This study used a within-subjects design. The purpose of this study was to examine cross-talk among monopolar surface electromyographic (EMG) signals from the superficial quadriceps femoris muscles. The "efficiency of electrical activity" technique for assessing muscle function uses monopolar EMG. Thus, knowledge of the potential for cross-talk among the superficial quadriceps femoris muscles when using monopolar recording will be valuable in rehabilitative settings. Fourteen healthy men (mean +/- SD age = 22.0 +/- 3.9 years) volunteered to perform submaximal to maximal isometric muscle actions of the dominant leg extensors in 10% increments from 10% to 100% of the maximum voluntary contraction (MVC). During each muscle action, monopolar surface EMG signals were detected from the superficial quadriceps femoris muscles. Three separate cross-correlations were performed to examine cross-talk among the vastus lateralis and rectus femoris, vastus medialis and rectus femoris, and vastus lateralis and vastus medialis. The peak cross-correlation coefficients ranged from R(x,y) = 0.182-0.944, with the greatest cross-talk occurring between the vastus lateralis and vastus medialis. In addition, the cross-correlation coefficients generally increased with force. These findings showed moderate to large degrees of crosstalk among monopolar surface EMG signals from the superficial quadriceps femoris muscles. Thus, the monopolar EMG signals from these muscles should be interpreted carefully and with the understanding that at least a portion of the signal from each muscle is due to cross-talk.