|Have you been running for nothin'?|
I do yet have a good reason to use the conditional, here. In view of the estrogen-related sexual differences in the adaptational response to exercise the fact that the Canadian researchers had 8 guys in their HIIT group and 5 men and 4 women in their work-matched steady group could be a serous design-flaw - a flaw which renders the comparison of 4x30s wingate all out cycling tests w/ 4 min break or a workmatched (see Figure 1) 4 min steady state exercise trial.
|Figure 1: Comparison of total workload, peak power output, mean power output, work duration and ratings of perceived exerction between the intermittent and steady state acute trials (Cochran. 2014)|
Acute vs. long-term effects - a question of total volume?
It is this comparison of the long-term effects which eventually yielded different results for the work-matched intermittent and steady-state regimen. For the acute training, on the other hand, there were no significant differences in any of the measured parameters (see Figure 2)
|Figure 2: When it's done at high intensities and with identical workloads intermittent and continuous exercise produce identical acute effects (Cochran. 2014)|
In the "long run", in this case 6 weeks, Cochran et al. did yet observe differences not between their HIIT and steady state protocols, but between their current and previous results by Burgomaster et al. (2008), where a similar protocol did yield the changes in mitochondrial enzyme markers that were absent in the study at hand.
As Cochran et al. correctly point out, a 1:1 comparison of their data to the findings of Burgomaster is not exactly scientifically valid. There was after all no direct direct comparison between the CONT and INT protocols in the study at hand. Still, a recent review by Bishop et al. (2013) suggested that training volume is more important for increasing mitochondrial content than training intensity and the volume in the previously cited study by Burgomaster was higher - 50% higher (6 vs. 4 wingates).
The volume difference alone may thus explain the missing changes in citric acid synthase activity and the disappointing absence of increases in GLUT4, MCT1 and MCT4, which would signal an increased propensity to use both glucose and fat as a substrate.
- Burgomaster, Kirsten A., et al. "Similar metabolic adaptations during exercise after low volume sprint interval and traditional endurance training in humans." The Journal of physiology 586.1 (2008): 151-160.
- Cochran, Andrew JR, et al. "Intermittent and continuous high‐intensity exercise training induce similar acute but different chronic muscle adaptations." Experimental physiology (2014).
- Ma, J. K., Scribbans, T. D., Edgett, B. A., Boyd, J. C., Simpson, C. A., Little, J. P., & Gurd, B. J. (2013). Extremely low-volume, high-intensity interval training improves exercise capacity and increases mitochondrial protein content in human skeletal muscle. Open Journal of Molecular and Integrative Physiology, 3, 202.