Background: Grip strength is a widely accepted clinical indicator of upper limb function. The grip strength measurement validity and clinical relevance of the American Society of Hand Therapists (ASHT) and suboptimal postures, such as slumped sitting, remain unclear. Objects: This study aimed to investigate whether sitting postures, including slumped posture, influence grip strength and upper limb muscle activation and to determine the clinical significance of grip strength assessment in the slumped position. Methods: Forty healthy adult males participated in a cross-over design study. Grip strength and surface electromyography of the triceps brachii, biceps brachii, anterior deltoid, and upper trapezius were measured in ASHT-recommended, slumped, and self-balanced postures. The Friedman test was used to compare outcomes across postures, and the relationship between grip strength and muscle activation was determined using Pearson correlation. Results: Grip strength significantly differed across postures (χ² = 25.37, p < 0.001), with ASHT posture having the highest values and slumped posture the lowest. Triceps brachii and upper trapezius showed significant variations in muscle activity across postures (p < 0.05). The electromyography-to-force ratio, representing neuromuscular efficiency, was the highest in ASHT posture. Grip strength strongly correlated with triceps brachii activity (r = 0.72, p < 0.001). Conclusion: Postural alignment significantly affects grip strength and upper limb muscle efficiency. These results highlight the clinical relevance that grip strength assessment should consider both ideal and habitual postures to improve the validity of functional evaluation and rehabilitation planning.

INTRODUCTION
MATERIALS AND METHODS
    1. Participants
    2. Experimental Procedure
    3. Measurement Tools and Procedures
    4. Statistical Analysis
RESULTS
    1. Comparison of Grip Strength Across Postures
    2. Comparison of Upper Limb Muscle Activation AcrossPostures
    3. Differences in Grip Strength to Muscle ActivationRatio Across Postures
    4. Correlation Between Grip Strength and Upper LimbMuscle Activation
DISCUSSION
CONCLUSIONS
FUNDING
ACKNOWLEDGEMENTS
CONFLICTS OF INTEREST
REFERENCES

• Sung-Hyun Kim(Department of Physical Therapy, College of Health and Welfare, Silla University, Busan, Korea) Corresponding author