From challenging workout DVDs for at-home use to demanding outdoor boot camp-style classes and grueling studio-based workouts, high intensity training is popular because it provides what people want: results!

High intensity workout programs are popular because they work. They are a time-efficient way to expend a lot of calories and sculpt well-defined muscles. However doing too much high intensity exercise too frequently without sufficient recovery can lead to a number of physiological outcomes that can actually keep you from reaching your goals. It’s important to remember that any exercise is a physical stress applied to the physiological systems of the human body. Known as the SAID Principle (Specific Adaptations to Imposed Demands), the type, volume, duration and frequency of exercise determines the type of stresses applied as well as the specific physiological outcomes that occur.

The body’s metabolism is responsible for converting the nutritional substrates from dietary intake into fuel for muscle contractions, and into new cells used to repair tissue damaged during exercise. During rest and low intensity exercise, energy in the form of Adenosine Triphosphate (ATP) is provided through aerobic metabolism. However, the energy for high intensity exercise is produced by the Phosphagen and Glycolysis energy pathways, which place a significant demand on the involved muscle tissue to rapidly deliver the necessary ATP to fuel muscle activity. Regardless of whether it is resistance training or cardiovascular conditioning, one of the outcomes of high intensity exercise is an extreme amount of metabolic stress. While high intensity exercise can provide many desired results, it is important to understand that it can also lead to the following six types of metabolic damage which could keep you from reaching your fitness goals:

1. Elevated blood lactate

When muscles involved in exercise can no longer meet energy demands through aerobic metabolism they will tap into the ATP-PC and Glycolysis energy pathways to produce ATP anaerobically (without oxygen). One by-product of anaerobic metabolism is lactic acid, which can accumulate quickly during high intensity exercise. The Onset of Blood Lactate (OBLA), commonly called the lactate threshold (LT), is a physiological marker that indicates an elevation in blood acidity, which can inhibit energy production and the ability to do physical work, leading to fatigue. When you feel that burning sensation in your muscles, it’s an indication of OBLA and a sign that it is time for a lower-intensity active recovery interval, however, regular High Intensity Interval Training (HIIT) can train the body how to tolerate working at OBLA as well as improving the ability to quickly remove lactate and other metabolic waste. 

2. Acidosis

Besides lactic acid, anaerobic exercise also elevates levels of hydrogen ions (H+), both of which increase blood acidity reducing the levels of oxygen and other nutrients available for aerobic energy production. In extreme cases acidosis can cause severe damage to muscle tissue resulting in a breakdown of muscle protein called myoglobin. When myoglobin is broken down and subsequently enters the blood stream this could ultimately lead to rhabdomyolysis. Rhabdomyolysis can inhibit normal function of the kidneys, potentially leading to hospitalization or possibly death, so it is extremely important to listen to your body and not push physical exertion past your normal comfort levels.

3. Gluconeogenesis

Protein is normally used to repair tissue damaged during exercise and promote the growth of new muscle. Carbohydrates are converted to glycogen and used for ATP production during anaerobic exercise. Fatty acids require oxygen and take longer to convert to ATP, making them an inefficient energy source for high intensity exercise. When high intensity exercise lasts for extended period of time, the body will convert protein to ATP in a process callused gluconeogenesis, reducing the amount of protein available for muscle growth. The process of converting amino acids (the building blocks of protein) to ATP elevates levels of ammonia, further increasing blood acidity and the risk of acidosis.

4. Increased levels of human growth hormone (HGH)

When muscle damage occurs due to exercise,  the body will produce higher levels of HGH to repair this damaged muscle. This is the metabolic response that most body builders hope for because this increase in HGHcan lead to an increase in muscle size. So if your goal is to add lean muscle mass, this is a good thing and one of the most important benefits of high intensity training. But if your goal is to simply lose weight, then all of that high intensity exercise could be having an opposite effect, increasing both muscle size and net bodyweight. One thing to keep in mind is that as you add lean muscle mass, you can increase your resting metabolism, elevating the amount of calories you burn at rest.

5. Increased glycogen storage in muscle tissue

High intensity exercise frequently relies on muscle glycogen to produce ATP. As a result of extended exposure to high intensity training, muscles become more efficient at storing glycogen for future use. Generally glycogen storage is not a bad thing because it provides necessary energy that can help you become more efficient at high intensity exercise. But it’s important to note that when stored in muscle tissue, one gram of glycogen holds approximately three to four grams of water. As your muscles become more efficient at storing glycogen, they also store water, which can lead to an increase in muscle size. If you are training to compete in aerobic endurance events this is not a bad thing because it can allow you to store more energy for your events. However, if weight loss is your goal then training your muscles to store glycogen (and water) may have an impact on the scale that you weren’t hoping for. Remember that weight is one number, but body composition may be a more important measure of overall health.

6. Overtraining

Excessive exposure to high intensity exercise without sufficient rest periods can lead to Overtraining Syndrome (OTS). Signs of OTS include reduced immune system function (leading to lingering colds or flu-like symptoms), elevated heart rate, sleeplessness, increased irritability, weight gain (despite exercise) and a significant decrease in physical performance. It can take anywhere from 24 to 96 hours to fully recover from a metabolically demanding high intensity exercise session. For optimal results, make sure you allow a full night’s sleep the evening prior to a high intensity training day and that your next workout is a low to moderate intensity session.

High intensity workout programs are popular and here to stay because they’re both dynamically challenging and extremely effective in helping many people achieve a variety of results, from weight loss to muscle growth. If you enjoy high intensity workouts continue doing them, but be aware that they do cause significant metabolic damage. It is essential to allow an appropriate amount of rest and recovery between physically demanding workouts.

Rest doesn’t mean simply sitting on the couch watching TV. Rest can refer to active rest, which can be almost any form of lower-intensity exercise. If you are sore the day after a hard workout, it may be due to an accumulation of metabolic waste (the lactic acid and hydrogen ions related to acidosis) in the involved muscle tissue. Elevating your heart rate with low to moderate intensity exercise the day after a high intensity workout can be an effective tactic for removing the metabolic waste; these lower-intensity exercise sessions are often called regeneration workouts because they can promote both recovery and muscle growth. A high intensity weight-lifting day can be followed by a yoga class or a bodyweight workout to help promote the recovery process. A high intensity cardio day can be followed with lower-intensity exercise like cycling at a moderate tempo or walking to help you stay active while recovering from the previous day’s workout.

Bottom line: Don’t stop your high intensity workouts, but make sure you adjust your fitness schedule so you can fit in active rest days to allow for optimal recovery. This simple step can help you move closer toward your goals.