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Electromyographic Analysis of Grip
Paige Fox, MD, PhD; Viet Nguyen, MD; Catherine Curtin, MD
Stanford University, Palo Alto, CA

Introduction: In the 1960s and 1970s, Dr. Charles Long and colleagues used electromyography (EMG) to describe the roles of intrinsic and extrinsic hand muscles in digit motion. Since that time, multiple advances have been made in EMG technology and testing. However, we have not revisited the results of these landmark studies. This study was designed to use state of the art video- EMG synchronized analysis to determine the dynamic use of extrinsic and intrinsic finger flexion during grasp in normal volunteers.
Methods: Under IRB approval, healthy volunteers responded to a brief questionnaire to obtain their age, handedness, hand function, profession and any prior hand trauma or hand surgery. EMG targeting the function of the flexor digitorum profundus (FDP), flexor digitorum superficialis (FDS), and intrinsics was then obtained under the direction of a board certified neurologist. Motions specific to each group of muscles were used to confirm probe placement. The subjects were then asked to make a "light fist" followed by a "tight fist". Time- locked digital video was obtained during testing. Three independent observers then analyzed the video-EMG synchronized data to determine the EMG pattern of muscle use during composite fist formation.
Results: Three patterns of muscle recruitment during grasp formation could be determined from EMG data. First, FDP was involved in the initiation of light and tight fist formation in 4 of 5 patients. In one unique patient the intrinsics initiated the fist motion with FDP recruited less 200 milliseconds later. Light fist formation primarily involved the FDP with either the FDS or intrinsics, but not both, in all patients. In contrast, both the FDS and intrinsics were recruited in all tight fist EMGs. However, the sequence of recruitment after fist initiation by FDP differed between patients in tight fist formation.
Conclusions: The flexor digitorum profundus muscle is the workhorse in composite fist formation irrespective of effort. However, the role of FDS and the intrinsic muscles is less consistent across patients. The recruitment of either FDS or the intrinsics during light fist formation is less predictable than initially reported. The sequence of recruitment of these muscles also varies between patients. We believe this information has the potential to improve patient rehabilitation after flexor tendon repair. The differential patterns and sequences of muscle recruitment could be used to create personalized hand therapy regimens to expedite recovery.


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