For the goal of dropping body fat and retaining muscle mass, doing cardio is obviously an important factor.where do you stand on the controversial issue of whether HIIT or LISS should be done for the above purpose?
High intensity interval training (HIIT) has gained great interest in recent years, due to the scientific community highlighting it as an effective and time-efficient exercise strategy . This is due to similar metabolic adaptations (e.g. carbohydrate and lipid oxidation), accompanied by enhanced maximal oxygen uptake and mitochondrial biogenesis being observed through shorter durations of exercise when compared to regular endurance exercise [2-4]. Mitochondrial biogenesis is the process by which new mitochondria are formed in the cell, with the mitochondria providing a major source for energy production (ATP) and regulating cellular metabolism. Subsequently an increased capacity for lipid oxidation (breakdown) by the mitochondria has been observed with HIIT and endurance training [5, 6].
Common HIIT protocols implemented utilize a number of “all out” sprints (4-7 x 30 secs) with varied rest intervals (1-2mins), performed 3 days per week . Thus the total length of a session is ~ 15-20minutes max. Of particular interest, is the positive increase in the capacity for fax oxidation which has been observed with HIIT, even after only a 2 week intervention [7, 8]. Indeed, it is the alterations in lipid metabolism and fat oxidation, which makes it an attractive strategy to reduce body fat.
These positive benefits of HIIT exercise occur with a reduced total work time required by the individual compared to endurance training [3, 9]. It is important to note the studies reporting reductions in subcutaneous fat have usually implemented HIIT for 12-15 weeks [10, 11]. In terms of muscle mass retention with HIIT, there appears to be limited data, but performing at a maximum intensity would place greater emphasis on the anaerobic energy system and thus type II muscle fibre utilisation 
At the other end of the intensity spectrum is low intensity steady state (LISS) exercise, in which greater emphasis is placed on the aerobic system and the exercise performed for a longer duration of time (e.g. ≥30minutes @ 50% VO2 peak) [2, 13]. Commonly, aerobic exercise is performed at 50-65% VO2 peak intensity but ultimately, it should be noted that this is steady state in nature. At this intensity, similar adaptations occur to HIIT as already highlighted above (mitochondrial biogenesis etc.) which culminate in enhanced muscle oxidative capacity and increases in maximal oxygen uptake [2, 14]. Furthermore, performing at this intensity range has been observed to maximize fat oxidation rates in both the trained and untrained state .
The major difference between HIIT and LISS is the total work time undertaken by each strategy, with endurance training creating a greater negative balance and greater total work time. Interestingly, 24-hour substrate oxidation and energy expenditure appears not to differ between 40-70% VO2max [16-18]and is similar between HIIT and traditional endurance exercise. Finally, there is emerging evidence for type I muscle fibre hypertrophy with aerobic exercise alone [19-22]been observed and thus with an adequate resistance training protocol in place, increasing both muscle fibre type sizes would seem pertinent. Thus, whilst during resistance training, type II muscle fibres will be predominately activated, endurance training can target type I activation and provide the best of both worlds for whole muscle hypertrophy.
Take home message:
Ultimately, the physiological adaptations for both exercise strategies are similar in nature and thus provide two methods which can be used to support an adequate resistance training protocol. Indeed, the use of either method maybe a situational factor. For instance, limited gym time where HIIT might be used vs. a longer time period for running, cycling etc. LISS may be more beneficial for creating a larger calorie deficit for fat loss and given the high intensity of resistance training, could be more suited for active recovery and reducing potential injuries. Overall, these two strategies offer positive benefits for creating an adequate stimulus for fat loss, and influence on skeletal muscle adaptations.
*It is important to note a potential concurrent training effect, with the cross transfer of endurance training and resistance training on muscle adaptation. Although this may only be experienced with an extremely large volume/intensity from both training components .
When it comes to weight loss I am of the opinion that HIIT should be the primary focus. LISS is traditionally used in bodybuilding circles but the research is pretty clear in dictating that HIIT provides better results in a timelier manner than LISS. The only issue with high intensity cardio is the impact that it has on the central nervous system. If HIIT is done too often I have noticed that my athletes will feel sluggish and their performance in the gym will start to suffer as well. The solution to this is to limit the amount of high intensity interval training that is performed to a reasonable range. If fat loss is still not sufficient with the current amount of HIIT being performed then I will either recommend that the athlete performs more intervals per session, or that the athlete performs some LISS cardio on other days for some added caloric expenditure.
It is not a matter of LISS being bad, it is just that HIIT is more effective. Ultimately I believe that there is room for both in a training program, but the adaptations that HIIT provides make it the go to option for fat loss cardio.
Do also Read
Is Circuit Training Better Than Cardio?
Why Doing Cardio Is Really Bad!
Sprint Workouts Are The One Key To Getting Lean Fast
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