If you read my first article on this topic, you have a good idea of how your body adapts to get stronger. There are both morphological as well as neurological adaptations.
So what exactly are these ‘morphological’ and ‘neurological’ adaptations to strength training? And most importantly, how much of an effect does each adaptation actually have on strength?
The goal of this article is to summarize the current research on adaptations to strength training, and how much of a relevant contribution each make (the “so what” of each mechanism). Why? Because people hear about these mechanisms, and design entire exercise programs around them. As with most things, people need to step back and look at the big picture. So here is a quick review, based on a comprehensive review article I recently read.
This is a dorky article, and most people don’t need to read it. But if you like this stuff, read on. Note that the content of this article may change over time as I read new research and elaborate on particular points.
The review article I am using the skeleton of my article: Folland, J. P., & Williams, A. G. (2007). The adaptations to strength training – morphological and neurological contributions to increased strength RID B-6817-2008. Sports Medicine, 37(2), 145-168. http://www.ncbi.nlm.nih.gov/pubmed/17241104
These are changes in the size, shape and structure of the muscles themselves.
Evidence for these adaptations:
Changes in whole muscle size: an increase in the anatomical cross sectional area (ACSA) is generally thought to be the major adaptation to resistance exercise. This is studied mainly through biopsies. However, the amount to which this increase occurs is highly variable between the muscles trained and even along their length.
Muscle fibre hypertrophy is regarded as the primary mechanism of increased CSA, through myofibrillar growth and proliferation. This is the increase in contractile material in existing muscle fibres, and is thought to have a large effect on increasing strength and size.
Satellite Cells are activated early in training. They proliferate and attach to existing myofibres. This increases the amount of myonuclei available to support the myofibrillar growth.
Hyperplasia (formation of new muscle fibres, as opposed to growth of existing fibres), is currently debated in the literature as existing to any significant degree.
Muscle hypertrophy seems to be greater in type IIa and IIb fibres. These are larger than type I muscle fibres, and likely has a significant effect on increases in muscle size.
Changes in fibre type. This is often quoted in the fitness world: Your muscle fibre types actually can change. However, the effect of this is subtle in the early phase (2-3 months) with no evidence that it continues. It seems to effect only hybrid muscle fibres (type I/IIa and IIa/IIx) and shifting them up to type IIa.
Physiological cross sectional area (PCSA) can actually change as well, with such examples as increased angle of pennation of muscle. I don’t quite understand how this happens, but it appears to be a significant contribution. I suppose when muscles get wider they increase their angle of pennation.
Muscle fibre density may be a factor, but there is currently no conclusive evidence for this actually occurring in response to training. Same conflicting results for the idea that there is an increase in the force production of the intrinsic contractile elements (no increase in ‘strength per muscle fibre’).
Tendon and connective tissue may grow in density, however, the effect on muscle strength and size is unclear.
Essentially, neurological changes are learning and coordination. These changes result in things like increased recruitment and activation of muscles, and are quite task-specific adaptations. Generally, they have a greater effect at the beginning of a program, but are never really as substantial as morphological changes. This shouldn’t really be a surprise since we are talking about strength here, not skill.
Specificity of adaptations. This implies that muscles are coordinated by the nervous system specifically to improve strength. This effect seems significant at first, less later.
Cross-over effect. Another highly referenced effect, where exercising one side of the body will improve strength in the other side. It is debated whether this is really increased activation or strength, but rather improved coordination and stabilization of your body during the task learned. Either way, this is only somewhat significant, again earlier in training.
Imagined contractions. These can actually have a significant effect, especially in the lower body (ambulatory) muscles.
Increases in Agonist Activation: Interestingly, evidence is increasing that untrained, healthy subjects are unable to fully activate their muscle fibres. How much this can be improved upon seems to depend on the muscle groups trained, type of contraction, the joint positions involved, and the complexity and novelty of the task.
Firing frequency: The evidence that this increases strength is unclear. It may, however, cause muscles to reach their highest level of force production faster during a movement.
Synchronization: this is the ability to activate the motor units of a particular muscle in sync to generate more force. Again, the theory as well as the evidence here is underwhelming.
Cortical adaptations: It seems there is increased activity in the primary motor cortex during low force muscle activity. However, in response to strength training, the cortico-spinal excitability seems to decrease. The authors of this paper assert this to be contradictory findings in light of the changes stated below (spinal). However, I contend that with training, cortical projections may simply require less excitability to influence lower motor neurons, as the reaction to this excitement may become potentiated in the spinal region. Thus, it still counts as a neurological adaptation leading to increased strength. But what do I know.
Spinal Reflexes: Excitability in the motor neuron pool of a muscle seems to increase with training. The evidence isn’t clear, however.
Antagonist Co-activation: When a movement occurs agonists and antagonists contract simultaneously, with the sum force going the the direction of the agonists pull. It has been shown in more complex tasks that the antagonist may decrease in activity, leading to increased strength. This effect isn’t as obvious in single joint movements. Again, the research is unclear.
A sentence that really stood out to me in the conclusion of this review article was: “The rapid rise in strength at the start of a training programme, within the first 2 weeks, which is primarily due to neurological adaptations, significantly increases the loading and training stimulus to which the muscle is then exposed.”
This is something I have thought about for a long time. If there is a learning/neurological phase at the beginning of a program, then are you really loading your muscles enough to cause them to adapt? If you are switching up your program too often, then are you basically resetting your learning curve every time, and never really getting to that muscle building phase? Yes, you are exercising, and learning movements, but are you really strengthening the muscles?
Effectively building muscle (which has many physiological/metabolic benefits, as well as possibly being the only way strength training can have generalizable, transferable benefits to other movements involving that muscle) should then require you to minimize or quickly transcend the ‘learning’ phase, so you can effectively tax the muscles. This is another reason why I believe strength training should be done with very simple, safe, multi-joint exercises that mimic highly generalizable movements (squatting, pushing, and pulling in a few different directions).
Anyway, I digress…
Hope you found this article informative. It’s dry right now, but I hope to build upon it as time goes on.