Here at SYSTM (previously Sufferfest), we do love intervals, suffering, and working so hard that we leave a puddle of sweat and pain underneath our turbo trainers after a good, high-intensity session. And, of course, these high-intensity sessions are vitally important to our training: improving fitness, getting faster and just generally feeling badass when we get them completed. But there is still an important place in our training for low-intensity and longer durations so let’s dive into why this is the case…
We may not have the time available to train like the pros, who will be doing 25+ hours of training a week. But, interestingly enough, we probably do a similar amount of intensity each week as they do at times. So, what do they do differently? The answer is: a lot of long, slow duration training - which is used to facilitate more potential gains from the high intensity work that they do.
We can improve our aerobic power via a few different methods but many of them come down to something called PGC-1α (Peroxisome proliferator-activated receptor gamma coactivator 1-alpha). One of the primary things that PGC-1α does is to increase mitochondrial biogenesis, therefore increasing the number of cells within our muscles that oxidise fat, carbohydrates and lactate to produce adenosine triphosphate (ATP) - which is what our body uses to energise all muscular processes . Another benefit of increased PGC-1α expression is that insulin-stimulated glucose transport is increased (also called increased insulin sensitivity) via GLUT4 (Glucose Transporter Type 4). Essentially, this means that we can deal with carbohydrate loads better without the associated blood sugar highs and lows, as blood sugars remain more stable.
The first method of activating PGC-1α that we’ll discuss occurs due to high-intensity exercise and is achieved via the AMP-K (adenosine monophosphate-activated protein kinase) enzyme. The AMP-K facilitated PGC-1α signalling occurs during high-intensity due to the ratio of AMP:ATP increasing. This is because ATP is being used at a rate faster than it can be generated, such as during very high-intensity exercise that is not sustainable. So, those hard interval sessions in SYSTM will be contributing towards improving your aerobic function.
However, long duration low-intensity training sessions can also cause PGC-1α signalling to occur in a different way. When we contract our muscles, calcium is secreted into the cytoplasm from the sarcoplasmic reticulum. This in turn activates calcium/calmodulin-dependent protein kinase (CAMKII), which is itself a signalling molecule for p38 mitogen activated protein kinase (p38 MAPK). p38 MAPK not only increases PGC-1α after the initial uptake of exercise, but also increases the expression of PGC-1α as exercise continues, thus improving mitochondrial biogenesis, oxidative capacity and aerobic performance. Due to the continuous nature of this signalling pathway, long duration exercise is more beneficial than shorter in this respect. The only way to conduct this exercise sustainably is to perform it at lower intensities.
Another benefit of long duration exercise is something called angiogenesis, or muscle capillarisation, which is the creation of new capillaries within the muscle, allowing more oxygen to be transported around the muscle and therefore improving oxidative capacity. This can also be facilitated via PGC-1α signalling to a lesser degree. However, this driver for angiogensis is vascular endothelial growth factor (VEGF), which is secreted from the muscle fibres to the muscle interstitium. Additionally, studies have found that, as intensity increases, VEGF activation decreases and angiogenesis is hampered, which is why low-intensity exercise is so important for angiogenesis. These benefits are theorised to occur more in the long duration exercise due to, again, the continued muscle contractions over far longer time durations than for, say, 60 minutes of high-intensity training.
So, what is low-intensity training? Traditionally, it is referred to as Zone 1 or Zone 2 exercise, something that is easily maintainable over long durations while being able to maintain conversations. Physiologically, it is the moderate exercise intensity domain which refers to exercise that is below LT (lactate threshold), where the primary fuel oxidised is fatty acids and lactate production does not increase from base levels. A general rule of thumb for this is exercise performed at less than 70% of Functional Threshold Power, but LT for individuals varies in terms of their percentage of FTP and can also change based on environmental factors such as heat and even fatigue. Additionally, sub-LT is often an intensity that maintains the Heart Rate to Watts ratio without cardiac drift occurring. Cardiac drift, also referred to as aerobic-decoupling, is when HR increases compared to power in a non-linear fashion. So, either power stays the same and HR increases, power drops and HR remains static, or power increases but HR increases at a rate that is exponentially higher than would be expected.
What does this mean for everyone reading this? Fortunately, you don’t need to be doing 4+ hour rides 5 times a week to reap the benefits of long duration low-intensity training. Getting one or two longer rides at a low-intensity in at the weekend will help you significantly with your aerobic fitness, as well as providing a good base to build upon using high-intensity sessions. So, although suffering and doing sessions to make the Pain Cave earn its name are important, don’t forget the importance of those long slow endurance rides when you can fit them in.
For further reading on PGC-1α, have a look at this research paper: Jung, S., & Kim, K. (2014). Exercise-induced PGC-1α transcriptional factors in skeletal muscle. Integrative medicine research , 3 (4), 155–160.
For more info on angiogenesis, this is a great read: Gliemann, L. Training for skeletal muscle capillarization: a Janus-faced role of exercise intensity? Eur J Appl Physiol 116 , 1443–1444 (2016).