Literature has shown that children have lower anaerobic capacity and oxidize more lipids during aerobic activity compared with adults. The purpose of the present study was to examine the effects of age on the activity of marker enzymes for anaerobic and aerobic metabolism in human skeletal muscle from relatively sedentary children and adults. The m. obliquus internus abdominis was analyzed for anaerobic [creatine kinase, adenylate kinase, and lactate dehydrogenase (LDH)] and aerobic (carnitine palmitoyltransferase and 2-oxoglutarate dehydrogenase) enzyme activities in 32 male individuals. The subjects were divided into two groups: children (3-11 y; n=20) and adults (29-54 y; n=12). LDH activity was higher in adults (118.2 +/- 20.1) compared with children (27.8 +/- 10.1) micromol.min(-1).g(-1) wet weight (p <0.0002). Creatine kinase activity was 28% (p <0.0003) lower in children than in adults, and adenylate kinase activity was 20% (p <0.006) lower in children than in adults. In addition, we found higher 2-oxoglutarate dehydrogenase activity in adults compared with children (p <0.04), with no effect of age on carnitine palmitoyltransferase activity (NS). When samples were expressed relative to protein content, only LDH activity remained significantly lower in children compared with adults (p <0.0001). In conclusion, the lower LDH activity observed in children compared with adults may partially explain decreased anaerobic and lactate generation capacity of the children studied. However, the mechanisms for the relatively deficient anaerobic enzyme activities of children are not clear.