Many debates still exist about which types of cardiovascular exercise are best for fat loss, muscle preservation and general health. High intensity interval training (HIIT) has gained a lot of popularity lately for a number of different reasons, including its convenience to include in a training plan due to the relatively short amount of time that is required to complete these sessions, as well as some other beneficial physiological effects.
The high intensity nature of HIIT creates some adaptations that are associated with improved health, muscle preservation and increases in mitochondrial density; these are our energy producing parts of the body’s cells. In some studies HIIT training has shown to cause muscle growth as the resistance or speed of movements cause a rapid and high energy is going to simulate more closely resistance training. This may also help preserve muscle tissue in a fat loss diet and if included as part of a muscle growth phase assist in the development of muscle tissue whilst still allowing for a cross over with cardiovascular training. This type of training has also shown to improve markers of health such as insulin sensitivity in a similar manner to resistance training.
HIIT training can be employed using not just classical cardiovascular equipment but the use of highbred training such as using battle ropes, sledges and modified strongman type activities which can help develop muscle tissue whist challenging many of the body’s systems that can benefit overall health whilst allowing for a varied training program. So at this point many of you may be thinking why ever bother using low intensity cardio at all?
Well firstly HIIT is very hard work! The fact is many people think that they are doing hit when in reality they are not. True HIIT is pushing your body to the very maximum for periods of typically 20-30 seconds, with enough rest to recover before the next bout. The fact is when HIIT is performed correctly it can take minutes to recover after a high intensity burst. If you are performing HIIT for long bouts, with limited recovery, then the chances are you are not actually performing true HIIT.
Secondly on a fat loss diet HIIT can appear to be more mentally demanding than LISS when energy levels are low, therefore if you’re just looking to increase calorie output LISS maybe more beneficial (30-40 minutes of LISS will typically still burn more calories than 10-15mins HIIT when performed correctly). Performing HIIT regularly in a calorie deficit will put a lot of stress on the body to recover, therefore for some people it may not be appropriate to use every day. HIIT also has the capacity to deplete glycogen stores, which may be fine if you are just using HIIT as your main workout, however for those who want to combine it with resistance training this may have a negative impact on your performance, so careful planning of your sessions (especially leg day!) is required to get the most out of the training you prioritise.
In terms of energy output the high intensity elements mean that in a shorter space of time you effectively burn more calories with HIIT, however with LISS you can sustain it for much longer so most people end up using more calories in this way. HIIT makes up some ground in terms of energy expenditure as it increases excessive post exercise oxygen consumption, this is a process where the body effectively increases its oxygen consumption and energy output to help repair and rebalance the body after the demands of a HIIT style training session.
Unfortunately excessive LISS training has been associated with reductions in metabolic rate which obviously isn’t an ideal situation for those looking to lose body fat or maintain a healthy metabolism. Therefore for those who perform daily LISS we suggest the inclusion of resistance training or combining it with HIIT to help negate some of these potentially negative effects.
Ultimately the kind of ‘cardio’ you do should be chosen by your goals, energy levels, the amount of time you have and most importantly what you enjoy! For most people looking to lose body fat a combination of HIIT for the muscle preservation and metabolic effects combined with LISS to increase overall calorie output and allow adequate recovery between HIIT sessions is a best of both worlds approach to exercise that is sustainable, varied and effective.
Dr Paul Rimmer
Nexus Fitness content.