Dynamic versus static grip strength: how grip strength changes when the wrist is moved, and why dynamic grip strength...

The synergistic relationship between wrist/forearm range of motion (ROM) and grip strength (GS) is arguably one of the most important aspects of hand function. Clinically, GS is measured with the wrist in a standardized static position, and the results of such tests have been deemed valid and reliable. The question remains, however, whether this static GS (SGS) measurement is an accurate indication of how an individual functionally grips objects--that is, most functional tasks require the fingers to grasp an object forcibly while moving the proximal joints such as the wrist and forearm. Therefore, further analysis of an individual's dynamic GS (DGS) during wrist/forearm movements may improve the clinician's understanding of hand function and provide more pertinent guidelines for assessing functional gripping, e.g., for vocational and avocational tasks and in designing workstations. The purpose of this study is twofold: to describe and assess a DGS testing device that utilizes optically encoded gyroscopes and a strain-gauge dynamometer to simultaneously measure GS and wrist/forearm position over real time; and to assess and compare grip force production differences in SGS and DGS in uninjured wrists, using this novel device. Twenty-nine uninjured wrists of men (n = 15) and women (n = 14)--age range, 21 to 43 years--were tested with the DGS device. Subjects were excluded if they had any previous wrist/forearm fracture, pain, or limitation of motion. The DGS device was designed and fabricated with two optically encoded gyroengines, a vertical gyroscope with two axes for measuring flexion/extension and radial/ulnar deviation, and a directional gyroscope with one axis for measuring supination/pronation, mounted on a strain-gauge dynamometer. The signals from the gyroscopes and dynamometer were processed by means of a data aquisition board and analog-to-digital circuitry and collected on a 486-MHz computer. The methods included repeated testing of each gyroscope axis to known angular measurements, and randomly assigned maximal gripping trials from the 29 subjects. The standard deviation of gyroengines is 1 degree to 2 degrees for each ROM axis. Maximal DGS is significantly less (mean 14%) than SGS, and SGS is 29% less than DGS at the same three-dimensional ROM positions. Gyroengines are feasible three-dimensional tracking devices that can be used to monitor wrist/forearm ROM in conjunction with GS.