Magnitude of the Endpoint Force of Flexor Pollicis Longus Muscle Increases with Grasp Opening

Joseph D. Towles, MS; Vincent R. Hentz, MD; Wendy M. Murray, PhD

Objectives: The thumb must be capable of producing adequate pinch force over a large range of thumb positions to achieve functional independence in a broad range of activities of daily living. Reconstructive surgery of the thumb after tetraplegia is designed to restore the ability to use the thumb to produce a pinch force over a functional range of postures. The design of these types of surgeries benefits from knowledge of how the magnitudes of the endpoint forces of thenar muscles are influenced by grasp posture. This study is intended to illustrate the effect of thumb posture on the magnitude of the endpoint force of the flexor pollicis longus muscle (FPL), which plays an important role as the thumb generates pinch forces.

Methods: We performed cadaveric tests on eight specimens to investigate the relationship between thumb posture and FPL’s endpoint force. During each experiment, we fixed anatomical specimens of the forearm and hand in neutral position. We attached an extension spring to FPL’s tendon and attached the endpoint (thumb-tip) to a six-axis force/torque transducer. Finally, we applied 10 N to FPL’s tendon and measured the muscle’s thumb-tip force in two thumb postures, narrow grasp - which is required to hold thin objects, such as credit cards and forks - and wide grasp - which is necessary to hold thick objects, like balls and bottles.

Results: In all specimens, the magnitude of FPL’s endpoint force increased significantly (p<.05) as the thumb’s position went from the narrow to the wide grasp. On average, the magnitude of FPL’s endpoint force increased by 126±49% (range: 55% to 167%, n=8, p<.05).

Conclusions: This work indicates that the magnitude of FPL’s endpoint force increases with grasp opening. A parallel computer simulation study reveals that the increase in magnitude with grasp opening is mainly due to the increase in extension of the metacarpophalangeal joint. Future work will determine whether this endpoint force-grasp opening relation extends to other thenar muscles. It is clear that the thumb’s posture plays a key role when pinch forces are produced. Hand surgeons are able to account for this as they stabilize joints and transfer tendons to improve the grasp ability of the spinal cord injured, for example. Veterans with tetraplegia involved in these surgeries may be able to perform more activities of daily living and, therefore, enjoy an increased quality of life.

Funding Acknowledgment: Rehabilitation R&D Service of the Department of Veterans Affairs, VA Merit Review B898-3RA