In Part-2 we looked at what hypertrophy is, the most effective type of training that activates the hypertrophic response and the relationship between strength development and muscle growth.  In this article we’ll look at what time frame do these changes occur? What’s the time-line for strength and hypertrophy development and when does neural adaptations end and hypertrophy start?

People new to resistance training (or who have not trained for several years) respond readily with significant improvements in strength and hypertrophy to virtually any program. Strength gains develop rapidly in untrained people. These improvements are clearly evident within the first 4-8 weeks of training [1] and neural adaptations are thought to play a prominent role in this early stage. It is presumed that muscle hypertrophy follows improvements in strength. This hypertrophy becomes evident 6 to 8 weeks into a program.[3]

The contribution of neural and hypertrophy adaptations to strength development can be influenced by the complexity of the exercise.[3] For example, one study has shown that over a 20 week training period, hypertrophy of the upper body muscles occurred during the first 10 weeks whereas the lower extremity and trunk muscles did not hypertrophy until the last 10 weeks.[3] The authors of this research believed that for simple motor tasks such as the bicep curl exercise, early gains in strength may occur concurrently with muscle hypertrophy. On the other hand, more complex movements associated with the trunk (e.g., bench press) and legs (e.g., leg press, squats) require a longer period for neural adaptation therefore there may be a delay in muscle hypertrophy.

Highly trained or experienced individuals have been defined as those with 2 years or more of consistent training experience.[4] In comparison to the amount of resistance training research published each year, there isn’t a lot of well-controlled studies that provide information about neurological and hypertrophy contributions in the development of strength in trained individuals. However, this is a key area I focused my research on.

When I first looked to the textbooks regarding time-courses generally, neural adaptations and changes in protein quality (i.e., myosin isoforms-part-1) provide most, if not all the early strength improvements observed in the initial stages of a training program. The contribution of neural factors is thought to diminish as training continues beyond 6 to 8 weeks.[5,6,7,8]

Muscle biopsy studies show that hypertrophy becomes evident typically after 6 weeks of training but this response is also suggested to plateau within 12 to 16 weeks of training.

According to most of the timelines proposed in the textbooks and academic literature [2,6,7,8] little or no significant improvements in strength or hypertrophy occur after 16 weeks of training! In fact, the use of anabolic drugs has been proposed as the only way a trained individual may experience significant hypertrophy.[6] Seriously, that’s pretty much all the documentation regarding muscle growth in the scientific literature! Surely we can do better than that!!

Most of the textbooks (and the major organizations that adhere to them) subscribe to the notion that the longer you train, the more your capacity for improvements diminishes. Obviously, that’s true to a certain point. However, consider the following before you put the mind-brakes on your muscle-building potential.

The fact is, the training studies, and the methodologies used to assess hypertrophy have been pretty lousy. For example, did you know that most (if not all) of the research that shapes the “textbook” recommendations for strength and hypertrophy training didn’t make any attempt to control or monitor nutritional intake! Not only that, but many didn’t even supervise the training! Clearly more insightful research can be done in this area.

We know muscles can keep growingeven after years of training. In a year-long training program, older men and women were shown to increase muscle strength within the first 8 weeks but muscle fiber hypertrophy did not occur until after 30 weeks of training – well after most of the strength gains had occurred.[9]

We also know that people who have trained for years typically have larger muscle fibers than those who have been training for 3–6 months.[10,11] Even more intriguing is the fact that even highly trained lifters (5 or more years of consistent training) can still experience significant neurological adaptations that result in strength improvements. This just goes to show, your muscles never stop learning.

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Creatine Stimulates Muscle Growth by Triggering Satellite Cell Formation

Additionally, well controlled studies that have used trained individuals, a personal-training approach to supervision and monitored nutrition intake have reported average increases in muscle fiber size of 35% (for all fiber types) and gains in lean mass of up to 6%.[11-15] In fact, these gains were quite typical in my clinical work with experienced bodybuilders – many that were veterans with years of experience.[11-15]

Obviously, there are limitations to the extent of any physiological adaptation that can be achieved from physical training. However, it is equally apparent that we’re only just scratching the surface regarding aspects that may improve the hypertrophy response, particularly in trained individuals.

At least one of the reasons for a lack of quality information about muscle growth is the methods used to assess muscle hypertrophy in the past have been quite crude. Even measuring the cross-sectional area of the muscle fiber itself (via muscle biopsy) still isn’t sensitive enough to detect the small changes in hypertrophy can and do occur after a series of workouts or even just one workout.

Remember, according to the text books, only 10 years ago, quality gains in muscle and strength after 16 or so weeks of training were thought only to be possible via the use of anabolic steroids! We now know this is far from true and in fact more sensitive methods are able to detect muscle hypertrophy after just one workout. Therefore, the typical limitations about building muscle that many drug-free lifters seem to accept, are in fact, unfounded and inaccuratesomething to consider before you put the mind-brakes on your own rate of bodybuilding progress.


  1. O’Bryant HS, Byrd R, Stone MH. Cycle ergometer performance and maximum leg and hip strength adaptations to two different methods of weight-training. J Appl Sport Sci Res 2: 27-30, 1988.
  2. Komi PV. Training of muscle strength and power: interaction of neuromotoric, hypertrophic, and mechanical factors. Int J Sports Med Suppl 7: l0-15, 1986.
  3. Chilibeck PD, Calder AW, Sale DG, Webber CE. A comparison of strength and muscle mass increases during resistance training in young women. Eur J Appl Physiol 77:170-175, 1998.
  4. Kraemer WJ, Adams K, Cafarelli E, Dudley GA et al., Progression models in resistance training for healthy adults. Med Sci Sports Exerc 34:364-80, 2002.
  5. Sale DG. Neural adaptations to strength training. In: Strength and Power in Sport, Komi PV Ed. Oxford: Blackwell Scientific Publications, 249-265, 1992.
  6. Moritani T. Time course of adaptations  during strength training In: Strength and Power in Sport, P. V. Komi Ed Oxford: Blackwell Scientific Publications, 266-278, 1992.
  7. Wathen D, Baechle TR, Earle RW In: Essentials of Strength and Conditioning: National Strength and Conditioning Association (NSCA). Baechle TR and Earle RW.  2nd Ed. Human Kinetics Champaign IL, Ch19:513-527, 2000.
  8. Fleck  SJ & Kraemer WJ. Designing Resistance Training Programs 2nd Ed. Human Kinetics Ch7, 131-63, 1997.
  9. Pyka G, Lindenberger E, Charette S, Marcus R. Muscle strength and fiber adaptations to a year-long resistance training program in elderly men and women. J Gerontol A Biol Sci Med Sci 49:M22-M27, 1994
  10. MacDougall J D, Elder GCB, Sale DG, Moroz JR, Sutton JR. Effects of strength training and immobilization on human muscle fibers. Eur J Appl Physiol 43: 25-34, 1980.
  11. Volek JS. Influence of nutrition on responses to resistance training. Med Sci Sports Exerc 36:689-96, 2004.
  12. Cribb PJ, Hayes A. Effects of supplement timing and resistance exercise on skeletal muscle hypertrophy. Med Sci Sports Exerc. 38(11):1918-25, 2006.
  13. Cribb PJ, Williams AD, Carey MF, Hayes A. The effect of whey isolate and resistance training on strength, body composition, and plasma glutamine. Int J Sport Nutr Exerc Metab. 16(5):494-509, 2006.
  14. Cribb PJ, Williams AD, Stathis CG, Carey MF, Hayes A. Effects of whey isolate, creatine, and resistance training on muscle hypertrophy. Med Sci Sports Exerc. 39(2):298-307, 2007.
  15. Cribb PJ, Williams AD, Hayes A. A creatine-protein-carbohydrate supplement enhances responses to resistance training. Med Sci Sports Exerc. 39(11):1960-8, 2007.
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Adaptations and Influences That Affect Muscle Growth – Part-3

by Paul Cribb Ph.D. CSCS. time to read: 7 min