Endurance athletes: fine tuning your engine for performance

If you compete in events lasting more than 10 seconds, maximizing endurance performance is likely of interest to you. But even if you’re a sprinter, thrower or jumper, where there are minimal demands on endurance, health can and should be a consideration as well. That’s why cardiovascular fitness is so important; it plays a huge role in ensuring optimal health as well as performance.

The best training adaptation

While the importance of endurance training is evident for all athletes regardless of sport, - what’s less obvious is exactly what training should be done, and what sort of improvements can be expected. As more and more research has been published, it’s become clear that the adaptation to exercise training is not universal. What’s also clear is that while the impact of different approaches is becoming better understood, it’s less clear what training formula athletes should use for the best training outcomes.

There are many different factors that influence performance adaptations. For example, does the intensity of training make a difference, and if so, when and to what degree? What about the duration of the training intervention? What about the frequency of training sessions? These are decisions that athletes have to make, and without clear answers, it’s just a guessing game.

Moreover, some of those factors aren’t necessarily related to the training stimulus. Instead, they relate to the individual performing the training. Does age matter? Does sex matter? Does training status and initial level of fitness matter? What about health status and the presence of chronic disease? Athletes bring all sorts of contextual factors to their training and these factors can have a very real impact.

These are critical questions because they can greatly influence what training decisions are made. The more informed those decisions are, the better they’ll be. And while there are countless studies on endurance training, most of us don’t have the time to sift through all the available evidence to come up with solid conclusions. Fortunately, a group of researchers decided to do just that.

New research

Considering the huge volume of research that exists on endurance training, it can be difficult to get a well-defined and comprehensive understanding of what the evidence suggests. With so much evidence, some of it is bound to be ambiguous or even contradictory. However, that doesn’t mean that there is no clear signal amidst the noise. To help clarify the impact of various contextual factors on improvements in mitochondrial content and capillarization, three Scandinavian researchers performed a systematic review and meta-regression on the available endurance training research⁽¹⁾.

In particular, they focused on the way the training affected mitochondrial adaptations. Mitochondria can basically be considered as the ‘energy factories’ of cells, with increased densities of mitochondria being associated with positive changes in endurance performance and health. In addition, they also looked at key enzymes involved in aerobic energy metabolism (ie the energy pathways that fuel endurance) to see what impact various modes of training had on these enzymes. They also investigated the protein content of one enzyme in particular (cytochrome c oxidase), which can be used as a marker of increased gene expression for cytochrome c oxidase production – ie whether the training was effectively switching on ‘endurance genes’.

What they did

After sifting carefully through all the previously published articles related to this topic, they found 353 studies containing 5,650 subjects relevant to mitochondrial adaptations, and 131 studies with 1897 subjects relevant to capillarization (improved blood flow through muscles). All these studies were investigating the impact of endurance training, high-intensity interval training (HIIT), continuous training, or sprint interval training.

In terms of mitochondrial adaptations, the researchers looked at changes in maximum oxygen uptake (VO2max), mitochondrial volume density, and the content and activity of various mitochondrial (aerobic) enzymes. For those interested, the following enzymes were analyzed- citrate synthase (CS), cytochrome c oxidase (COX), hydroxyacyl-coenzyme A dehydrogenase (HADH), succinate dehydrogenase (SDH), oxoglutarate dehydrogenase, malate dehydrogenase, and protein content of COX subunit 1, 2, and 4. For capillarization adaptations, the researchers were interested in three main outcomes. They looked at the cross-sectional area of muscle fibers, the capillary density (how many capillaries within a given volume), and the number of capillaries per muscle fiber.

While the researchers primarily focused on mitochondrial and capillarization adaptations, there were multiple factors the researchers also analyzed to determine how each factor impacted the magnitude of adaptation to exercise training. To determine the influence of different types of endurance training, the researchers looked at differences between low-intensity endurance training, high-intensity interval or endurance training, and sprint-interval training. They also examined the effect of the duration of a training intervention, comparing shorter and longer training programs. Finally, they looked at the impact of the frequency of training on adaptation.

From a population standpoint, multiple groups were compared.  The researchers looked at differences between subjects who were less than 35 years old and subjects who were older than 55 years old. They also compared how the degree of adaptation changes as the subjects initial level of fitness differed, comparing unfit subjects to progressively more fit subjects. As some research has demonstrated sex-related differences in outcomes, these comparisons were made as well. Lastly, healthy subjects were compared to those who had chronic health conditions or diseases.