Purpose This work presents methods developed for the analysis of dynamometric and electromyographic signals acquired during sustained maximal isometric hand grip tasks. In these analyses, we try to find a temporal pattern that could indicate the beginning of the decrease in hand grip strength. This information is essential for choosing the best isometric activity time for hand rehabilitation. Methods The technique applied to the dynamometric signal was to find the time interval that precedes the drop of the isometric force RMS (root mean square) curve to values lower than the effective value. The technique applied to the surface electromyography (sEMG) signals was the identification of the instant when the lowest value of the median frequency of the signal occurs. The dynamometry data collection was carried out bilaterally in 19 men and 21 women. The electromyography data collection was carried out bilaterally in only 19 women. Results The statistical results (5% significance level) for the dynamometry data indicated that the time from the beginning of the test to the beginning of the decay of strength was approximately 7.6 3.2 s, while the results for the sEMG data indicated that the time from the beginning of the test to the appearance of the lowest value of the median frequency was approximately 10.1 2.9 s. Conclusions The method of using the RMS values of the Force, presented by this work, found results of approximately 7 s, while the method most currently used (Median Frequency) found values of approximately 10 s. These results may help the rehabilitation professional to decide with more security the isometric time of exercises. Supplementary Information The online version contains supplementary material available at 10.1007/s42600-023-00262-2.