Resistance exercise enhances the molecular signaling of mitochondrial biogenesis induced by endurance exercise in human skeletal muscle.
http://www.ncbi.nlm.nih.gov/pubmed/21836044?dopt=Abstract

Abstract

Combining endurance and strength training (concurrent training) may change the adaptation compared with single mode training. However, the site of interaction and the mechanisms are unclear. We have investigated the hypothesis that molecular signaling of mitochondrial biogenesis after endurance exercise is impaired by resistance exercise. Ten healthy subjects performed either only endurance exercise (E: 1h cycling at ~65% of VO(2max)) or endurance exercise followed by resistance exercise (ER: 1h cycling + 6 sets of leg press at 70-80% of 1 repetition maximum) in a randomized cross-over design. Muscle biopsies were obtained before and after exercise (1 and 3h Post cycling). The mRNA of genes related to mitochondrial biogenesis (PGC-1α, PRC) and substrate regulation (PDK4) increased after both E and ER, but the mRNA levels were about 2-fold higher after ER (P<0.01). Phosphorylation of proteins involved in the signaling cascade of protein synthesis (mTOR, S6K1 and eEF2) was altered after ER but not after E. Moreover, ER induced a larger increase in mRNA of genes associated with positive mTOR signaling (cMyc and Rheb). Phosphorylation of AMPK, ACC and Akt increased similarly at 1h Post (P<0.01) after both types of exercise. Contrary to our hypothesis, the results demonstrate that resistance exercise, performed after endurance exercise, amplifies the adaptive signaling response of mitochondrial biogenesis compared with single-mode endurance exercise. The mechanism may relate to a crosstalk between signaling pathways mediated by mTOR. The results suggest that concurrent training may be beneficial for the adaptation of muscle oxidative capacity.

Just read the abstract, quite interesting. I’m unaware of any long term studies examining the effect of combined training on mitochondrial density or function, though.

I think Dr. Stuart Phillip had a study which showed that in beginners resistance training tend to induce endurance adaptations too. But it slowly wears off. He kid odf mentions it in the podcast that near;lifter posted.

Yeah, RT helps VO2max in beginners, but I’ve never seen anything about mitochondrial function.

I meant mitochondrial proteins. And a big part of the increase of VO2 max in begiiners is increase in peripheral adaptation (increase in mitochondrial density). This is from the Phillips review:

Recent data suggest that the MPS response to resistance
exercise becomes more “refined” with training such that available
substrates are preferentially directed toward the synthesis
of different protein fractions depending on the exercise stimulus
(48, 123). For example, our laboratory recently demonstrated
that in untrained individuals the performance of an
acute bout of resistance exercise, and to a lesser extent aerobic
exercise, stimulates global protein synthesis (48, 123). These
data suggest that a novel stimulus disrupts homeostasis such
that the signal to the protein synthetic machinery is unrefined
and turns on all muscle proteins (myofibrillar and mitochondrial).
Following training, however, the acute synthetic response
becomes more finely tuned to the specific mode of
exercise in that resistance exercise preferentially stimulates the
synthesis of myofibrillar proteins whereas aerobic exercise
stimulates the synthesis of the mitochondrial fraction (48, 123).
How this is accomplished is uncertain since changes in phosphorylation
of many commonly studied signaling proteins did
not shed insight into mechanistic underpinning of the response

That’s interesting, but it’s still just acute data. We need long term studies to examine if mitochondrial respiration actually improves from RT. If it does, I would guess it would be a beginner thing, as I doubt RT would be a big enough stimulus in endurance athletes.