Purpose: The purpose of this study was to evaluate if surface electromyography (sEMG) of the Vastus Lateralis (VL) muscle during maximal incremental ergocycle test (ET) and isometric contraction (MVC) could be used to detect peripheral muscle abnormalities in 14 CHF (Ejection fraction = 25.93 ± 7.02%), 25 COPD (FEV1 = 40.52 ± 21.24% pred.), and 16 age matched healthy subjects. Method: Subjects executed isometric contractions of the quadriceps at various % of MVC while sEMG was recorded over the belly of the VL at the location of the future biopsy. The next day, the subjects performed an ET while sEMG was recorded over the freshly biopsied VL muscle. Median Frequency (MDF) and integrated electromyography (iEMG) were subsequently analysed using Acknowledge v.3.2 software. A discriminant analysis (Quadratic model) divided the population in 39 subjects with muscle abnormalities and 16 subjects without evidence of muscle abnormalities. The discriminatory variables established from healthy subjects were % and surface area of type I muscle fibres, the whole quadriceps muscle surface area from tomodensitometry imaging and the ratio of the activity the enzyme phosphofructokinase over the 3-hydroxyacyl CoA dehydrogenase (PFK/HADH). RESULTS: ROC curves analysies revealed a sensitivity of 81.3% and a specificity of 58.3% using ΔMDF/wattmax values obtained during the ET (Max MDF-Min MDF expressed per unit of wattmax achieved). Results were slightly better using ΔiEMG. (Sensitivity: 85.7% & Specificity: 66.7%). Furthermore, at 80% of MVC, the MDF expresses a sensitivity of 71.4% and a specificity of 83.3% while iEMG showed a sensitivity of 57.1% and a specificity of 85.7%. CONCLUSION: Since previous studies from De Luca et al. (1985), Moritani et al. (1998), and Gerdle et al. (2000) have shown that the sEMG spectrum is undoubtedly influenced by metabolites, fibre typing, and the state of fatigue of the muscle, we conclude that these data show a great potential for sEMG as a non invasive tool to identify patients with muscle abnormalities. Sponsored by Bayer. inc.