Turn up the heat to improve strength

 8/11/16 England’s Courtney Lawes with team physio during training Action Images via Reuters / Paul Childs Livepic 

Recovery from injury means regaining strength, but injuries also necessitate load modification. Therefore, a load of 60% to 80% of 1 rep max (RM), needed to stimulate hypertrophy and make strength gains, might be too great of a strain on injured tissues. The opposite approach, performing high reps of a low load, may be challenging in the clinic environment due to the time required to complete a significantly higher number of repetitions of each exercise. Another tactic is the use of blood flow restriction training (BFR), which we will explore in an upcoming article. The limitations of BFR include practitioner expertise and, again, the high number of repetitions required.

Researchers in Japan wondered if there might be a more straightforward way to stimulate strength gains while limiting the strain and load on tissues. Capitalizing on the concept that heat stress alone causes muscle hypertrophy, they explored the literature pairing heat stress with low load exercise. They discovered that many of the various protocols were unfeasible for application in the clinical setting. Therefore, they undertook an experiment to determine the effect of low-load resistance training performed after the application of heat stress using a protocol that would be practical for clinicians.

The Japanese team randomly assigned 30 recreationally active young men to an experimental or a control group(1). All subjects underwent baseline measurements, which determined the 1RM of a lying triceps extension exercise as well as triceps muscle thickness of their dominant arm. The study protocol applied heat via a hydrocollator hot pack heated to 75ºC and wrapped in a towel, to the triceps muscle of the heat-stress subjects’ dominant arms for 20 minutes. The participants then performed a supine triceps extension exercise with a dumbbell. The weight was 30% of each individual’s 1RM. They exercise consisted of three sets of eight reps with 60-seconds of rest between sets.

The control group performed the same exercise protocol at the same weight but without the application of the hot-pack. Both groups participated in training sessions three days per week for six weeks for a total of 18 sessions. The investigators remeasured the 1RM of all subjects every two weeks and adjusted their training load to reflect strength gains. After six weeks, the researchers again measured the 1RM and muscle thickness of the dominant triceps muscle.


All subjects were statistically similar in their baseline measurements at the start of the experiment. The heat-stress group demonstrated statistically significant gains in strength via the 1RM measurement at two, four, and six weeks compared to baseline. Also, the 1RM measurement at six weeks was significantly more than that taken two weeks prior. The 1RM measures in the control group were not significantly different at any point in the intervention. Likewise, the heat-stress group showed a significantly greater muscle thickness after the intervention compared to baseline, while the control group’s measurements remained nearly the same.

Practical implications

It’s not surprising that the control group showed little change in either strength or muscle mass. One wouldn’t expect muscle growth with only three sets of eight reps performed at 30% of 1RM. However, the significant gains made with the same protocol combined with the addition of heat is encouraging. The researchers developed a protocol that acknowledges that injured tissues require load modification yet also need enough load to make strength gains.  Heating the tissues for only 20 minutes and keeping the exercise regime to three sets of eight reps is very do-able within the time and resource constraints of a clinic session. This experiment demonstrated that this protocol continues to stimulate both strength and muscle growth for at least six weeks.

The application of this approach to early post-surgical rehabilitation may speed recovery and improve outcomes. Many surgeons restrict movement and stress around the incision site for the first six weeks to protect the wound. This protocol may facilitate strength gains with low-load exercises to isolated muscles, such as the quadriceps after knee surgery while maintaining the integrity of the wound-healing process. It may also allow injured athletes to improve the strength of the damaged muscle without applying stress that impedes healing.


  1. BMC Musculoskeletal Disorders. 2019;20:603


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