AIM: The purpose of this study was to assess maximal grip force in two conditions of voluntary muscular contraction (MVC) and electrical stimulation superimposed on voluntary muscular contraction (SES) to better understand mechanisms and effectiveness of electrical stimulation of the hand. There is conflicting evidence regarding the effectiveness of electrical stimulation superimposed on voluntary muscular contraction on improving maximal force. Increased knowledge of the physiologic and mechanical effects of electrical stimulation applied during voluntary muscular contraction can lead to refinement of its clinical application. METHODS: Twenty subjects (36+/-13 years; 17 males and 3 females) participated in this study. All subjects were undergoing physical therapy within a hand rehabilitation center. They were instructed to randomly perform three grip determinations in both voluntary muscular contraction and superimposed electrical stimulation conditions to elicit maximal grip force of the unaffected hand. Force was assessed using a handheld dynamometer. Subjective force and contraction were assessed just after sessions as well as pain and discomfort using a visual analogue scale. RESULTS: The mean force values were 22+/-7 kg and 30+/-1 kg for the superimposed electrical stimulation and voluntary muscular contraction conditions, respectively. Analyses of the force measures showed that force was weaker in the superimposed electrical stimulation condition (P<0.001). Patients rated their pain and discomfort at 0+/-0 mm and 4+/-2.9 mm, respectively. CONCLUSION: Maximal grip force is reduced when electrical stimulation is superimposed to voluntary muscular contraction. This result could be explained by unbalanced muscular synergies at the hand due to SES, confirming these synergies as essential to produce maximal grip force.