Just to add my two cents worth - first, there are scant longitudinal data concerning fiber type proportions across the lifespan - most of the data is cross-sectional. This means one cannot distinguish between a prolonged effect of exercise on fiber type and simply biological predisposition to endurance sport being conferred for those having a higher proportion of oxidative muscle fibers (i.e., high performing endurance athletes may have always had low type IIx proportion). Second, it is well known that type IIx fibers decline when a sedentary person engages in exercise training, and this is also true of resistance training (Reviewed by Andersen and Aagard, Scand J Med Sci Sports 2010), so even if endurance trained individuals lose type IIx fibers, this effect is not distinct from resistance training. Third, motoneurons contain boatloads of mitochondria, so your point about mitochondria is overlooking a key source of where mitochondrial dysfunction is likely to have the most impact on aging skeletal muscle: the motoneuron and perhaps also the perisynaptic Schwann cells that envelop the motoneuron terminals at the neuromuscular junction. In this respect, there is very convincing data showing that mitochondria in the motoneuron nerve terminals degenerate with aging (Garcia et al. J Neurol Sci. 2013), likely because the rate of their replacement is not keeping up with the accumulation of damage (but, at this point we do not know where those mitochondria come from - the transit time of a mitochondrion made in the motoneuron cell body where the nucleus resides is far longer than the half life of a mitochondrion, so this seems an unlikely way to replenish mitochondria in the motoneuron terminals - perhaps they are donated by the perisynaptic Schwann cells?). Do I think we have proven at this point that mitochondria are the most important cause of muscle degeneration with aging? No, and nor do I think we have a clear sense of the hierarchy of impact between muscle fiber mitochondria and mitochondria in the perisynaptic Schwann cells and motoneurons, but your counterarguments are built upon several misunderstandings of the literature. We have a lot of work to do so keep an open mind 
1 Like