Background: Surface electromyography (sEMG) can be used to provide real-time muscle performance data and desirable feedback to patients and clinicians in rehabilitation or sports injury settings. However, a comparison of electrical activity patterns of muscles with different fiber type compositions has not been conducted, and the relationship between electrical outputs with muscles of different sizes is relatively unexplored. PURPOSE: This study was designed to evaluate and compare the recruitment and activation patterns of large and small muscles with different fiber type combinations (proportion of type I and II fibers) during isometric contractions using sEMG. METHODS: Participants (n=10, mean age: 24.4 ± 2.5 years) with no history of musculoskeletal injuries or cardiovascular problems were asked to perform progressive isometric contractions at 25%, 50%, 75%, and 100% of their maximal voluntary contraction (MVC) level. sEMG data was collected from the abductor pollicis brevis (APB) of the thumb and vastus lateralis (VL) of the upper leg. Raw force output, mean integrated EMG (iEMG), normalized electrical activity output, and median frequency were compared between these two muscles and four contraction levels. RESULTS: APB, which consists of a higher percentage of Type I fibers, generated more electrical activity (mean iEMG: APB = 1187.57 ± 117.41 uV·s, VL = 175.93 ± 18.00 uV·s, p<0.001), but less force output at maximal contraction (p<0.001). Normalized electrical activity showed significantly different recruitment patterns between the two muscles (p<0.001), and the power spectra indicated a higher median frequency for APB (mean MDF: APB = 100.45 ± 3.30 Hz, VL = 77.24 ± 1.22 Hz, p<0.001). CONCLUSION: sEMG has been used as a biofeedback tool to help both patients and clinicians in documenting muscle recovery following sports injuries. However, it is important to realize there are muscle differences based on size and type of fiber. Changes in electrical activity used as an indication for strength improvement cannot be generalized across muscles. Smaller, predominately Type I muscles may require a higher muscle performance target during rehabilitation. Supported by NSF EFRI 1332329.