Multi-directional strength and force envelope of the index finger

Objective. The purpose of this study was to biomechanically evaluate the motor function of the index finger based on multi-directional strengths. Design. An experimental apparatus was developed to measure force production of a digit at various points of force application along the digit and in any direction of force application within the transverse plane of the longitudinal axis of the digit. Background. Most existing tests of hand motor function are subjective, semiquantitative, and/or non-specific. Methods. Eight normal subjects with asymptomatic hands were tested. Maximum voluntary isometric contraction forces were measured at the middle of the proximal phalanx of the index finger in 16 directions that were evenly distributed within 360°. Results. The highest force, 110.7 (SD 9.0) N, was generated in flexion, while the lowest force was generated in extension. The forces in extension, abduction and adduction were 37.6%, 97.9% and 79.3% of the flexion force, respectively. The area of the force envelope was 25 739 (SD 3688) N–N. The average percentage quadrant areas, relative to the total force envelope area, for extension–adduction, extension–abduction, flexion–abduction, and flexion–adduction were 12.9%, 20.4%, 36.0%, and 30.8%, respectively. The average percentage quadrant areas for extension, abduction, flexion, and adduction were 9.1%, 33.3%, 35.6%, and 22.1%, respectively. Conclusion. The current study provides an advanced level of quantification of hand motor function.