Evaluation of the Torque–Velocity Test of the BTE-Primus as a Measure of Sincerity of Effort of Grip Strength

Abstract

An inverse linear relationship exists between torque and velocity in the mid-ranges of an isotonic maximal contraction in a single joint movement (such as the elbow and knee). We hypothesized that submaximal effort does not produce a linear torque–velocity relationship because replicating a submaximal isotonic contraction requires an enormous amount of proprioceptive feedback and the nervous system may not be able to accurately replicate both force and speed of contraction. If this hypothesis is true, the torque–velocity test of the BTE-Primus would be an effective method for assessing sincerity of effort. The purpose of this study was to examine if differences exist in the linear torque–velocity relationship between maximal and submaximal grip strength effort. Due to the fact that a test is not valid unless it is reliable, an additional purpose was to calculate the test–retest reliability of velocity during isotonic contraction using the torque–velocity test of the BTE-Primus' grip tool. Participants included 32 healthy, right-hand dominant (16 male, 16 female) persons, aged 20–50 years (mean age 25 ± 8.0), with no history of upper-extremity injury. The subjects participated in two days of grip-strength testing (approximately two weeks apart) and were instructed to exert maximal effort with both hands on one day, and to feign injury with one hand on the other day. Each day included two sessions of testing, which consisted of performing the “torque–velocity test” on the BTE-Primus grip attachment (#162). We randomly assigned the feigning hand (dominant vs. nondominant) and the effort (maximal vs. submaximal). The test administrator was blinded to the level of effort. On each day, four isotonic grip-strength tests were performed at loads of 20%, 30%, 40%, and 50% of isometric test scores. Three repeated isotonic grip strength trials were performed at each load and the average was plotted. One plot was generated for the maximal effort and another for the submaximal efforts. Average torque was plotted against the average velocity at each of the four loads and for each level of effort (maximal vs. submaximal). The linear relationship of the torque–velocity curve was examined by performing regression analysis, calculating the intercept, slope, correlation coefficient (r), and the coefficient of determination (r²) for each curve. Paired t-tests were used to compare the intercept, slope, and r² between maximal and submaximal efforts. Bonferroni correction set the alpha level at 0.0167. Sensitivity and specificity values were calculated for linearity (r²) and a Receiver Operator Characteristic (ROC) curve was constructed to obtain the optimal sensitivity and specificity combination. In addition, test–retest reliability was determined for velocity of maximal isotonic effort using Intraclass Correlation Coefficient. Significant differences between maximal and submaximal efforts were found for the intercept (t = 5.069; p < 0.001) and for linearity as expressed by r² (t = 5.414; p < 0.001). Mean r² was 0.89 for maximal effort and 0.53 for submaximal effort. The slopes of maximal and submaximal efforts were not significantly different (t = 0.14; p = 0.888). The ROC curve revealed the optimal combination of sensitivity (0.69) and specificity (0.72) values. Test–retest reliability of maximal isotonic grip effort for velocity was r = 0.843. The differences in intercepts suggested that velocity was greater during maximal effort. Greater r² values indicated greater linearity for maximal efforts than submaximal efforts. These findings suggest that the torque–velocity test of the BTE-Primus can distinguish between maximal and submaximal efforts during grip-strength testing. However, the test misclassified 31% of submaximal effort and 28% of maximal error, for a total error of 59%. Therefore, this test does not posses adequate sensitivity and specificity values to justify its use in the clinic.