Improvement in Isometric Strength of the Quadriceps Femoris Muscle After Training with Electrical Stimulation DAVID M. SELKOWITZ The purpose of this investigation was to determine if training isometrically with electrical stimulation (ES) alone would significantly increase isometric strength of the quadriceps femoris muscle. The relationships between the strength changes and the relative force and duration of training contractions were also studied. An experimental group (Group 1) and a control group (Group 2), 12 subjects in each, underwent pretesting and posttesting to obtain their maximum voluntary isometric contractions (MVICs). Group 1 trained with maximally tolerable isometric contractions induced by ES, three days a week for four weeks. Results showed that although both groups demonstrated increases in isometric strength of their quadriceps femoris muscles, training isometrically with ES produced a significantly greater increase (p < .01) than not training with ES. The relative strength improvement in Group 1 was positively and significantly correlated with training-contraction intensity and duration. The relative increase in isometric strength, using only ES, may be determined by the ability of the subjects to tolerate longer and more forceful contractions. Suggestions for further research and implications for the clinical use of ES for strength-training are discussed. Key Words:zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Electric stimulation, Muscle contraction, Physical therapy. Exogenous electrical stimulation (ES) of nerve and muscle is used for muscle reeducation, 1 delay of atrophy, 2,3 temporary reduction of spasticity, 4,5 and reduction of contractures 6 and edema. 7 The use of ES for muscle strengthening in research and clinical practice has become increasingly popular in re- cent years. Training isometrically with ES is reported to increase isometric 8-11 and isokinetic 9, 12 quadriceps femoris muscle strength in healthy subjects. Patients recovering from knee ligament surgery 2 and patients with chondromalacia patellae 13 have also shown increases in isometric strength of the quadriceps femoris muscle after ES training. Experimental procedures have been based in part on ES research of isomet- ric strength increases in the quadriceps femoris muscle of athletes by investigators in the Soviet Union (to date unpub- lished in the United States). 8 Several investigators have dem- onstrated the effects of ES training on muscle physiology. 2, 14, 15 Munsat et al demonstrated significant increases in muscle fiber size and changes infibertype composition in the rectus femoris muscle of patients with contractured hamstrings after the use of chronic stimulation with implanted electrodes. 6 Stanish et al reported that ES training prevented a reduction in myofibrillar adenosine triphosphatase (ATPase) activity (associated with contractile force and speed) in the quadriceps femoris muscle of patients who had their knees immobilized after surgery. 3 Laughman et al, 10 Currier and Mann, 11 Currier et al, 16 and Massey et al 17 demonstrated no significant differ- ences in strength gains between groups training with ES and those training with only voluntary contractions. Comparing studies and drawing valid conclusions concern- ing the use of ES is difficult because of inadequate standard- ization of experimental procedures. Procedures involving an experimental group training with ES in addition to other forms of exercise, 2,3 an experimental group receiving ES su- perimposed on voluntary contractions, 11, 16 or an experimental group receiving ES as the only method of exercise 8-13, 17, 18 are reported. Recent studies have compared groups training with ES with groups training with only voluntary contractions (VC), 10,11 and with groups training with ES superimposed on voluntary contractions (ESVC). 11 Romero et al applied ES to the quadriceps femoris muscle bilaterally (simultaneously) and tested strength isometrically and isokinetically. 9 Halbach and Straus 12 and Massey et al 17 used dynamic testing modes to compare groups training isometrically with ES with those training dynamically without ES. Kots's notes, 8 Johnson et al, 13 and Halbach and Straus 12 did not report a control group for comparison with an ES group. In the literature on ES strength training, ES contractions have always been isometric. Of the previously mentioned studies, only Massey et al 17 applied ES to a muscle group other than the quadriceps femoris muscle. Other varying training factors include the frequency and period of training sessions and the duration and intensity (relative force) of training contractions. Electri- cal stimulation factors of frequency, amplitude, pulse width, and waveform have also varied. Characteristics of the ES and the training protocol have not always been reported thor- oughly nor have either been well-controlled. 19 Mr. Selkowitz is Staff Physical Therapist, Sports Health and Preseason Evaluation Center, Merritt/Peralta Medical Center, 2935 Telegraph Ave, Oak- land, CA 94609 (USA). He was a graduate student in the program for a Master of Science in Physical Therapy at Sargent College of Allied Health Professions, Boston University, Boston, MA, at the time this study was conducted. This project was supported in part with a grant from the Dudley Allen Sargent Research Fund. This article was submitted March 28, 1983; was with the author for revision 37 weeks; and was accepted August 1, 1984. 186 PHYSICAL THERAPY