Biomechanical Analysis of Dynamic Gripping in Manual Laborers Exhibiting Work-Related Scapholunate Instability Signs

ABSTRACT Background: Scapholunate instability (SLI) represents the most prevalent form of carpal instability and is increasingly recognized as a clinically significant occupational condition in manual-labor populations. Repetitive forceful gripping, sustained wrist loading, and awkward joint postures inherent to construction, manufacturing, and heavy industry may predispose workers to progressive scapholunate ligament compromise; however, the specific biomechanical mechanisms underlying dynamic grip force transmission in symptomatic laborers remain poorly characterized. Objective: This study aimed to quantify dynamic grip force profiles and wrist kinematics in manual laborers exhibiting clinical signs of scapholunate instability; compare these biomechanical parameters with asymptomatic worker controls; and identify occupation-specific loading patterns that may contribute to scapholunate ligament compromise. Methods: A cross-sectional observational design was employed. Forty-two male manual laborers, mean age 36.4 (SD 7.2 )years; mean occupational exposure 9.8 (SD 4.1) years, were recruited from construction and surgical instrument manufacturing sites in Sialkot District, Pakistan. Participants were stratified into a symptomatic group (n = 21) based on positive Watson scaphoid shift test, dorsal wrist pain, and functional limitation, and an asymptomatic control group (n = 21). Dynamic grip force was measured using a calibrated Jamar dynamometer across five standardized trials. Wrist kinematics were captured during a simulated gripping task using electrogoniometry. Surface electromyography (sEMG) recorded flexor digitorum superficialis, flexor carpi ulnaris, and extensor carpi radialis longus activity. Scapholunate gap was confirmed radiographically. Statistical analyses included independent samples t-tests, Pearson correlation, and logistic regression ( = 0.05). Results: Symptomatic workers demonstrated significantly reduced grip force (28.4 (SD 5.6) kg versus 41.7 (SD 6.3) kg; p < 0.001) and elevated wrist ulnar deviation during peak gripping (22.3 (SD 4.1) degrees versus 14.6 degrees (SD 3.8) degrees ; p = 0.002) compared with controls. sEMG amplitude of flexor digitorum superficialis was paradoxically elevated in the symptomatic group (mean difference 18.3 microvolts; 95% CI 11.4-25.2), suggesting compensatory hyperactivation. Scapholunate gap correlated positively with years of occupational exposure (r = 0.67; p < 0.001). Logistic regression identified wrist ulnar deviation angle (OR = 2.14; 95% CI 1.43-3.19) and grip force asymmetry (OR = 1.87; 95% CI 1.21-2.89) as independent predictors of instability signs. Conclusions: Manual laborers with work-related scapholunate instability signs exhibit distinct biomechanical signatures during dynamic gripping, including reduced force output, abnormal wrist posture, and compensatory neuromuscular recruitment. Occupational exposure duration is a significant predictor of radiographic scapholunate dissociation. These findings support the integration of ergonomic risk screening and early biomechanical assessment into occupational health protocols for manual labor populations in low- and middle-income settings. Abbreviations: CI: confidence interval, DRUJ: distal radioulnar joint, EMG/sEMG: (surface) electromyography, FCU: flexor carpi ulnaris, FDS: flexor digitorum superficialis, MSD: musculoskeletal disorder, OR: odds ratio, ROM: range of motion, SL/SLI: scapholunate/scapholunate instability, SLIL: scapholunate interosseous ligament, SLAC: scapholunate advanced collapse, WMSDs: work-related musculoskeletal disorders, SD: standard deviation. Keywords: scapholunate instability; dynamic gripping; manual labor; wrist biomechanics; carpal kinematics; occupational ergonomics; grip force; electromyography; Watson scaphoid shift test; work-related musculoskeletal disorders