What is metabolic stress?

Metabolic stress is understood as an accumulation of metabolic products or metabolites, sometimes referred to as waste products, such as lactate, phosphate and hydrogen ions (H+) that when there is a low availability of oxygen to muscle cells during exercise exercise, walk hand in hand with hypoxia.

Lactate is the acid that causes acidosis in the muscles, and is primarily responsible for the “burning” sensation called “pump” during weight-bearing training.

This “burning” sensation and metabolic stress appear to be related, although metabolic stress is also increased when “cleaning” of metabolic waste products is inhibited.

The current theory is that metabolic stress contributes to the growth of muscle mass in several ways, however the evidence obtained so far that this metabolic stress has a relevant role in muscle growth is based solely on “in vitro” studies. » with muscle cells in isolation, and not in real people undergoing training. Therefore, there is no clinical evidence that metabolic stress plays a relevant role in muscle growth in people, independently of mechanical stress.

The origin of metabolic stress

All training that is carried out with loads leads to an increase in muscle hypertrophy and strength, as long as it involves producing muscle force throughout repetitions with muscle contractions.

When a muscle contracts repeatedly it becomes fatigued, that is, it reduces its ability to produce force as a voluntary response, understanding two different types of fatigue:

• Central fatigue: which occurs in the central nervous system.
• Peripheral fatigue: which occurs at the muscular level.

These two types of fatigue are sub-divided into several quite complex processes. The premise that muscle growth could only be obtained by providing peripheral fatigue through heavy load training has always been accepted, since all clinical studies have found that muscle fibers can detect mechanical tension through mechano-sensors.

Upon detecting this mechanical tension, a sequence of anabolic signals is triggered that generate muscle protein synthesis, from which the volume of muscle fibers is increased, popularly known as muscle hypertrophy.

However, recently it has been observed that training with light loads leading to muscle failure also precipitates a high level of fatigue that can generate muscle growth very similar to that of training with high loads, observing that light loads cannot generate muscular hypertrophy if they are not carried out until muscular failure is obtained.

This could determine that the weight of the loads is not a relevant factor for hypertrophy, but muscle failure, regardless of the load used. A large number of research studies have found that it is fatigue that actually contributes to muscle growth, through the accumulation of metabolites that occur when training with loads until muscular failure, regardless of the weight that these loads have.

This accumulation of metabolites is what is now believed to cause metabolic stress in muscle fibers, and consequently generate anabolic signaling, which is very similar to that generated by mechanical tension.

In other words, the fact that the hypertrophic response with low loads associated with an increase in metabolic stress has been detected, seems to indicate that metabolic stress and its consequent accumulation of metabolites are truly responsible for the growth of muscle mass.

Differences between fatigue and metabolic stress

Fatigue is an inhibition of the muscle’s ability to generate voluntary force. This inhibition can occur either by a reduction in the signal sent from the nervous system, called central fatigue, or by a reduction in the ability of the muscle to produce force, called peripheral fatigue.

The central fatigue of the motor units concurs by an inhibition of the signal sent to the brain through the spinal cord.

It is also precipitated by an increased response to an afferent stimulus, ie a nerve signal from peripheral receptors, which reduces the excitability of motor neurons.

Peripheral fatigue located at the muscle level is stimulated due to the inhibition in the activation of individual muscle fibers, either by an inhibition of the sensitivity of actin and myosin filaments to calcium ions, or by an inhibition in the release of calcium ions from the endoplasmic reticulum.

For years, it was thought that the effects of peripheral or muscular fatigue were caused primarily by the accumulation of lactate, which occurs during anaerobic glycolysis, and later by the associated release of hydrogen ions (H+), which leads to acidosis. However, recent studies have found that this increase in metabolites, frequently called waste products, are not very relevant in the fatigue process, and that other factors are more relevant.

Does metabolic stress generate hypertrophy?

The first original investigations investigating the mechanisms of hypertrophy after resistance training did not take into account metabolic stress as a possible cause of hypertrophy.

Over time, research began to find that the accumulation of metabolites could provide a stimulus for hypertrophy. Not long after this hypothesis began to be developed, calling this accumulation of metabolites caused by resistance training as metabolic stress.

The term was coined by the North American researcher Brad Schoenfeld, and from that moment to the present the premise that metabolic stress is a factor that leads to hypertrophy has been introduced, regardless of the mechanical tension used.

They also found that bodybuilding training using moderate loads to muscle failure, with short rest periods, tended to generate greater metabolic stress than heavy load training with high weights.

As a consequence of this research finding, new routines were developed that generate greater metabolic stress through training with light loads until muscular failure.

How does metabolic stress work?

Metabolic stress is generated by the accumulation of metabolites as a result of training, thus contributing to hypertrophy, according to the following parameters:

• Greater recruitment of motor units: the recruitment of muscle fibers occurs gradually with low threshold motor units in the first area, and later with high threshold motor units in the second area, to sustain the demand for force in muscle contraction during the training. Although heavy loads activate a broad spectrum of fibers, current studies have found that metabolic stress increases the recruitment of high-threshold fibers at light loads.
• Systemic release of hormones: it has also been found that an acute rise in anabolic hormones after training as a result of the accumulation of metabolites generated by metabolic stress increases hypertrophy. Any exercise capable of inducing metabolic stress is associated with an increase in growth hormones after training.
• Muscle cytokine production: metabolic stress affects muscle growth by increasing anabolic myokines and decreasing the production of catabolic myokines. Fundamentally this has been observed with IL-6, or Interleukin 6, a myokine that acts as a cell signaling for hypertrophy.
• Inflammation of muscle cells: another consequence of metabolic stress associated with hypertrophy has also been observed, consisting of increased fluids, or hydration of muscle cells, which appears to act as a regulation of cell function. Several studies carried out in this regard have found that this hydration of the cells increases protein synthesis and decreases protein catabolism.
• ROS release: the release of different forms of reactive oxygen, known as ROS (Reactive Oxygen Species) are released in response to hypoxia (lack of oxygen), but also to the increase in metabolites produced by metabolic stress during exercise training. Current studies suggest that ROS release has consequences on hypertrophy.

Frequently asked questions about metabolic stress

How is metabolic stress defined?

Metabolic stress is defined as an accumulation of glucose metabolites such as lactate, hydrogen ions, inorganic phosphorus or creatine, which are generated from anaerobic glycolysis and are associated with muscle contraction.

How is metabolic stress stimulated?

Everything seems to indicate that metabolic stress is stimulated by those workouts whose exercises lead to muscle failure, regardless of mechanical tension, that is, it is not the weight that is relevant, but muscle failure after enough repetitions.