Exercise Thresholds and Energy Systems in Rehab

When it comes to providing the best physical therapy for runners and patients, I rarely hear energy systems as being a target of the plan of care. Strengthening, cardio, mobility, sure. All of the above. But what’s a lactate threshold or critical power? Is that something from Dragon Ball Z (Over 9000, anyone?)? Let’s talk about it.

Patients come to us with different abilities to handle exercise, and their abilities change with injuries and deconditioning. We can use the distinction in exercise thresholds to assess what’s sustainable versus unsustainable while prescribing dosages of workouts, consider why we’d want to use both forms, and look at how this pushes patient resiliency further using patient examples. 

Picture Patient A: Hasn’t exercised consistently or at all for the past year or longer, highly inflamed joint pain and low tolerance to loading, and poor cardiovascular endurance. Perhaps some chronic systemic diseases like diabetes or metabolic disease.

Patient B: Marathon runner who had to take 8-10 weeks off from impact due to a bone stress injury. They’ve had some measurable decreases in their cardiovascular fitness and even some loss of strength or muscle mass. 

First, let’s define sustainable versus unsustainable exercise. There’s nothing inherently better or worse about either mode, it simply gives us a way to further contextualize your exercise.

1. Sustainable Exercise: Sustained metabolic rate for a given time and task. Lower intensity. Your body can match the demands for oxygen with this.

2. Unsustainable Activity: Unsustainable metabolic rate for a given time and task. Intensity and effort are higher, demands your focus. Most things, especially resistance training, in the gym are unsustainable. Oxygen in is outpaced by the oxygen out.

   
   

We can use exercise thresholds, specific points of intensity where your physiology appreciably changes once the threshold is crossed. This may look like heart rate, VO2, blood lactate levels, or the humble talk-test. Heart rate zones can be loosely based on these thresholds, however take these with a grain of salt. HR zones are specific to the exercise of choice: Your biking HR zones are different from heart rate zones while running. 

Furthermore, HR zones don’t perfectly reflect your physiologic response to exercise. One person’s 70% could be an easy pace, right below the lactate threshold. For another, their 70% could be above their critical power level, which is Zone 4 and rather unsustainable over a period of time (likely 30 minutes or so). The takeaway is that context matters here. 

So what to do? Here are better options for measuring exercise thresholds that reflect better your reality on the track or in the gym:

1. Lactate Threshold: Let’s define lactate. It’s a product of metabolizing carbohydrates. There’s always some lactate present in your system, but early on in a low intensity exercise you’re primarily using fat as a fuel source. Once carb metabolism beings, lactate production exceeding your muscle cell’s metabolism results in lactate shuttling into the bloodstream. From here it goes to plenty of other organs like the liver for metabolism. This is a lactate measured value, but I like the talk-test to pinpoint the shift in intensity that marks where lactate production exceeds clearing. As exercise intensity increases, CO2 production increases as a byproduct. Being able to talk gets affected as removing CO2 becomes the respiratory system’s priority. To test this yourself: Start at an easy pace. Every 2 minutes, increase the pace a little bit. In the final 30 seconds of each interval, recite a basic phrase like the ABCs or the "Happy Birthday" song. After speaking, ask yourself if talking was “comfortable” (below lactate threshold, metabolically sustainable), “a little labored but okay” (sustainable but riding the line of critical power), or “hard to talk” (we’re starting to hit failure if we hold this long enough). That transition from comfortable to discomfort is the pace at which you hit your lactate threshold. Surprised at how low that actually is? Fun fact: lactate accumulation is the result of your muscle cells not being able to keep pace with the metabolic rate, it’s not the primary limiting factor on your pacing. Say it with me: lactate is not the reason for your fatigue or power reduction. 

2. VO2 Max: Yet another lab-measured threshold. VO2 max is when oxygen saturation of the muscles has been reached. There’s a true max, in which a person must be well trained in, and then there’s a “VO2 peak,” in which you have a functional limitation of muscle endurance more than cardiorespiratory fitness preventing you from reaching a true oxygen saturation. Few people would be in the position to test their VO2 max. Instead, VO2 max is an emergent value from a person’s cardiovascular performance, leaner body mass, and overall health. If your 5k run time is improving, then yes your VO2 max has likely improved. Don’t overly focus on where your VO2 max is or fall for fancy workouts specifically targeting VO2 max. Instead, consider that all those running workouts promising to improve your VO2 max actually will improve your overall performance or work capacity. 

3. Critical Power/Critical Speed: This is the big one. This is the point at which you’re operating at the point of no return for power output (or speed if measuring on a run), and you’ve got minutes to spare. Almost all gym activities fall under this category as well. At this point, the harder you work the more quickly you’re expending energy reserves and rapidly depleting yourself. Working on your critical power unlocks the hard tasks needed for peak performance gains.

   
   

Now for the rehab component:

For Patient A, a flight of stairs may be working above her lactate threshold. We need to try to delay the point at which climbing stairs is the lactate threshold. Functional activities have to be well below your metabolic fitness so that we aren’t being continually overworked during the day. This is all about raising your bare minimum ability during your day and your workouts. No one wants getting up off the sofa to be near their 1RM squat effort. 

For Patient B: They need to keep their lactate threshold as high as they can while working around the limitations of their tissue-specific injuries. While there’s reduced transference in modalities, we want to mimic their chosen modality as closely as possible. I like to use their sub-lactate threshold pace as the target for improving their base: more mitochondrial density, capillary density of tissues, etc. We know that cardiovascular peripheral and central adaptations can detrain rather quickly, so let’s use this knowledge to counter losses and keep Patient B on track. 

Seeing some changes but still want a more individualized approach to your rehab? If you want to learn how to take control with some expert guidance and treatments to optimize your performance and longevity, then that’s exactly what we provide at Zenith Performance Physical Therapy. Call or text us at 562-502-1767 to schedule a free phone consultation and come in for an evaluation!

Thomas Bilodeau, PT DPT, USAW-L1

Zenith Performance Physical Therapy 

ICE Certified Specialist: Orthopedics, Fitness Athlete, Older Adult