¹Friedrich-Alexander-Universität Erlangen-Nürnberg & Universitätsklinikum Erlangen, Internal Medicine 3 – Rheumatology and Immunology, Deutsches Zentrum Immuntherapie, Erlangen
²Universitat Jaume I, Department of Mechanical Engineering and Construction, Castellón de la Plana
³Friedrich-Alexander-Universität Erlangen-Nürnberg, Institute of Applied Dynamics, Erlangen
⁴Charité – Universitätsmedizin Berlin, Med. Klinik mit Schwerpunkt Rheumatologie und Klinische Immunologie, Berlin

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Introduction: Rheumatoid arthritis (RA) and psoriatic arthritis (PsA) patients experience a decline in grip strength over the disease course, despite clinical remission [1]. The underlying neuromuscular changes contributing to functional impairment remain unclear. This study aimed to identify differences in forearm muscle activity (extensor digitorum communis (EDC), flexor digitorum superficialis (FDS)), between RA, PsA patients and healthy controls (HC) during a standardized power grasp.

Methods: RA (ACR/EULAR 2010) and PsA (CASPAR) patients from the Internal Medicine 3 outpatient clinics, Universitätsklinikum Erlangen (Germany), were included during routine clinical visits, and HC recruited via social networks (Ethics #357_20B). Clinical standard scores for disease activity were assessed (RA: Disease-Activity-Score 28 (DAS28); PsA: Disease Activity in PSoriatic Arthritis Score (DAPSA)). Muscle activity was recorded using electromyography (EMG) of the FDS and EDC during a standardized power grasp (Vigorimeter, KLS Martin Group, Germany, Figure 1 [Fig. 1]). EMG amplitude parameters indicative for muscle activation (root mean square (RMS), maximum (MAX)), and waveform parameters linked to muscle effort and motor unit recruitment (new zero crossing (NZC), enhanced wavelength (EWL)) were calculated [2] and compared between groups (linear mixed-effect models, stratified by sex, adjusted for age, hand dominance; participant ID as random intercept term).

Results: 187 participants were included (HC: n=43, 22/21 f/m, age 54±16; RA: n=69, 47/22 f/m, age 53±14; PsA: n=75, 38/37 f/m, age 54±14). Despite low disease activity, female PsA patients showed reduced grip strength and lower EWL in FDS and EDC compared to HC and RA. Female RA patients had higher RMS compared to PsA and lower grip strength than HC (Table 1 [Tab. 1]). No significant differences were found in male participants (all p>0.05).

Conclusion: These findings align with previous research showing increased RMS values in functional tasks in female RA patients [3], suggesting greater muscle activation to compensate for functional impairments, whereas PsA patients show lower muscle effort (EWL), reflecting limited strength output. The observed deficits in hand function in patients who are mostly in clinical remission are likely due to disease-related neurophysiological changes. EMG analysis serves as a valuable tool to detect these adaptations of muscle function and highlights different compensatory mechanisms in female PsA and RA patients.

References

[1] Rydholm M, Book C, Wikström I, Jacobsson L, Turesson C. Course of Grip Force Impairment in Patients With Early Rheumatoid Arthritis Over the First Five Years After Diagnosis. Arthritis Care Res (Hoboken). 2018 Apr;70(4):491-8. DOI: 10.1002/acr.23318
[2] Jarque-Bou NJ, Vergara M, Sancho-Bru JL. Does Exerting Grasps Involve a Finite Set of Muscle Patterns? A Study of Intra- and Intersubject Variability of Forearm sEMG Signals in Seven Grasp Types. IEEE Trans Neural Syst Rehabil Eng. 2024;32:1505-14. DOI: 10.1109/TNSRE.2024.3383156
[3] Brorsson S, Nilsdotter A, Thorstensson C, Bremander A. Differences in muscle activity during hand-dexterity tasks between women with arthritis and a healthy reference group. BMC Musculoskelet Disord. 2014 May 15;15:154. DOI: 10.1186/1471-2474-15-154