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Current research indicates that eccentric exercises are effective in promoting muscle hypertrophy and various physiological adaptations. However, the evidence for hyperplasia in humans remains limited and inconclusive. Further studies, particularly those involving direct measurement of muscle fiber number, are needed to determine whether eccentric exercises can induce hyperplasia in humans.
Eccentric exercises, characterized by muscle lengthening under tension, have been widely studied for their effects on muscle hypertrophy and physiological adaptations. While hypertrophy, the increase in muscle size, is well-documented, the potential for hyperplasia, the increase in the number of muscle fibers, remains a topic of debate. This article explores whether eccentric exercises can induce hyperplasia in humans by reviewing current research findings.
Eccentric vs. Concentric Muscle Actions
A systematic review and meta-analysis compared the hypertrophic effects of concentric (muscle shortening) and eccentric (muscle lengthening) actions. The study found that eccentric muscle actions resulted in a greater effect size for muscle growth compared to concentric actions, although the results did not reach statistical significance. The mean percent change in muscle growth favored eccentric actions (10.0%) over concentric actions (6.8%)1. This suggests that eccentric exercises are more effective in promoting muscle hypertrophy, but the evidence for hyperplasia remains inconclusive.
Physiological and Neural Adaptations
Eccentric exercises are known to induce various physiological and neural adaptations. These include muscle hypertrophy, increased cortical activity, and changes in motor unit behavior, all contributing to improved muscle function2. However, the literature primarily focuses on hypertrophy rather than hyperplasia. The initial muscle damage and subsequent repair processes associated with eccentric exercises could theoretically stimulate hyperplasia, but direct evidence in humans is lacking.
Molecular Mechanisms
Research on animal models provides insights into the molecular mechanisms that might support hyperplasia. A study on rats demonstrated that acute eccentric exercise activates hypoxia-inducible factor (HIF)-1α, which in turn upregulates vascular endothelial growth factor (VEGF) and endothelial nitric oxide synthase (eNOS) expression. These factors are involved in angiogenesis and muscle adaptation3. While these findings highlight the complex molecular responses to eccentric exercise, they do not directly confirm hyperplasia in human muscles.
Can eccentric exercises cause human hyperplasia?
Chantal Coles has answered Unlikely
An expert from Murdoch Childrens Research Institute in Cell Biology, Immunology, Proteins, Stem Cells
The number of skeletal muscle fibres are fixed at birth so myofibre hyperplasia is thought to occur only during embryonic development and very early in post-natal development. During post-natal development these myofibres grow in size through a process called hypertrophy where the size of the myofibre increases to cause an increase in muscle size. This can happen from two processes 1) increase in protein synthesis and 2) addition of myonuclei from satellite cells – what we refer to as skeletal muscle stem cells. Eccentric muscle contractions are known to cause myofibre hypertrophy mainly through an increase in protein synthesis but also through activation and recruitment of satellite cells (muscle stems cells). The activation of satellite cells predominantly occurs following muscle damage when regeneration is required. There are eccentric studies that have not shown activation of satellite cells following eccentric muscle contraction. It is unlikely hyperplasia is involved.
Can eccentric exercises cause human hyperplasia?
Simon Walker has answered Unlikely
An expert from University of Jyväskylä in Exercise Physiology, Biomechanics
There are two theoretical mechanisms through which a muscle could increase its size; 1) hypertrophy, which is the increase in size of a single muscle cell (aka myofibril) or 2) hyperplasia, which is the increase in number of muscle cells.
Hyperplasia occurs during fetal development, but seemingly ceases at birth or shortly after birth. Hence, the total number of muscle cells that a person has is largely genetically predetermined. If a person has more muscle cells within a muscle, then they also have a great potential for overall muscle growth, assuming that muscle cells can all grow to a similar maximum size (which is not true in reality). Therefore, if hyperplasia occurs post-development it would be advantagous to increase or maintain muscle mass, particularly in cases where there is a loss of muscle cell number, such as aging or certain diseases.
Hyperplasia has been shown to occur during loaded stretching in animals. For example, hyperplasia of muscle tissue has been shown in adult birds that had weights attached to their wings. However, in adult humans, hyperplasia has not been demonstrated robustly despite interest in the topic in the 1980s and 1990s. Counting muscle cell number is of course extremely challenging, since we cannot remove a whole muscle from a living human.
In summary, we cannot know for certain whether hyperplasia dose or does not occur, but the evidence suggests that it is NOT the cause of muscle growth in adult humans. If hyperplasia was to occur in adult humans, then (based on the findings in animals) a likely stimulus for this could be the extremely heavy loads used during eccentric exercise.
Can eccentric exercises cause human hyperplasia?
Matt S Stock has answered Likely
An expert from University of Central Florida in Musculoskeletal Science, Exercise Physiology, Motor Learning
There is certainly evidence to suggest that skeletal muscle hyperplasia is a mechanism which explains changes in whole muscle size in animal models. The data in humans is a little less clear and likely will remain so, simply because of the difficulty in studying this topic. Indirect evidence of hyperplasia has been demonstrated in studies comparing the number of muscle fibers in bodybuilders vs. controls.
Excellent reference: https://www.ncbi.nlm.nih.gov/pubmed/8107539