In a recent study published in the Journal of Strength and Conditioning Research, investigators were out to find good ways for the average exerciser to measure something called internal load. Unlike external load, which is amount of weight you’re moving, the internal load is a measure of the stress response in the body. According to the study, internal load determines whether exercise will result in a favorable training adaptation or overtraining, illness, and even worse, injury. Sounds like something we should all be paying attention to.


In this study, the researchers used two factors to measure internal load under laboratory conditions, and compared these to four methods that are easier to use in a gym setting. Namely, they used cortisol and lactate as the lab measures of internal load. Cortisol is a major hormone responder to stress, often viewed negatively for its catabolic (muscle reducing) effects. Lactate is a metabolic waste product caused by exercise. Together, the higher the quantities of these molecules, the more stressful the exercise. Too much of either indicates the body has been pushed too far. 



Breaking Muscle Shop

Now, while we have plenty of good measures of internal loads, such as heart rate for running, what we lack in athletics is a good way to measure internal load accurately across multiple activities. With increasing popularity of athletic events like CrossFit competitions, this is a good time to find a better a way to measure internal load without lab tests. Here are the four other methods that were tested and compared with the lab assessments:


  • Volume Load (VL): This is the one we most commonly use as athletes or coaches. It is essentially written as an equation of the load you moved, multiplied by the times or distance you moved it.
  • Session Rate of Perceived Exertion (SRPE): This is a value some people use to judge training load, and many more think about without realizing it. This is how difficult a training session felt, measured on a scale of 1 to 10 or similar.
  • Rate of Perceived Exertion Load (RPEL): This is the SRPE multiplied by time. It is, perhaps, more accurate than SRPE because it includes a volume measure (time) as well.
  • Rate of Perceived Exertion Load 2 (REPL-2): This measurement is the same as RPEL, but the rest periods are subtracted from the time measurement. This is to account for the large rest lengths during weight lifting.


So with numerous acronyms in tow, let’s get to the results and eliminate a few of them for easier reading. All of the measurement methods could be used to accurately determine weight lifting intensity. They all increased as the percentage of one rep max decreased, and were highest at 55% of one rep max. However, these values didn’t correlate well to cortisol levels. There were some correlations to lactate, especially for VL and RPEL-2.


The researchers suggested that the internal load of regular weight lifting might best be predicted by the method we’ve been using all along, VL. However, for other athletic activities where VL might not work very well, RPEL-2 shows excellent promise, with similar correlations to internal load as VL. While more research is needed, any athlete who keeps detailed records of their training may want to begin using VL now to see how it corresponds to athletic success.



1. Kyle Genner, et. al., “A Comparison of Workload Quantification Methods in Relation to Physiological Responses to Resistance Exercise,” Journal of Strength and Conditioning Research, DOI: 10.1519/JSC.0000000000000432


Photo courtesy of CrossFit Impulse.

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