Many people start working out to increase muscle size or become stronger and capable of lifting heavier weights. Strength training with external resistance can increase strength and stimulate muscle growth simultaneously. However, there is a distinct difference between training for maximum force output (strength) and an increase in muscle size. Resistance training alone does not induce muscle growth; the training stimulus has to cause either metabolic or mechanical fatigue to initiate the physiological mechanisms responsible for increasing muscle mass. 

Momentary muscle fatigue is a signal that an appropriate amount of either metabolic or mechanical stress has been applied. Metabolic and mechanical demand on muscle tissue can stimulate the mechanisms responsible for muscle growth. Therefore, knowing how to apply the stress in the right way is essential for achieving an optimal response from a workout program. If you have a client stuck at a plateau and you’re looking for a way to kick-start his or her workout program, knowing how to create the proper stress stimulus on muscle tissue can help you achieve the results your client wants. 

Here are eight things to know about how to place the appropriate demands on muscle tissue to achieve your client’s desired results:

1. Muscle growth and improvements in strength require activating high numbers of fast twitch (Type II) muscle fibers. Fast twitch fibers are capable of producing significant levels of force and are frequently involved in anaerobic energy production, which makes them most susceptible to both mechanical and metabolic damage. Type IIb fibers are completely anaerobic because they used stored adenosine triphosphate (ATP) to provide the energy to produce a high amount of force in a short period of time. Type IIa fibers produce energy from glycogen in a process known as glycolysis, which can take place both with (aerobically) and without (anaerobically) oxygen. Fast twitch fibers have a greater diameter (cross-width) than slow twitch fibers and are responsible for the hypertrophy, or increased fiber size, of a particular muscle.
2. Mechanical stress refers to the physical forces applied to muscle fibers. Resistance training causes microscopic damage to muscle tissue, which in turn signals the biochemical reaction to produce new satellite cells responsible for repairing the mechanical structures and building new muscle proteins. This is how strength training to the point of momentary muscle fatigue initiates the physiological mechanisms responsible for muscle growth.
3. Metabolic stress is caused by a muscle producing and using the energy required to fuel contractions. Moderate- to high-intensity, high-volume strength training programs use the glycolytic system for producing the energy required for the involved muscles to work. Anaerobic energy is muscular energy produced without the presence of oxygen. The result of sustained anaerobic glycolysis is an accumulation of lactate, inorganic phosphates and hydrogen ions, which elevate blood acidity leading to an effect known as acidosis. There is a strong relationship between blood acidosis and the elevated levels of the hormones that support muscle protein synthesis. When a muscle works to the point of fatigue or “failure,” it has expended its supply of available energy. This leads to metabolic stress on the involved tissue.
4. Mechanical stress is an important and essential stimulus for creating exercise-induced muscle growth. Metabolic stress may also be responsible for signaling the body to initiate the physiological mechanisms responsible for muscle growth. Similar to the age-old quest of which came first, the chicken or the egg, we’re not sure which plays a greater role in muscle growth—mechanical or metabolic stress—because both occur simultaneously, making it difficult to identify which is more important. However, using a weight heavy enough to cause momentary fatigue after eight to 15 reps, combined with short between-set rest intervals, will create both the mechanical and metabolic stimulus that could lead to the desired adaptations.
5. When fast twitch muscle fibers create energy from anaerobic glycolysis, it can lead to an important adaptation responsible for influencing muscle size. As muscle cells continually use glycogen for fuel, they will adapt by storing more glycogen during the recovery phase. One gram of glycogen will hold up to 3 grams of water when stored in muscle cells. Exercising to momentary fatigue not only elevates mechanical damage to the muscle, it can also deplete stored muscle glycogen. This results in an increase in muscle size once it is replenished.
6. One of the long-term adaptations to strength training is an increase in muscle fiber cross-width, which is also known as hypertrophy. As the cross-sectional area increases in size, the fibers have more surface tension and become capable of generating higher amounts of force. Muscles with a larger cross-sectional area of individual muscle fibers are capable of producing greater amounts of force. Myofibrillar hypertrophy refers to the increase in size or thickness of individual actin and myosin protein filaments, which can improve the force-production capacity of the myofibrils. Myofibrillar hypertrophy does not lead to larger muscles; rather, it results in thicker muscle fibers capable of generating more force. Sarcoplasmic hypertrophy is an increase in the volume of the fluid-containing proteins responsible for tissue repair in the intercellular space that surrounds an individual muscle fiber. Sarcoplasmic hypertrophy can cause the cross-sectional area of muscle fibers to increase, but most of this growth is due to the volume of the sarcoplasm and non-contractile proteins not directly involved with force production. Despite a common misperception that lifting weights can lead to an immediate increase in muscle size, it can take eight weeks or longer for significant muscle growth to occur, even in a well-designed program.
7. Compound, multijoint movements with free weights such as barbells, dumbbells and kettlebells involve a number of different muscles and can generate metabolic and metabolic stress during training. This is especially true when performing eight to 15 repetitions, with momentary failure occurring at the last repetition. Selectorized machines that focus on muscle-isolation or single-joint movements allow mechanical stress to be placed directly into localized tissue. Free weights involve a number of contributing muscles, which can increase the metabolic demand, while the stabilization provided by machines allows for heavier loads on specific muscles. Short- to moderate-duration rest intervals of 30 to 120 seconds help create a sizable metabolic demand. Completing three to four sets per exercise ensures an effective amount of stress on the involved tissues. The tempo of movement should be relatively short (i.e., one to two seconds) during the concentric phase of muscle action, and should be longer (i.e., two to six seconds) during the eccentric phase to ensure adequate mechanical strain. Slower muscle-lengthening (eccentric) actions place a tremendous amount of both mechanical and metabolic stress on muscle, which makes it an effective approach when exercising for muscular growth. Specifically, lengthening under load can create higher levels of damage, which leads to a rapid rise in protein synthesis to repair the damage.
8. Drop sets are another technique for creating muscle growth. A drop set involves using a set weight for as many reps as possible and then, when fatigue occurs, lowering the amount of weight used and continuing. Drop sets keep a muscle under tension for an extended period of time, which means they can induce significant levels of metabolic and mechanical stress. However, drop sets can also create extreme discomfort and should only be used by experienced weightlifters at the end of a workout. 

It is not completely clear whether muscle growth is the result of mechanical or metabolic

overload. What is known is that the mechanisms for muscle growth are best achieved when an exercise is performed to the point of momentary muscle fatigue. To stimulate muscle growth, a strength-training program must be applied in a manner that places a significant amount of mechanical stress on the muscle tissue, while also creating a sizable metabolic demand. 

Identifying whether a metabolic or mechanical stimulus is more appropriate for a client interested in muscle growth will take some trial and error. Some clients may be capable of tolerating the discomfort that comes with training to muscle failure, while others may prefer a heavier load for fewer reps to induce mechanical stress. While both mechanical and metabolic stimuli result in muscle growth, both can also lead to significant muscle soreness. If a client wants to increase muscle size or get stronger, he or she must understand what is involved and be willing to put forth a tremendous effort, thus’ making the phrase “no pain, no gain” completely appropriate.