Resistance training is a key strategy for improving motor skills and metabolic health throughout life. The adaptive responses of skeletal muscle mass and strength to training against external resistance are well documented and crucial for anyone wishing to modify their body composition and maintain their physical functional capacities, throughout their lives.
Exercise-induced muscle hypertrophy is traditionally regarded as a slow, gradual process, especially in comparison with the rapid strength gains that can be achieved. This disparity is largely attributed to the initial neural adaptations that occur when training begins, with significant muscle growth contributing more substantially as training persists. The relationship between increased muscle size and strength gains is still a hotly debated topic among scientists and trainers.
One of the intriguing aspects of resistance training is the plateau in muscle growth that seems inevitable after a certain period of training. Research suggests that the majority of muscle hypertrophy occurs in the first few years of training, then slows down, while strength may continue to increase even after muscle size appears to stagnate. This phenomenon raises questions about the physiological limits to muscle mass accumulation and the factors that contributed to this growth limit.
Exploring the reasons why muscle growth stagnates - whether due to reaching a genetic maximum or to other physiological constraints - is crucial to the development of effective training strategies. Researchers have therefore attempted to unravel the complex interplay of factors leading to the muscle growth plateau experienced by many bodybuilders over the course of their training lives.
Muscle growth is initially rapid in beginners. However, this growth inexorably reaches a plateau, attributed to the attainment of the genetic potential for muscle size. This explanation, however, oversimplifies the underlying physiological processes involved. Beyond genetic predisposition, several other physiological factors contributed to this plateau, such as altered anabolic signalling pathways, the balance between muscle protein synthesis and degradation, the impact of energy balance on muscle adaptation, and age.
Anabolic signaling pathways play a crucial role in muscle growth by regulating myofibrillar protein synthesis. Over time, with regular training, these pathways may become less responsive to stimuli provided by strength training, leading to a decrease in the rate of muscle growth despite continuous effort. Factors such as the mTORC1 pathway, crucial for protein synthesis, and the Ubiquitin-Proteasome system, responsible for protein degradation, are closely involved in this process. It is this balance between anabolic and catabolic processes that determines the overall balance of muscle proteins and, consequently, muscle growth. What's more, the "anabolic resistance" becomes more pronounced as we age, suggesting a gradual increase in the body's resistance to mechanical stimuli over time.
Energy balance, i.e. the relationship between caloric intake and expenditure, significantly affects muscle protein synthesis and degradation. Indeed, scientific research shows that caloric restriction can increase muscle protein breakdown, underlining the importance of adequate nutrition to support muscle growth. Fitness enthusiasts, particularly those with a low percentage of body fat, may be more likely to experience a decrease in muscle size if energy and protein intake are not sufficient to meet training demands.
Muscle cells, like all cells, undergo a complex life cycle involving phases of growth, DNA replication and division. However, mature skeletal muscle fibers are considered post-mitotic, meaning they no longer divide but can grow in size through the fusion of satellite cells. These satellite cells are crucial for muscle repair and growth, as they donate nuclei to muscle fibers, thereby increasing their genetic material, which is necessary to support larger muscles. The process of myonuclear accretion by satellite cell fusion is a critical aspect of muscle adaptation, enabling the maintenance of the myonuclear domain, which is essentially the volume of cytoplasm each nucleus can support with the genes and proteins required for cell function and growth.
The concept of molecular brakes is also evoked. These are mechanisms that can regulate or limit muscle growth to prevent uncontrolled expansion. These brakes could be factors such as myostatin, a protein that inhibits muscle growth, or cell cycle regulators that prevent satellite cells from proliferating uncontrollably. The interplay between these growth-promoting and inhibitory signals ensures that muscle growth does not exceed the body's capacity to support it, both in terms of structural integrity and metabolic demands.
This suggests that there are limits to cell growth, beyond which further growth becomes inefficient or even detrimental. The mechanisms underlying this self-regulation are still being explored, but they underline the complexity of muscle adaptation and the balance between growth and maintenance required for optimal function. This would explain both why it's so difficult to progress once a certain level of muscle mass has been acquired, and why it's so difficult to maintain that level.
Muscular adaptation changes over the course of an individual's life, highlighting the nuanced interplay between aging, anabolic resistance and the potential benefits of introducing resistance training at an early age.
As explained above, the phenomenon of anabolic resistance involves a muscle response to growth stimuli (such as resistance training and protein intake) that diminishes with age. This resistance has a significant impact on muscle protein synthesis, making it more difficult for older adults to maintain or gain muscle mass compared to their younger counterparts. Several studies have shown that, for the same protein synthesis, seniors need to ingest a greater quantity of protein. The etiology of the development of anabolic resistance is probably multifactorial, including epigenetic changes, reduced satellite cell numbers, reduced capillarization, reduced anabolic hormones, altered amino acid delivery, increasing insulin resistance and age-related physical inactivity, which can exacerbate all the factors mentioned...
Since aging is a limiting factor in building muscle mass, many authors and trainers have suggested starting strength training at a younger age. The aim would be to spend more time in ideal physiological conditions to maximize muscle growth potential through early exposure to strength training. This early onset could attenuate the effects of anabolic resistance encountered later in life, suggesting that the timing of training initiation could influence long-term muscular adaptation. The idea is that early training would build a more solid foundation for muscle mass and strength that would be maintained longer into adulthood, potentially offsetting the decline generally associated with aging.
What's more, at the molecular level, the nuclei acquired from satellite cells during a period of training are permanently preserved, even during periods of complete detraining. This suggests that muscles can retain a "memory" of previous growth, facilitating more efficient adaptations to future training stimuli. This concept is particularly interesting in the context of early training, as it implies that muscles exposed to strength training at a younger age could respond more robustly to training later in life. And the muscle gains achieved earlier in life would be greater than the muscle gains achieved by a middle-aged adult, beginning resistance training.
Understanding the impact of aging on muscle growth, and the potential benefits of early resistance training, offers valuable insights for the development of age-appropriate training programs. For sports and health professionals working with older adults, this knowledge underlines the importance of tailored exercise prescriptions that take into account the diminishing anabolic response. In addition, they highlight the potential long-term benefits of encouraging physical activity and strength training from an early age, not only for immediate health outcomes, but also for maintaining muscular health and functional capacity well into old age.
Research into the plateau of muscle growth with strength training highlights the complex interplay of genetic, physiological and lifestyle factors that influence our ability to build and maintain muscle mass over time, and explains why muscle development can stagnate, despite continuous training efforts.
There is probably a genetic "ceiling" that leads cells to regulate their own size to ensure optimal functioning. This operating logic is reinforced by physiological mechanisms, such as anabolic resistance and the balance between anabolic and catabolic processes, which govern muscular adaptation, enabling the body to develop a muscle mass that is adapted to its motor requirements, but whose energy consumption is not too high and does not require costly additional efforts.
Starting strength training earlier in life also appears to be a strategic advantage, potentially offering a buffer against the anabolic resistance encountered with aging and enabling a higher base for muscular health to be built that can be maintained into later life.
So, the question of whether or not to reach one's full genetic potential during one's lifetime doesn't really make sense for a natural athlete. Depending on the age at which a person starts training, their regularity over the course of their life and their nutrition, the real objective is to limit the effects of time on our bodies as much as possible. It's therefore important to remember that resistance training should be viewed from a long-term health perspective. By adopting the principles of early and regular training, tailored exercise programming and adequate nutritional support, it will be easier to overcome the physiological challenges of muscle growth.
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