REF-1532 · Pilegaard, Saltin & Neufer, 2003 · The Journal of Physiology · Paper · PubMed · Free full text

Does one session do anything to the mitochondria?

In seven men, the PGC-1alpha gene went 10- to over 40-fold more active within 2 h of a 3-hour knee-extensor session, then settled. Gene signal only, no performance.

Main caveat: Read from the abstract only. The full text could not be opened.
thin evidenceMeasured on a mixed group

Seven men trained one leg by knee extension for four weeks. At the end of that block they did 3 h of two-legged knee-extensor exercise, so both the trained and the untrained leg worked in the same session, and biopsies came from the vastus lateralis of both legs at rest and then at 0, 2, 6 and 24 h into recovery.

PGC-1alpha transcription rose 10- to more than 40-fold and PGC-1alpha mRNA 7- to 10-fold (P < 0.05), with the peak inside the first 2 h of recovery; the authors describe the increase as transient. The rise was greater in the trained leg, which was working at a lower share of its own capacity.

Nuclear respiratory factor 1 (NRF-1) mRNA, a gene that PGC-1alpha drives in cell culture, did not change. Hexokinase II, mitochondrial transcription factor A, PPAR-alpha and calcineurin A-alpha and A-beta mRNA were about 2- to 6-fold higher at 6 h of recovery in the untrained leg and did not change in the trained leg.

Before the session the trained leg had the longer time to exhaustion in a test the abstract describes as 2 min of maximum resistance, and more hexokinase II, citrate synthase and 3-hydroxyacyl-CoA dehydrogenase mRNA.

Our reading: this is one of the first human demonstrations that a session switches on the master regulator of mitochondrial growth for a few hours, which is why the base cites it for the principle and not for any practice.

Protocol (from the abstract; the full text could not be opened)

Who: n = 7 men. Nothing further is given in the abstract — no age, body mass, VO2max or training history.
Training block: 4 weeks of one-legged knee extensor exercise training. The abstract gives no frequency, duration, intensity or progression, and does not say how the trained leg was chosen.
Test session: 3 h of two-legged knee extensor exercise, performed at the end of the training block. Intensity and work rate are not in the abstract. Because the same absolute load was shared, the trained leg worked at a lower relative workload, which the authors state.
Biopsies: vastus lateralis of both legs — one at rest, then at 0 h, 2 h, 6 h and 24 h into recovery.
Measures named: PGC-1alpha transcription and mRNA; NRF-1, hexokinase II, mitochondrial transcription factor A, PPAR-alpha, calcineurin A-alpha and A-beta mRNA; citrate synthase and 3-hydroxyacyl-CoA dehydrogenase mRNA; time to exhaustion before exercise. Statistics: P < 0.05; no method, no sample-size calculation and no effect sizes in the abstract.
Funding and interests: the record lists support from the US National Institute of Arthritis and Musculoskeletal and Skin Diseases (grants R01 AR045372 and AR-45372) and is also indexed as non-US government support; the work came from the Copenhagen Muscle Research Centre and the August Krogh Institute. A conflict-of-interest statement could not be read.

Key Results

MeasureWhat was found
PGC-1alpha transcription10- to more than 40-fold higher after exercise (P < 0.05), peak within 2 h
PGC-1alpha mRNA7- to 10-fold higher (P < 0.05), peak within 2 h
Trained vs untrained legPGC-1alpha activation greater in the trained leg, at a lower relative workload
NRF-1 mRNAno effect of exercise
HKII, mitochondrial transcription factor A, PPAR-alpha, calcineurin A-alpha and A-beta mRNAabout 2- to 6-fold higher at 6 h in the untrained leg (P < 0.05); no change in the trained leg
Before exercise, trained vs untrained leghigher time to exhaustion (2 min maximum resistance) and higher HKII, citrate synthase and 3-hydroxyacyl-CoA dehydrogenase mRNA
Transient, but the numbers are not shown. The abstract calls the increase marked and transient and names 0, 2, 6 and 24 h as the recovery biopsies, so the time course exists in the paper; the values at 6 and 24 h are not in the abstract, and how far transcription had fallen by the next day cannot be read here.
Ranges, not means. "10- to >40-fold" and "7- to 10-fold" are ranges across time points or subjects; the abstract does not say which, and gives no spread for any of them.
The leg difference is a comparison, not an experiment (inference). Legs differ in training state by design, and both did the same session, so a larger response in the trained leg is an association within each man; no randomised allocation of legs is reported in the abstract.

Mechanisms

Measured: a short-lived rise in the transcription and mRNA of PGC-1alpha after one long session, larger in the trained leg; no change in NRF-1 mRNA; a rise in several metabolic genes at 6 h in the untrained leg only.
Authors' interpretation: PGC-1alpha may coordinate the switching-on of metabolic genes in human muscle after exercise, consistent with its role as a transcriptional coactivator. They call the NRF-1 result interesting, since cell-culture work had PGC-1alpha driving it.
Inference: the authors' own word for the rise is transient, which is why the base treats each session as a pulse rather than a state (PHYS-18, where the measured return to baseline within 24 h comes from another biopsy study).This paper measures the pulse, not the adaptation: it has no mitochondrial content, no enzyme activity after the session and no performance, so the step from "the gene was switched on" to "the mitochondria grew" comes from other work (REF-465, REF-457, REF-463).
Inference: the PGC-1alpha response was not blunted in the leg with four weeks of training behind it, while the other metabolic genes rose in the untrained leg only, which argues against the idea that a trained muscle stops answering. One trained state, four weeks, one session type: nothing here tells an athlete how much of the signal survives after years of training.

Practical Application

Does one session do anything to the mitochondria? In seven men, the PGC-1alpha gene went 10- to over 40-fold more active within 2 h of a 3-hour knee-extensor session, then settled. Gene signal only, no performance.
Good for: explaining why consistency matters more than any single session. The response is a pulse that peaks within about two hours and passes, so the next session, not the last one, is what builds the adaptation (PHYS-18).
Good for: answering an athlete who fears that training stops working. Four weeks of training did not blunt this response; it was larger in the trained leg, at a lower relative load.
Not good for: any dose. There is no comparison of intensity, duration or session type here, so it cannot support zone 2 over intervals, or the reverse, and it says nothing about weekly hours (SYNTH-26 carries the base's practice on that).
Not good for: anything about sleep, the night after a session, fuelling or the gap between hard sessions. None of these was studied; a 3-hour daytime session and four recovery biopsies is the whole design.

Limitations

Read from the abstract only. The full text could not be opened. The training block, the work rate in the 3-hour session, the statistics, the 6-h and 24-h values, the subjects' age and training status, and the conflict-of-interest statement are all unknown here.
n = 7 men. No women, and the abstract reports no spread for any result, so how consistent the response was between men cannot be judged.
A knee-extensor model, not endurance training. Three hours of two-legged knee extension loads a small muscle mass with no whole-body cardiovascular or thermal strain; a long ride or run is a different stimulus. Nothing in this design transfers to a session an athlete would recognise.
Training status unknown, athletes not studied. The "trained" leg is four weeks of single-joint work, not an athlete's training. Whether a highly trained athlete's muscle answers the same way was not tested here.
mRNA is not adaptation. Transcription and mRNA were the outcomes. Mitochondrial protein, mitochondrial content, enzyme activity after the session, VO2max and performance were not measured, and gene responses do not always track the adaptation that follows.
The leg comparison is not randomised. The abstract does not report how the trained leg was chosen, and the two legs differ in more than their training state (relative load in the test session, baseline mRNA).
One session, one time course. No second visit, no dose comparison, no control condition without exercise, so the rise is attributed to the session by the within-subject time course alone.
How this entry was checked and changed
Why this level

a mechanistic biopsy study whose outcomes are gene transcription and mRNA with no performance outcome after the session (rubric C+); read from the abstract, because the full text could not be opened. By the rubric's rule for foundational work it keeps its own level, and any synthesis pooling the later replications carries the higher one