Knowing how metabolic stress in resistance training works is critical for any personal fitness trainer to know. With advanced knowledge around metabolic stress, you’ll be able to understand the mechanisms of muscle growth. This, in turn, will help you create balanced training programs for clients. 

The primary mechanisms as part of a program design for hypertrophy are muscle damage, mechanical tension, and metabolic stress. Knowing what metabolic stress is and how it works in relation to the other hypertrophy mechanisms will ensure that your clients get the best results possible. Use this guide to better understand what metabolic stress is and how you can leverage it for clients. Metabolic stress is a critical mechanism to achieve an adaptation of hypertrophy, or skeletal muscle enlargement.

As you learn about metabolic stress, remember that bodybuilders aren’t the only types of clients who can benefit from this form of strength training. In fact, clients who want to lose body fat can also benefit from the development of lean muscle mass. With more muscle tissue, clients are able to increase their metabolism, allowing them to burn more calories throughout the day. Additionally, a lower body fat percentage is favorable in the prevention of other chronic conditions such as metabolic syndrome, type 2 diabetes, and cardiovascular disease.

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What is Metabolic Stress?

To know how metabolic stress is affected during a strength training workout, you have to first know what this mechanism is. First, metabolic stress should be differentiated from mechanical stress. Mechanical stress, also known as mechanotransduction, is the mechanism of converting mechanical energy into chemical signals. The mechanoreceptors in the sarcolemma stimulate anabolic intracellular muscle protein synthesis. Think of mechanical stress as “outside in.” Mechanical stress is created by performing repetitions of a heavy load, through a full range of motion, for a specific period of time. For example, lifting a barbell and performing biceps curls. In this instance, the weight and the motion refer to the mechanical stress on the muscle. Thus, why you can think of it as “outside in”.

Metabolic stress, on the other hand, refers to metabolism. And, the simple definition of metabolism is the chemical processes going on within a living organism to maintain life. Inside, the compounds of carbohydrates, fats, and proteins serve as potential energy. The chemical process of breaking down these compounds to release kinetic energy is known as metabolism. 

Thus, metabolic stress during resistance training is the stress placed on the muscle from the metabolites of these compounds. And, therefore, you can consider metabolic stress during exercise an “inside out” perspective.

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How Does Metabolic Stress Build Muscle During Resistance Training?

During resistance training, veins are repeatedly constricted by working the muscle. 

This occlusion of blood supply to working muscles reduces the amount of cellular oxygen available to make ATP, thus increasing the anaerobic metabolism contribution. There are many theories about how and why this leads to muscle growth.

Theory #1

One theory says that this excess of metabolites, namely lactate, intramuscular phosphocreatine (PCr), inorganic phosphate, and hydrogens can cause intracellular swelling. The cell treats the excess metabolites as a threat and initiates anabolic signaling reactions to stimulate the strengthening of its organelles and ultrastructure, leading to muscle growth. In other words, the cell grows from the inside out. You might refer to this as “muscle pump.” The muscle cell is swelling inside.

Theory #2

Another theory is that the accumulation of these metabolites simply increases muscle activation in a larger proportion of muscle fibers increasing the amount of mechanotransduction or “outside-in” stimulus. In this case, the believed reason for the increased activation is due to the fatiguing effects of hydrogen and inorganic phosphate accumulation in the tissue. When muscles fatigue and exercise continue, the CNS will recruit more motor units. 

Other theories meant to explain muscle hypertrophy linked to metabolic stress include the acute rise in anabolic hormones, an increase in signaling proteins (cytokines), and the influence of reactive oxygen species. 

One could lean toward the first theory, as recent research has shown that higher-volume resistance training causes hypertrophy in the sarcoplasm, not the contractile components (myofibrils). In addition, blood flow restriction methods have shown to hypertrophy muscle without any added mechanical loading. 

Ultimately, pinning down the primary mechanistic driver of muscle hypertrophy from metabolic stress is complex and remains largely misunderstood. The good news is, we don’t need to fully understand the primary driver to leverage this mechanism in client sessions.

Safely Training to Maximize Metabolic Stress and Muscle Hypertrophy

Higher repetition volumes will lead to greater metabolite accumulation and, thus, greater metabolic stress. Much research, old and new, confirms this assumption. We can also look to bodybuilders, who tend to be large humans, often larger than powerlifters, because they more regularly train using higher volumes and shorter rest periods.

The work of Brad Schoenfeld has also taught us that heavy loads with few repetitions and lighter loads lifted to failure induce similar rates of hypertrophy. 

When building training programs designed to induce metabolic stress keep a few safety concerns in mind:

  • Programming with heavy loads with shorter rep ranges is a time-efficient method to induce hypertrophy but it also induces central nervous system fatigue. When used too often, it can lead to joint discomfort. 
  • On the other hand, training too often using lighter loads and lighter weight may not induce enough muscle fiber deformation (muscle tension) to maximize muscle size. 

Finding a balance between load size and frequency keeps your clients safe while also helping them see muscle growth.

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Programming Strength Training Routines

The amount of training needed to maximize metabolic stress will depend on the client.

For example, when working with athletes, this type of training should be used sparingly in the prime movers such as the lats, leg musculature, and chest and shoulder muscles. The road to improving athletic performance goes through power and power is most negatively affected by higher volumes and shorter rest periods. 

For physique clients or those who want to add lean body mass, combining heavy days with metabolic stress days can limit CNS fatigue, maximize hypertrophy, and, more importantly, protect your clients’ joints. 

Below are some training tips to maximize hypertrophy by incorporating metabolic stress.

  • Keep the rest phase at the end of the eccentric phase short. The goal is not muscle damage nor is it to lengthen the sarcomere, so do not accentuate a slow tempo on the eccentric phase of the movement. Keep moving. 
  • Employ shorter rest periods. The research suggests that there is not a significant difference between 30-second rests and two-minute rests. However, a difference has been shown between 30 seconds to two minutes when compared to five-minutes. Keep the side conversations between rest periods brief. 
  • Try not to focus on hitting a specific number of repetitions. Focus on the quality of the contraction.
  • Try “pump training,” which is an excellent way to hypertrophy muscle via metabolic stress. In this method, many repetitions are completed at a constant and sometimes fast speed, with short rest intervals. When combined with blood flow restriction, this method can be very effective for hypertrophy. It’s also complementary to many training programs. 
  • Try compound sets or multiple sets of different exercises working the same muscle. That brief period of rest between sets will take the sting out of the workout and improve motivation to keep going.

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Focus on Growth—But Keep it Fun

Metabolic stress can help clients reach their hypertrophy goals—but don’t forget to keep it fun. This training can be tough work. Pushing your clients to uncomfortable limits can cause you to lose clients, so don’t forget to bring in variety, stay in tune with what they need, and focus on movements that are as effective as they are safe.

The mechanisms involved in hypertrophy training are complex. And, a traditional personal training certification will not give you the exercise physiology underpinnings needed to achieve maximal muscle growth. The science behind what induces a specific adaptation for the human body to make a change is something you’ll learn through an exercise science degree or an exercise physiology degree. Likewise, exercise science degree career opportunities are much more expansive than a traditional fitness training certification. 

When you get a degree in exercise science from Lionel University, you don’t just learn how to design effective exercise programs. You also learn how the body responds to change. This allows you to become a fitness expert and apply this knowledge to training, similar to the tips we listed in this article. Further, the doors to fitness job opportunities are wide open. Regardless of whether you’re pursuing an associates degree, bachelor’s degree, or master’s degree, specialists in exercise science are in demand.

As you go through your degree program at Lionel, you’ll also earn your personal training certification and Master Trainer certificate in the first few months. This means you can start working as a personal trainer while you finish your program! And, with the help of financial aid, earning your exercise science degree is even more of a possibility. 

Check out our programs and contact Lionel today!