The Whole Picture Blog 2 Of 5

The Whole Picture: Physical Stress

BY Simon Wegerif

In part two of the five part series, Simon Wegerif explains how physical stress adds to the whole picture of athlete development.

This is Part Two of a five-part series on total load, the cumulative training and life stress an athlete experiences while training.

It might seem odd to think about training (which most of us love) as “physical stress,” but, as we talked about in the first part of the series, it’s the body’s reaction to stress which determines the all-important adaptation during recovery.

It’s only by balancing the amount of physical stress (training) with our ability to recover that athletes can reach optimal adaptation. Too little stress means we adapt slowly (or not at all), and too much will result in getting sick or injured sometimes for an extended period.

On the other hand, planned periods of overload followed by planned periods of recovery is termed overreaching. Functional overreaching (FOR) occurs when we get the balance right, and performance increases following a period of recovery:

unctional overreaching

Non-Functional overreaching (NFOR) occurs when the training is either too intense, too extended, or both. Not only does recovery take much longer, but no performance gains result meaning the whole experience was a (painful) waste of time:

on functional overreaching

External vs. Internal Load

When it comes to the physical component of total load, we need to measure both external and internal loads.

External load measures training objectively (i.e. how much, how long, how often):

  • Power
  • Speed or pace
  • Distance
  • Time or duration
  • Frequency

Internal Load measures the resulting stress response to that external load:

  • Perceived effort or exertion (RPE)
  • Fatigue, soreness, mood changes
  • Cortisol and CRP production
  • Resting heart rate and and heart rate variability (HRV)

Training Stress Score (TSS) is sometimes considered an external measure, but it is really an internal measure because it is calibrated relative to the capability of your body to manage stress at a particular point in time. Training Stress Balance (TSB) and the related Acute: Chronic load ratio metric are also internal load measures.

A key point to remember is that our reaction to a given amount of physical stress does not remain constant; it varies with life experiences, bio-rhythms and many other factors.

The body’s reaction to different types of training

In a comprehensive review paper in Sports Medicine, Jamie Stanley et al. showed how heart rate variability (HRV) can provide excellent insight into how an athlete’s body reacts to both individual and accumulated training loads. They concluded that it is intensity that has the largest effect on how the body perceives training stress, finding an almost perfect relationship between blood lactate and post-exercise reduction in HRV:

After analyzing the results of eight separate studies on endurance athletes, Stanley et al. concluded that parasympathetic markers (HRV) of recovery required:

  • < 24 hrs for low intensity exercise (zones 1-2)
  • 24 – 48 hrs for threshold level exercise (zones 3-4)
  • > 48 hrs for high-intensity exercise (HIIT, zones 5-6)

Training and resilience

As mentioned earlier, the response of the body does not remain constant, and one interesting aspect of this is how resilience is built by regular training. Professor Tim Gabbett is an expert at the forefront of understanding how changes in training loads make athletes more or less susceptible to injury and illness.

No one will be surprised to hear that rapid increases in physical stress or training load also increase the risk of illness and injury. What is more interesting is how resilience, the relationship between external applied load and the athlete’s response, can be developed.

On the negative side, rapid declines in training load over a period of four to six weeks reduce resilience. These reductions can significantly increase the risk of injury when loads to which the athlete was previously accustomed are restored. Conversely, though, a steady increase in chronic training loads (CTL) actually builds resilience and reduces the risk of illness and injury. The figure above shows how elite athletes, who are accustomed to very high chronic loads, might respond to higher absolute load.

Alternatively, the moving average of daily HRV measures (baseline) can also be used as an indicator of resilience. In the example below notice that, following a series of six consecutive training days in a recreational athlete, the HRV baseline moves up indicating higher aerobic fitness. It also indicates an increased ability to tolerate high absolute loads and an increase in resilience.

A screenshot of a cell phoneDescription automatically generated

To summarize, external load (i.e. “what did the athlete do?”) produces a response (internal load) that is not fixed but instead depends on the condition of the athlete at that particular point in time. It is important to measure both external and internal loads, acutely and chronically, to understand when to increase loads safely as well as when to back off.

Internal load measures need to include a combination of subjective measures such as fatigue, soreness, mood, rate of perceived exertion, and objective metrics like heart rate or hormone measures. High chronic loads confer increased resilience and fitness provided the buildup has occurred at a rate that the athlete can safely tolerate. Training stress balance and the Acute: Chronic training load metrics are good choices to monitor a safe rate for this.

In all but the most elite athletes, physical stress from training is not the number one contributor to total load, and in the next part of this series, we will be looking at the important contributions that mental and emotional stress add to the whole picture.


Stanley, J., Peake, J.M. & Buchheit, M. Sports Med (2013) 43: 1259.

Br J Sports Med. 2017 Oct;51(20):1451-1452. doi: 10.1136/bjsports-2016-097298. Epub 2017 Jun 23.

Schwellnus M, et al. Br J Sports Med 2016;50:1043–1052. doi:10.1136/bjsports-2016-096572

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About Simon Wegerif

Simon Wegerif is a serial entrepreneur, inventor, and biomedical engineer. He was previously an executive with Philips Electronics in the UK and Silicon Valley. Simon is a competitive cyclist and has also completed a number of triathlons including Ironman distance. He created ithlete, the leading, scientifically founded HRV app in 2009 after identifying an opportunity for using HRV in his own training. He is considered an expert on the topic, having read over 1000 papers and frequently consults with industry experts.