Paper Review: Should You Add Sprints To An Endurance Ride?
The Study:
Quick Synopsis
Recently, we received an email about the paper above, looking to get our thoughts on the inclusion of sprints within a longer endurance ride.
We’ve typically advocated keeping power well-controlled when completing endurance rides, aiming to keep your power below your ‘first ventilatory threshold’ (VT1), or typically between 55-75% FTP or 60-70% Max HR.
The rationale for this is that VT1 is an intensity where fat oxidation is maximised. When you step beyond your VT1, you will start to use more carbohydrates for fuel, which lifts the rate of lactate production, and in turn suppresses fat oxidation.
As developing your capacity to oxidise fats is often a key goal of endurance rides, it makes sense to keep your power at an intensity that maximises use of this system and avoids unnecessary lactate production, which may suppress this system.
However the study linked above, which is by Bent Rønnestad and colleagues, looked at whether inclusion of sprints within a long endurance ride can enhance the adaptations you’d usually expect from such a ride.
Let’s take a look at what they found and whether their results seem to contradict our general guidance to keep power consistently low if looking to improve fat oxidation.
Methods
This study involved 12 male elite-level cyclists, training 13 ± 8 hours/week with VO2max in the region of 73.4 ± 4.0ml/kg/min.
After various baseline tests and familiarisation activities, participants completed two 4H indoor rides, on separate occasions.
During one 4H ride, the participants kept their power at 50% VO2max (i.e. a Zone 2 intensity) for the entire 4H ride.
During the other 4H ride, participants included a block of 3x 30S sprints, with 4M recovery every hour over the first 3H of the ride. These sprints were very close to maximal efforts (typically 91-93% of each athlete’s 30S maximal power). Between the sprints power was held at ~50% VO2max (although power output was very slightly adjusted so that in both trials, the participants completed exactly the same amount of work overall).
The researchers looked at various markers within the muscles and blood of the participants at 20-mins, 3-hours and 24-hours after the ride. These markers tell the researchers about the likely adaptations that may result from the training. The researchers also looked for signs of fatigue in the athletes, using the ‘isokinetic leg extension’ test, which measures the amount of torque the leg can produce at different speeds.
The study was very carefully controlled so that other factors such as food, supplement intake, and time of day would not interfere with the results.
Key Findings
Overall, the researchers found some indication that adding sprints to an endurance ride may enhance the aerobic adaptations from such a ride.
In particular, the endurance + sprint session resulted in higher levels of mRNA molecules known to have a role in improving fat oxidation and developing muscle capillaries.
As a key example, the endurance-only session resulted in a 2.8-fold increase in PDK4 20-minutes after the session (a molecule responsible for improving fat oxidation capacity). However, the endurance + sprint session resulted in a whopping 11.9-fold increase in PDK4 at the same time point!
The researchers also found that although the sprint training resulted in greater muscle fatigue shortly after the session, 24-hours later, there was no measurable muscle fatigue remaining.
In spite of the above, not all the results were positive. For example, the sprint session resulted in a suppressed response of a very key mRNA molecule (PGC-1α), which is responsible for increasing mitochondrial density. The sprint training also appeared to suppress the the levels of mRNA molecules linked with ion transport.
Strengths & Limitations
One of the major limitations of this study is that it only looks at markers in the blood and muscle in order to infer likely adaptations, but does not measure actual adaptations. In a follow-on study, it was shown that including sprints in 5 endurance rides over a 14-day training camp results in improved 30-second and 5-min maximal power vs performing exclusively low-intensity riding. The researchers also found the sprint training helped to maintain the levels of proteins that play a role in muscle contraction. However, in this study, there were no discernible effects on key markers of aerobic fitness such as VO2max or efficiency, so it’s possible that these improvements were a result of improved anaerobic capacity, rather than improvements in aerobic fitness.
The timing at which the blood and muscle samples were taken may have hidden some of the benefits of the sprint and/or endurance training. Indeed the authors of the paper argue that the timing at which the samples were taken might be one of the reasons they didn’t see signs of increased PGC-1α (a signal for improved mitochondrial density) in response to the sprint training.
In addition, this study was performed among elite athletes, and it’s not clear whether and to what extent lower-level athletes would respond. In particular, elite athletes recover quicker than the average cyclist, so including sprints within an endurance ride may be notably more fatiguing for less-well trained athletes and the additional recovery required may negate the training benefits.
Of course, this study is also in men only, and we know that women can respond quite differently to training interventions, particularly in relation to fat/carbohydrate combustion, so it’s not clear whether we’d see the same patterns in females.
Key Take-Aways
1. Adding sprints to endurance rides may have some benefits and doesn’t seem to cause notable fatigue in elite or at least well trained cyclists.
This suggests sprint training may be helpful to include within your training, particularly during times where other training is mostly low-intensity, or when you’re already training as much as you have time for and are looking for a strategy to increase your training stimulus.
Less well-trained cyclists should take care with sprint training to avoid accumulating too much fatigue which may impact subsequent, and more important sessions.
2. It seems that adding sprints to endurance rides may not suppress the stimulus for improved fat oxidation, and in fact may enhance this training effect.
So does the general guidance that you should control your power during endurance rides no longer hold true?
It’s not entirely clear. However what is clear is that this study can’t be used to support riding in a wholly unstructured manner, including sprints or power surges whenever you want to.
Crucially, in the study, the sprints are arranged in blocks with considerable periods of well-controlled low-intensity riding between. This may be an important feature when it comes to developing fat oxidation ability so as to avoid lactate levels being elevated throughout the majority of the ride (which acts to suppress fat oxidation).
So the bottom line is that, if you’re looking to improve your fat oxidation ability, then blocks of sprints within an otherwise well controlled endurance ride might enhance this training effect. But sporadic sprints or surges throughout a ride without periods of well-controlled riding is still probably best avoided.
