HIIT For Endurance Athletes?

High intensity interval training (HIIT) has become increasingly popular in recent times and has given rise to fitness endeavours in the likes of CrossFit and F45. With the increase in people partaking in HIIT, also comes an increase in the research around the effectiveness of this form of exercise to increase endurance, power, and speed from beginner to elite level.

HIIT is relevant to everyone, not just the elite athletes but also the average recreational participant as it adds huge health benefits. HIIT has been suggested to be the secret ingredient to increasing performance potential. HIIT consists of repeated intervals of training at intensities above lactate threshold alternated with short low intensity rest periods. Basically, stressing your overall system (musculoskeletal (bone, muscle, and connective tissue), energy reserves, mental capacity, and autonomic nervous system) to create a response in hormonal, cellular and biochemical pathways in various organ systems in your body. HIIT often looks like cross training within a training programme and can be used to trick the system, allowing some systems or elements to rest while training other systems. This can help avoid over training or burnout which is the result of inadequate recovery. However, if applied poorly it can lead to burnout, loss of training gains, injury, and illness, and therefore needs to be balanced and individualised.

Within our muscles the contractile component that does all the work is made up of a combination of fast and slow twitch fibres, and the combination of these is dependent on our individual physiology and the anatomy and function of the muscle in question. For example, our postural or stability muscles have a larger proportion of slow twitch fibres compared to our quads or hamstrings which are more geared towards movement and power.

Slow twitch fibres are more fatigue resistant, use more fat stores for fuel and are typically for slower and longer periods of work at lower power intensity. In contrast, the fast twitch fibres have a lower resistance to fatigue, rely on carbohydrate stores for fuel and are found mostly in our big power muscles. HIIT, by nature of the exercise, targets the fast twitch fibres. The physiological response we are wanting to achieve with HIIT is increasing the mitochondria cells to become more resistance to fatigue, allowing their output to be sustained at higher intensities for longer. Typically holding the intensity after 30 sec -5 mins becomes unsustainable and a decrease in power output is seen as this training targets the fast twitch fibres which are more fatigable than the slow twitch fibres and trigger the brain to rest. By alternating the high intensity with low intensity, we can repeat these intensities for a long period of time to elicit the physiological changes we are after.

There are two main advantages to HIIT, the first being this cellular adaptation which increases the number of mitochondria in the muscle cells and therefore increases the fibres resistant to fatigue. And secondly, you increase the efficiency of the heart and cardiac output and therefore the ability to oxygenate the blood and get blood to the working muscles. The heart will adapt differently to HIIT compared with long slow distance training. With HIIT you get a larger return of blood to the heart, creating a larger stretch within the ventricles or chambers of the heart. This allows a larger recoil, resulting in a stronger muscle. Overall, this increases the stroke volume of the heart (amount of blood pumped in every beat) and the cardiac output (volume of blood pumped per minute). In turn this leads to an increase in V02 (total oxygen pumped around the body). The combination of the increased mitochondria and V02 allows for maximal oxygen uptake.

 There is a vast array of combinations to implement HIIT and while the opportunity to get all the miles under your belt for upcoming events or goals is limited, mixing it up with HIIT might surprise you with the train smarter not harder approach. The great thing about HIIT is that the benefits can be applied to any sport, as fast twitch fibres and your cardiovascular system are always needed. Depending on the sport, this will influence which variables or activities you might opt for, or the work to rest ratios you choose.

 3 targets for HIIT to optimise performance through eliciting a different physiological response are the aerobic zone or oxidative zone or the cardiovascular systems ability to pump oxygen to working muscles for aeriobic energy and to cause fatigue resistance, anaerobic or glycolytic, blood lactate, sugar system and the neuromuscular and musculoskeletal system which will translate as muscle soreness or recruitment you get within a session. You can target all of these or one of these to emphasise the training effect you are after. If you target one, it gives other systems a chance to recover allowing you to keep up the positive stress on the system and not overload or damage other systems. Basically, beat the system!

 So where you are rehabbing injury, we can not only prescribe exercises to allow injury recovery, but also help adjust your programme for the desired training effect while resting the neuromuscular and musculoskeletal system where indicated. For example, a bike session for a runner will allow the cardiovascular system to receive positive stress but rest the musculoskeletal and neuromuscular system as the muscle contractions required are different to that of running.

 There is skill in the approach to implement the specifics of a HIIT session to get the desired physiological responses. Being in tune with your body and how it recovers is a pretty good place to start to monitor if you are overdoing it. But it’s important to balance perceived effort with any other variables you use to track load response such as training output (pace, speed, power, distance etc.), HR at different intensities, or HRV if that is something you are used to using. Things to monitor are sleep, nutrition, stress and performance. We can also test to see objectively where your energy systems are operating, to prescribe these sessions with certainty. If you want to learn more about this head over to our physiological testing page.

At different training loads you will get different physiological responses so there is no one size fits all approach. Chances are if you gravitate to one type because its easiest, it is where you are most adapted, so mix it up a bit to widen your capacity and potential. But remember new stress can take longer to recover from so listen to your body and get in touch if you need a hand. 

Sarah Pelham is one of our talented Sports Lab physiotherapists. She’s the kind of person that you want on your pub quiz team; she has a wealth of knowledge and a broad range of skill sets.