Four groups of patients with low back pain and a group of healthy subjects participated in a study to investigate the effects of administering three different muscle conditioning protocols on the functional output of the neuromuscular system. Quadriceps muscle weakness was identified in patients suffering from low back pain. The weakness was attributed to pain and its associated effect on reduced activity and disuse. The muscle conditioning methods used were: functional electric stimulation at low frequency of 20 Hz (group S20), functional electric stimulation at 50 Hz (group S50), and isometric training (group ISOM). Two groups served as controls. The first group (CONT1) consisted of low back pain patients that did not go through any of the previously mentioned special muscle conditioning programs. The second control group (CONT2) consisted of healthy subjects. Changes in the output of the neuromuscular system were quantified through measures of maximum voluntary contraction force, limb girth, and several measures of electromyographic activity. Results suggested that both functional electric stimulation and isometric exercises were significantly effective in increasing muscle strength over the experimental period of two weeks. Moreover, for the sample and conditions used in this study, functional electric stimulation at 20 Hz was more effective in increasing muscle strength than that at 50 Hz or isometric exercise. Isometric exercise was effective in increasing motor unit recruitment of both the exercised and the contralateral muscles. The results suggest that the underlying mechanism of effectiveness of functional electrical stimulation, in this case, is related to peripheral factors with no significant involvement of higher levels of the central nervous system or the brain. A conceptual model for the underlying mechanism of functional electric stimulation action is proposed based on the findings of this study. It was concluded that electric stimulation, when used according to the protocol described in this study, is a very effective and a viable muscle strengthening technique
