Although explosive isometric contraction provides little work toward the outside, force-time parameters of the rising phase of the force-time curve may be able to predict muscle power. The purpose of this study was to examine the relationship between muscle power with work (power grip) and force-time parameters during the rising phase in explosive isometric grip. Fifteen healthy young adult males participated in this study. Power grip was measured using loads of 20-50% MVC (peak isometric force). Subjects pulled explosively on a grip bar held with the second digital joints without the thumb. Peak power was calculated from peak velocity and load. Explosive isometric grip was measured using a hand dynamometer. Time-series data of both tests were sampled by an analog-to-digital interface. Both tests were performed with the subjects seated with a sagittal and horizontal position of the arm supported by an armrest. Peak power in the power grip test tended to be larger with an increase of the load, but there was no significant difference between 40% and 50% MVC. Only the peak power in 50% MVC significantly correlated with peak grip force (r = 0.52, p < 0.05). The force-time parameters related to the peak rate of the rising force phase in explosive isometric grip significantly correlated with the peak powers (30-50% MVC) (r = |0.58-0.78|). Peak rate of the rising force phase in explosive isometric grip may be useful for predicting muscle power with loads between 30-50% MVC.

Individual differences in muscle contractile speed during static explosive muscle contraction are reflected in the developmental phase of the force-time curve. The purposes of this study were to clarify the properties and reliability of the inflection point of force-time, statistically dividing speed during static explosive grip into two phases and to assess the relations between that inflection point and others. Static explosive grip data were measured two times with a 5-min. rest (sampling frequency; 100 Hz). 32 healthy, young men (age: 15.5 +/- 0.8 yr., height: 173.9 +/- 7.3 cm, body mass: 71.5 +/- 11.2 kg) participated. 8 static explosive grip parameters were selected: time of reaching, integrated area, and quotient values of the integrated areas up to 0.25, 0.5, and 1.0 sec. divided by maximal grip force. The inflection point was calculated statistically from two regression lines fitted to a developmental phase and the almost steady-state phase of reaching maximal grip force by applying a two-phase regression model. The reliabilities of maximal grip force, time of reaching 90% of maximal grip force, and the integrated area until 0.5 sec. and 1.0 sec. after the onset of grip were good (ICC=.77 to .93). The time of reaching an inflection force value appeared at 0.3 sec. after the onset of grip, corresponding to 80% of maximal grip force, and the reliabilities of the parameters regarding inflection point were good (ICC=.77 to .95). The time determined by boundary data between the former and the latter regression data set and the regression coefficient during the developmental phase correlated significantly with the time of reaching 90% of maximal grip force, the integrated area, and the quotient values of the integrated areas up to 0.25, 0.5, and 1.0 sec. divided by maximal grip force (rs=-.78 to -.96 and -.75 to 0.88, respectively, p<.05). However, these parameters did not correlate with maximal grip force. A force during the developmental phase and maximal grip force can depend on different physiological factors. The time determined by boundary data between the former and the latter regression data set and the regression coefficient during the developmental phase are useful parameters for evaluating static explosive grip.

The purposes of this study were to clarify the reliability of two types of grip exertions, explosive grip exertion and voluntary grip exertion up to the maximal grip strength and to examine their force patterns using force-time parameters. Subjects were 100 healthy young male volunteers (age: 17.8+/-2.50 yr.) who had no upper limb injury. Grip strength was measured two times with voluntary grip and then two times with explosive grip. 11 parameters derived from the force-time curve were selected to measure the developmental phase of muscle contraction. The reliability of maximal grip strength in explosive and voluntary grip exertions was very high (intraclass correlation = .95, .93). The difference between two trials in explosive grip tended to be smaller than that for voluntary grip, and reliability of the exertion pattern was higher for explosive grip than voluntary grip. The times to reach 90% of the maximal grip strength and the maximal grip strength in explosive grip were shorter than those in voluntary grip. The other 8 force-time parameters had higher values in explosive grip than voluntary grip and higher reliabilities. The force-time parameters reflect the properties of explosive exertion. The results suggest the possibility that static explosive strength could be evaluated using these force-time parameters.