It was about a year ago when I watched an exercise video a colleague had sent me. The fitness enthusiast filmed a HIIT training workout for their followers. The workout consisted of various lower body weight plyometric based exercises. It had only been a couple of rounds in when the individual jumped in the air, landed and immediately dropped. Everything came to an abrupt haul. The motivation to push others into the next jump was quickly shifted to protection mode. The individual was curled up in a ball, tensing every muscle as if their body went in to protective mode to shield anything else from occurring. In the end, it looked like the individual had injured their lower leg when landing.

        High-intensity interval training, or for short HIIT, is an approach taken to improve aerobic capacity. Interval training consisting of short bouts of higher intensity work, supplemented with lower intensity rest. It has become a popular approach when time is limited. For instance, research has shown that completing a 10-minute HIIT training workout can have similar effects that training for 30-minutes has (Gibala et al., 2006). Now, there are many variables that this is dependent on, but the bigger picture is work harder for less time and reap the similar benefits.

        HIIT has shown to be an effective form for conditioning when time plays a factor in exercise. Over the years, high intensity interval training has been a field of interest in pulmonary diseases and rightfully so. Individuals battling pulmonary conditions spend a lot of time taking care of their health. From respiratory treatments to clinic and hospital visits, time always plays a factor. Why not see if HIIT training can be a complimentary approach when you don’t have a lot of time? This all lines up, however we have to think beyond the time allocated to work out and also look at the exercises and their impact on the whole body and not just the acute cardiovascular and peripheral muscle affects. Unfortunately, some people are unaware of the aftermath it can leave on the body if not properly programmed.

         So, this leaves us with the question of, “why did this person’s leg get injured?” They were only performing body weight plyometrics. There was no external load, so you wouldn’t think it would be that big of a stressor to injure them. In all reality, there are numerous variables, so many that we could sit over a cup coffee and discuss them all, but I am going to focus on the impact HIIT training has on the musculoskeletal system and why plyometric based HIIT training isn’t something you just program because of the positive benefits it has on the lungs and heart. We must think beyond the acute satisfaction we get after a brutal sweat pouring workout that HIIT hits you with and think about what it is doing to the body over a longer period of time. It is not what we do today that defines us, but it is our choices over the course of our lives that defines who we are. Why do people always talk about creating a legacy before they die? Because, it leaves a mark on time that is more powerful and impactful than one acute accomplishment. The same goes for HIIT training.

         The goal for those seeking to maximize lung function and aerobic capacity should be looked at beyond the initial conditioning component. What people don’t tend to realize is the magnitude of stress that plyometric based HIIT training has on the musculoskeletal system. The compression forces on the hip joint during the support phase of walking can reach 3 to 4 times body weight (Hall, 2011). If you weigh 100lbs, that is 300lbs to 400lbs of compression forces at the hip joint. We haven’t even factored in acceleration and plyometric based forces.

         When you start to increase the speed and power of the movements forces on the joints increase. This can lead to 9 times body weight in forces placed on the body during each repetition. Now, to really grasp how much stress your body is undergoing you will have to factor in the type of exercises, a persons’ movement mechanics, height, weight, etc. It is not so black and white but if the exercises involve ballistic based exercises than you can guarantee they are going to apply a lot of forces to the joints. If the body doesn’t have the adequate musculoskeletal tensile strength required for the demand or task, then an acute or chronic injury will occur. This is what you can assume happened to the fitness enthusiast or trainer who ended up with an injured lower leg. The body wasn’t able to handle the ground reaction forces and an acute injury occurred.

         It is no secret that individuals with pulmonary conditions can have decreased muscle mass, bone mineral density, and decreased exercise capacity (Cielen, Maes, & Gayan-Ramirez, 2014). Improving musculoskeletal strength in individuals fighting pulmonary conditions should be at the upmost importance in exercise programming. Strength training not only improves mechanical muscle function, it can also improve postural stability and control, increases bone mineral density, and connective tissue strength (Hong & Kim, 2018). All important factors in improving health in pulmonary conditions, but also important in reducing the forces and stressors placed on the body during HIIT training.

          The structure of the muscle fibers, the angle of the fascicles within the muscle, and the metabolic structures of the fibers affect how force production is determined and produced. The ability to take a mass, overcome inertia and create force production is a multifactorial process. Force production is mass times acceleration which means that in a situation where maximal effort is required, there must be a high rate of neuromuscular output and musculoskeletal strength and control. Individuals with COPD share commonalities with individual’s cystic fibrosis. Dyspnoea, exercise intolerance, deconditioning, decreased bone mineral density, decreased muscle weakness and skeletal muscle tissue mass are common (Divangahi et al., 2009).

        The linear growth and decline in growth over time in pulmonary conditions is a multi-factorial equation that is very important to factor in when HIIT training is being considered. Hussey et al. (2002) observed a significant decrease in peak torque and peripheral muscle strength in CF. Moco et. al. (2012) found “muscle-related abnormalities in oxygen metabolism,” in individuals with CF. As the demand for effort increases, the body’s ability to maintain the biomechanical advantage in intervals declines increasing the stress placed on the body and just another reason why creating foundational support through strength training is very important.

         Another factor that is not often considered is rest period between max exertion plyometric. HIIT training utilizes neurological demand at a different rate and frequency compared to aerobic training. The force exerted during a movement is dependent on the amount of motor neurons excited, the mechanical advantage, and the firing rate frequency. During plyometric intervals this would required an increase in motor unit recruitment and increase in neuron excitability. This influx causes a change in metabolic concentration and over time increases fatigue and decreases the quality of movement.

         We all have heard quality over quantity and in the case of HIIT training for someone who doesn’t regularly strength train, that concept is even more important. Think of the 1^st jump squat in a 20 second interval and the last jump squat in that same interval. The height, acceleration, and biomechanics respectively will not be the same. You will even see that your feet do not land in the same place at the end. The body gets tired, which also increases the stress factors to the body.

          So, you have to ask yourself. What is the goal? How much time do you have? And what do you have available to use to reach that goal? If you read this and realize you do not incorporate strength training into your workouts and are going to start, but don’t want to get away from HIIT training because you enjoy it; don’t worry, you don’t have to. Think about reducing ground reaction or foot contact. For example, hoping on a bike and doing interval training is great because there are no ground reaction forces being applied. Kettlebell swings are probably one of the best to transition from the bike to the introduction of foot contact because you are able to start building a ground force capacity. The whole idea and concept is to build a strong musculoskeletal foundation before integrating plyometric HIIT training. There are too many situations where people put the cart before the horse. They may get away with doing HIIT training for a period of time but it always shows its impact on the body in some form or fashion when it becomes too much. The negative impacts can be revealed through aches and pains within the joints, stiffness in the back, and/but not limited to zombie like unregulated aggravation from neurological fatigue.

         HIIT training is a beneficial approach to improving aerobic capacity, and potentially lung function. It has been used as a tool for airway clearance and often used to burn unwanted stress. There is no getting around the fact that HIIT training requires strength, control, and movement frequencies that cannot be sustained or maintained without adequate musculoskeletal strength and recovery. There are many ways to integrate HIIT training and the evaluation of the risk to reward of integrating plyometric HIIT training when ground reactions force volume is high.  Progress interval training accordingly with not only cardiovascular and cardiopulmonary capacity in mind, but also with the body’s foundational strength and biomechanical capabilities required to perform a task and eventually reach the target goal.

References

 Cielen, N., Maes, K., & Gayan-Ramirez, G. (2014). Musculoskeletal disorders in chronic obstructive pulmonary disease. BioMed Research International.

Divangahi, M., Balghi, H., Danialou, G., Comtois, A. S., Demoule, A., Ernest, S., Haston, C., Robert, R., Hanrahan, J. W., Radzioch, D., … Petrof, B. J. (2009). Lack of CFTR in  skeletal muscle predisposes to muscle wasting and diaphragm muscle pump failure in  cystic fibrosis mice. PLoS genetics, 5(7).

Gibala, M. J., Little, J. P., van Essen, M., Wilkin, G. P., Burgomaster, K. A., Safdar, A., Raha, S., & Tarnopolsky, M. A. (2006). Short-term sprint interval versus traditional  endurance  training: similar initial adaptations in human skeletal muscle and exercise  performance. The Journal of Physiology, 575(Pt 3), 901–911.

Hall, S. J. (6^th e.d.). (2011). Basic Biomechanics. McGraw-Hill.

Hong, A. R., & Kim, S. W. (2018). Effects of resistance exercise on bone health. Endocrinology and Metabolism, 33(4), 435–444.

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