Bossi et al. (2019) Optimizing Interval Training Through Power-Output Variation Within the Work Intervals. IJSPP.
TLDR: Varied intensity intervals allowed riders to spend more time above 90% VO2max, without altering the average power of the 6 x 5 minute intervals. This resulted in a greater training stimulus for the same dose of training.
What if you could get more bang for your buck from VO2max sessions without changing the average power, duration or effort of the session?
This is exactly what a group of researchers from the UK and Norway wanted to find out.
Background
Why do cyclists care about VO2max? Why do we try and improve it through training?
VO2max is a key determinant of endurance performance - it sets the ceiling for the capacity of our aerobic system. If an athlete is limited with their VO2max, this can often make it hard to drive improvements in their threshold power (e.g. critical power or FTP).
One of the common training methods to improve VO2max is to design sessions that allow athletes to maximise time spent above 90-95% VO2max. There are lots of ways to go about designing these sessions - for example, you could do repeated blocks of 40-20s or implement longer 4-5 minute intervals. However you go about these sessions, they are tough workouts - your legs are burning your heart and lungs feel like they are working near their capacity.
What did they study?
The researchers took a group of well-trained cyclists who were training ~11 hours per week and had a solid baseline level of aerobic fitness (VO2max = 69 ml/kg/min and FTP was approx. = 3.5-4 W/kg).
They prescribed the sessions based on maximal aerobic power (MAP). This is the power output during the last 60 seconds of a ramp test (starting at 50/100W and increasing the power by 25W each minute until exhaustion). It is typically the power that riders can hold for 4 ± 2 minutes.
They asked the riders to complete 2 different interval sessions and measure a range of physiological variables during each session. Both sessions had 2.5 minutes of easy recovery between intervals.
1. Constant Intervals = 6 x 5 minutes @ 84% MAP
2. Varied Intervals = 6 x 5 minutes @ varied power output - 3 x 30 second bursts @ 100% MAP, followed by 1-1.5 minutes @ 77% MAP.
What did they find?
The varied intervals allowed the riders to spend 68% more time above 90% VO2max (6 min 40 s vs. 4 min 46 s).
The varied intervals lead to a greater minute ventilation - meaning that athletes were breathing harder during the varied intervals compared to the constant intervals.
There was no difference in exertion between the two sessions - riders found both sessions equally as taxing.
Implementing Bossi intervals in training
The results from the study suggest that varying the intensity during high-intensity interval sessions may be an effective method to get more out of each session without altering the average power or effort level of the intervals. You're getting a greater training stimulus for the same dose of training.
One thing to note is that there hasn't been a follow-up study conducted yet to see if these varied intensity intervals translate into better long-term training outcomes (i.e. improvements in VO2max and threshold power). However, the acute responses the researchers have shown in this study suggest that the Bossi intervals are a valuable strategy in the toolbox for endurance athletes and coaches.
To go about implementing the Bossi intervals in your training, this table shows you how to calculate target powers for the different intensities during the 5 minute intervals. The % of 5 min power refers to the average power you would typically complete 6 x 5 minutes. The example uses the average 6 x 5 minute power of 300W.
As with all things in endurance training, context matters. There are endless approaches to improve performance, and in this case, VO2max. For example, across a 4-week training block, you might stand with 4 x 5 minutes @ constant power and progress that to 6 x 5 minutes @ varied power by the end of the block.
Another important consideration with VO2max intervals is managing training stress. These are taxing sessions and need to be implemented strategically, otherwise it's easy to end up overcooking it and putting yourself in a hole. For many, 1-2 x VO2max sessions per week (along with additional lower intensity riding) will be enough to drive improvements.
As always, thanks for reading and feel free to get in touch with questions.