The Hold-Relax Technique (HRT) is used in physical therapy and in sports for muscle relaxing and stretching. The present study compares HRT with a purely cognitive technique, which is called Isometric Sensorimotor Task (IST). The effectiveness of these two different therapeutic approaches in producing relaxation of hamstrings of 14 healthy subjects, aged between 20 and 40 years, with different degrees of muscle shortening was evaluated. Surface electromyography activity measurement before and after the isometric contraction, showed a greater muscle relaxation IST-related compared to that HRT-associated. Furthermore, in some subjects, HRT produces an increased muscle activity in contrast to what the interpretative models based on muscle inhibition predict. This high variability in HRT efficacy, confirmed by recent scientific data, it allows to design an interpretative model of this technique and, generally, of a post-isometric muscle relaxation based on a cognitive theory.