I was recently asked whether HIIT training is safe for children to perform on a regular basis. I was not able to give a definitive answer, however. Despite my personal proclivity for HIIT, my gut answer was ‘no’. This, of course, awakened my curiosity and I subsequently hunted for an answer.
The reason I instinctively thought it wasn’t safe is that for fitness vocational qualifications, we have to stress that children and adolescents should avoid prolonged anaerobic exercise (and high impact work) due to under-developed anaerobic metabolism, poor thermoregulation, and potential damage or overuse injuries to the musculo-skeletal system (such as the growth plate injuries or osteochondritis).
Looking through various awarding body manuals confirmed this with statements such as:
- Avoid explosive power during the performance of complex movements (such as squats and lunges) until the mid-teenage years and/or until technical performance has been mastered with continual supervision. Does this mean that if a 10-year can perform lunges with perfect technique, they can then safely add explosive power to this movement?
- Strenuous work with young adolescents can lead to overuse injuries and epiphyseal plate damage. How hard is ‘strenuous’?
- Excessive weight-bearing exercise can result in musculo-skeletal injuries such as Sever’s disease. What constitutes an ‘excessive’ amount?
These manuals also state interval training and even plyometrics can be beneficial to adolescents, if they are closely supervised and programmed and implemented well by appropriately qualified and experienced trainers. Whilst HIIT training does not need to include plyometrics, it covers exercises that involve leaps, bounds and jumps (for example) which often are.
I next consulted my books. In my beloved NSCA Essentials of Strength Training and Conditioning book they state children and adolescents may benefit from plyometrics (and exercises that resemble plyometrics). They add, however, that depth jumps (stepping off platforms) and other high-intensity lower body exercises are contra-indicated to prevent possible epiphyseal plate damage. They also stress that the child should be emotionally mature enough to accept and follow instructions.
Any plyometric programme should progress gradually from drills that are relatively simple to more complex ones, with quality of movement being the key determinant of rate of progression. The NSCA stresses that any training programmes for children and adolescents should be multidimensional in that they should not just focus on one component of fitness or training approach. They should be varied in volume, intensity and type throughout the year, include adequate rest and recovery, and be personalised to each child. If children/adolescents are using HIIT training as their sole or key mode of training, it will undoubtedly increase risk. But if part of a carefully structured and varied sports-specific training programme, the risks may be somewhat mitigated. Sports performance typically involves some sort of speed or power development – I remember doing prolonged jumping drills when I was a teenager for volleyball, and my kickboxing training was essentially non-stop HIIT and impact training! However, my training in these sports was well-balanced and varied as I had good coaches.
ACSM in their Guidelines for Exercise Testing and Prescription book state vigorous exercise should only be attempted by those who have safely participated in moderate exercise, and that excessive amounts of vigorous intensity should not be attempted by pre-pubescent children due to their immature skeletons. They also add that children have a considerably lower anaerobic capacity compared to adults, limiting their ability to perform sustained vigorous intensity exercise. Children are not ‘mini’ adults; though their physiological responses to exercise are qualitatively similar to that of adults, there are important quantitative differences that should be heeded.
All the key recommendations from ACSM and other key organisations advocate moderate to vigorous training for children and adolescents. Definition of terms is where one of the key problems lie. I often see training deemed ‘HIIT’, which consists of vigorous intensity exercise (aerobic) in which work intervals are quite long and rest intervals are shorter (a negative rest period); which might meet the recommendations. Other ‘HIIT’ training may involve near-maximal or maximal work (predominantly anaerobic) with short work intervals with longer rest periods between bouts (positive rest periods) – this would probably pose greater risks for injury and development issues as you’re working at near-max/max intensity and need a greater fitness foundation to attempt it safely. This interpretation of HIIT would seem to fall outside the recommendations. However, the NSCA do state that this may be necessary for sports performance, and say it can be alright if care is taken and training varied.
At this point, I still had no conclusive answer, so I consulted my journals to see if they could shed any light – no. Most of the journal articles I read discussed the benefits of HIIT in children and adolescents, or how it was beneficial in the fight against obesity. The prevailing message seemed to be HIIT training improved the variables measured, but more research is needed. I’ve copied some links for research articles in the public domain below:
For me, the key questions when looking at a particular HIIT session would be:
(1) What do they mean by HIIT? What are the work:rest ratios, intensities and total number of reps? What exercises are included? How much impact?
(2) What is the context for the session? Is it part of a balanced sports-specific programme or one-off ‘children’s fitness’ sessions?
(3) What are the age ranges of those taking part? Do you have pubescent children and adolescents in the same group? If so, is exercise differentiated for them?
(4) What is the session leader’s qualifications and experience in delivering HIIT? What safety measures have they put in place? Is training individualised to cater for differing developmental issues? What is the instructor:child ratio? Is this sufficient to ensure close supervision and monitoring? Do the leaders know what to look for during HIIT (e.g. warning signs that may warrant cessation of training)? Is training periodised or well-structured so there is sufficient recovery built-in? How are they going to prevent overuse injuries or overtraining? Do the leaders know about paediatric anatomy and exercise physiology issues?
(5) How are the children screened to ensure it is safe to do HIIT with them? What are the thresholds for determining a child is ready for HIIT? Do they take into account other activities/sports each child is participating in to personalise HIIT and prevent overuse injuries/overtraining? What base/foundational fitness levels have been developed in children to ensure a safer transition for higher intensity work?
I’m sure I’ve missed out loads (such as equipment, training environment to include surface, ventilation, etc), but the above list were the things that immediately spring to mind.
My quest continues. I have much research to do to find a satisfactory answer.