
• History of neuromuscular disease, polyneuropathy, or plexopathy
• Systemic diseases such as diabetes, hypothyroidism, or chronic liver or kidney disease
• History of cancer, rheumatoid arthritis, active anticoagulation therapy, chemotherapy, or exposure to neurotoxins
• Recent upper limb soft tissue or bone injury (trauma, fracture, or surgery of the arm or neck).