Randall Parker comments on recent research on the biochemical effects of exercise on aging tissue: "The reversal isn't complete. Aged muscles are still weaker and their gene expression patterns are still different than youthful patterns. We need to know why the aged muscles do not fully regain youthful strength when exercised. ... The resistance training exercise does not simply substitute for a lower level of exercise in the elderly. Younger control subjects who did not exercise much had youthful gene expression profiles even though they didn't exercise much. ... We need a gene therapy delivery mechanism that can deliver replacement mitochondrial DNA (mtDNA) into muscle tissues. Such a therapy would help us answer the question of whether accumulated mtDNA damage is a substantial cause of muscle aging. I hope the answer is 'yes' because methods to replace mtDNA will be much easier to develop than methods to replace nuclear DNA (i.e. the DNA in chromosomes in the nucleus of cells). Why the difference in difficulty? The nucleus contains over 2.9 billion base pairs of DNA whereas the mtDNA contains about 15,000 base pairs. So development of methods to deliver replacement mtDNA should be a relatively simpler task."