Mechanisms of Muscle Growth

Muscle growth is a highly complex issue often times muddled with anecdotal opinions and views. However, at the COR boot camps and personal training, we attempt to understand all the mechanisms and potential mechanisms for muscle growth. Muscle growth isn’t always the initial goal of a client, especially one seeking weight loss, but muscle (lean body mass) is an essential part of weight management and fat loss.

It is believed the three main mechanisms for increasing muscle are (Schoenfeld 2012):personal training and soreness
  1. Mechanical tension
  2. Metabolic stress
  3. Muscle damagepersonal training and soreness
These three methods are not typically considered by the common personal trainers or strength coaches, as many oversimplify the “no pain, no gain” mantra. More interestingly, soreness is a common correlate with muscle damage and metabolic stress, but does soreness build muscles?

Mechanisms of Muscle Growth During Muscle Damage

Physiologically, muscle fiber hypertrophy (growth) is believed to be a result of contractile protein proliferation, and muscle fiber enlargement (Vierck 2000Arnoldflex). A positive net protein synthetic response with adequate energy availability is also needed to facilitate further growth (Miyazaki 2009). Simply put, if the adequate nutrients and energy are not present, hypertrophy (muscle growth) cannot occur!Group Training3

In adults, muscles do not undergo significant cell replacement and growth, making microtrauma essential for muscle growth. The repair is facilitated by satellite cell activation, proliferations, and fusing with existing muscle fibers (Toigo 2006).

Exercise, nutrition, and satellite cells are all complement each other, as they cannot function without each other. Satellite cells mediate hypertrophy, but exercise (or some muscle damage) is needed to signal these satellite cells. Nutrition (energy) is necessary to activate the entire process.

However, individual differences do exist, as some people can perform the same exercise program and eat the same meal, with varying hypertrophic results. This is due to the hormones, gene expression and other factors causing individuality.

Once again, microtrauma is a necessary step for muscle growth. When a microtrauama occurs, calcium ions rush into the cell (Sorichter 1999). This stimulates satellite cells via the hepatocyte growth factor (HGF) and nitric oxide (NO) (Tatsumi 2010). Satellite cells may potentially be activated by calcium ion flux (Hara 2012).

HGF secretion is proportional to the extent muscle damage (Tatsumi 1998). Sateillite cell activation is also specific to the fiber that has become damaged and satellite cells of one fiber do not respond to adjacent fibers (Chargé 2004).

Eccentric exercise increases satellite cell content and activation of Type II muscle fibers (Cermak 2013).

Inflammation may also trigger several cytokines, while white blood cells such as neutrophils and macrophages invade the damaged tissue and release several growth factors, all of which may regulate satellite cell activity (Toigo 2006).

The most notable cytokines which stimulate growth factors and the anabolic response are cytokine interleukin-6 (IL-6), insulin-like growth factor (IGF-1), and HGF.

Also, the use of NSAIDs inhibits the pathway and impairs the hypertrophic response (Schoenfeld 2012). Indeed the use of NSAIDs following eccentric exercise reduces satellite cell activity for up to eight days (Mikkelsen 2009), the likely mechanism decreasing the hypertrophic response.

But, Is Soreness Necessary from Personal Training, Exercise, and/or Boot Camps?

Despite what you man think, soreness is not necessary for muscle growth. However, it often does accompany muscle growth, especially at the beginning of a resistance training routine. Soreness is a poor indicator of muscle damage, when muscle damage is a better indicator of muscle growth (Nosaka 2002). Unfortunately, monitoring muscle damage in a gym or clinical setting is difficult, resulting in many using soreness. Use soreness as a guide, but realize it is muscle damage, one mechanism of muscle growth, which is the key indicator for hypertrophy!

References:
  1. Cermak, N M, et al. “Eccentric exercise increases satellite cell content in type II muscle fibers.” Medicine and Science in Sports and Exercise 45, no. 2 (2013): 230-237.
  2. Chargé, S B, and M A Rudnicki. “Cellular and molecular regulation of muscle regeneration.” Physiological Reviews 84, no. 1 (2004): 209-238.
  3. Hara, M, et al. “Calcium influx through a possible coupling of cation channels impacts skeletal muscle satellite cell activation in response to mechanical stretch.” American Journal of Physiology – Cell Physiology 302, no. 12 (2012): C1741-C1750.
  4. Mikkelsen, U R, et al. “Local NSAID infusion inhibits satellite cell proliferation in human skeletal muscle after eccentric exercise.” Journal of Applied Physiology 107, no. 5 (2009): 1600-1611.
  5. Toigo, M, and U Boutellier. “New fundamental resistance exercise determinants of molecular and cellular muscle adaptations.” European Journal of Applied Physiology 97, no. 6 (August 2006): 643-663.
  6. Tatsumi, R. “Mechano-biology of skeletal muscle hypertrophy and regeneration: possible mechanism of stretch-induced activation of resident myogenic stem cells.” Animal Science Journal 81, no. 1 (2010): 11-20.
  7. Tatsumi, R, J E Anderson, C J Nevoret, O Halevy, and R E Allen. “HGF/SF is present in normal adult skeletal muscle and is capable of activating satellite cells.” Developmental Biology 194, no. 1 (1998): 114-128.
  8. Nosaka, K, M Newton, and P Sacco. “Delayed-onset muscle soreness does not reflect the magnitude of eccentric exercise-induced muscle damage.” Scandinavian Journal of Medicine & Science in Sports 12, no. 6 (2002): 337-346.
  9. Schoenfeld, B J. “Does exercise-induced muscle damage play a role in skeletal muscle hypertrophy?” Journal of Strength and Conditioning Research 26, no. 5 (2012): 1441-1453.
  10. Schoenfeld, B J. “The use of nonsteroidal anti-inflammatory drugs for exercise-induced muscle damage: implications for skeletal muscle development.” Sports Medicine 42, no. 12 (2012): 1017-1028.
  11. Vierck, J, et al. “Satellite Cell Regulation Following Myotrauma caused by Resitance Exercise.” Cell Biology International 24, no. 5 (2000): 263-272.
  12. Miyazaki, M, and K A Esser. “Cellular mechanisms regulating protein synthesis and skeletal muscle hypertrophy in animals.” Journal of Applied Physiology 106, no. 4 (2009): 1367-1373.