Diagnostics of mitochondrial disease requires a combination of clinical evaluations and biochemical characterization. However, the large normal variation in mitochondrial complex activity limits the precision of biochemical diagnostics. Thus, identifying factors that contribute to such variations could enhance diagnostic accuracy. In comparison, inbred mice demonstrate much less variations in brain mitochondrial activity, but a clear reduction with age. Interestingly, pretreatment of mouse brain mitochondria with the detergent dodecyl maltoside abolishes the reduction. We therefore postulated that DDM pretreatment could be valuable tool for distinguishing between variations caused by posttranslational modifications and those caused by genetic heterogeneity. In this study, we evaluated the effects of age, DDM sensitivity, oxidative damage and single nucleotide polymorphism on biochemical complex activity and the proteome of human muscle mitochondria, which serve as reference standards for mitochondrial diagnostics. Our results indicate that mtDNA variants are the primary contributors to the diversity in biochemical activity in human muscle mitochondria from healthy individuals.