The Deep-Dive Series: How We Get Stronger
- June 14, 2018
The human body is designed to adapt to the load that’s put on it- to get stronger, we simply need to take advantage of the mechanisms ingrained in our biology.
Our body is constantly breaking down tissue and rebuilding it. Gaining strength comes down to increasing its rate of building tissue, to make it surpasses its rate of tissue breakdown, thereby creating a positive balance.
The reason exercise and nutrition get so much emphasis in a health regimen is because, together, they help us achieve this positive balance. Exercise breaks down our body’s tissue and stimulates it to rebuild as a stronger structure. The nutrients from our food complement this process by providing the materials (or, sometimes, even the stimulus) needed for the rebuilding.
Read on to take a deep dive into these processes and discover their path to a stronger, fitter body.
The force generated by a muscle cell
When we talk about the strength of our ‘muscles’, we’re mainly referring to one specific type of muscle: our skeletal muscles, which are attached to our skeleton and are under the voluntary control of our nervous system.
Skeletal muscles are one of the two forms of ‘striated’ muscle tissue (the other being cardiac muscles). If you were to look at a skeletal muscle cell, a.k.a. a muscle fibre, through a microscope, you would see that it’s striped. Those stripes are composed of myofibrils: parallel, thread-like structures that essentially control the power generated by a muscle cell through their proteins, actin and myosin.
When a muscle cell is activated by our nervous system, actin and myosin kind of slide past each other, contracting the cell and generating a ‘power stroke’ – that’s what generates force. The total force exerted by a person is the sum of all the power strokes simultaneously taking place in all their muscle cells.
Building our strength primarily comes down to increasing the size of these muscle cells and adding more actin and myosin for stronger power strokes.
Increasing the size of a muscle cell
When we perform strength-training exercises, the stress placed on the involved muscles creates microscopic tears in them. These are seen as sites of injury by our immune system, which sets the repair process in motion.
In this, immune cells flow to the sites of repair in waves.
First come the mast cells and neutrophils, which help clear up the debris from the muscle’s microtears and even produce ‘cytokines’ – chemical messengers that play an important role in the rest of the immune response.
One of the many functions of cytokines is to bring in pro-inflammatory compounds, to create inflammation around the microtears and increase the permeability of blood vessels, allowing blood to flow into the area and bring in more immune cells.
As the blood’s flow increases, the second wave of immune cells comes in, made up of T-cells (part of our adaptive immune system) and macrophages, to clear up damaged cells and produce more cytokines.
We need these many cytokines because they activate and direct the actions of satellite cells: precursor cells, found on the outer surface of a muscle fibre, which ultimately repair our damaged skeletal muscle.
Satellite cells are dormant in undamaged muscle cells, but when activated by cytokines, they turn (or “differentiate”) into a specialised cell type called myoblasts.
Myoblasts begin making proteins (from amino acids brought in by the bloodstream), including myosin and actin, and begin to take on the characteristics of a muscle fibre. (This period lasts for about 48 hours after the muscle tear, which is why it’s so important to consume protein during this period after exercising.)
Eventually, the myoblasts begin to form lines and fuse into the muscle fibre, donating their contents as they do, thereby repairing the damage and adding to the size of the muscle fibre and its components. (During these final stages of repair, the third wave of immune cells comes in, to reduce the inflammation through anti-inflammatory compounds).
The extent to which a muscle fibre can grow is influenced by the presence of testosterone and growth hormones, both of which increase in production when we exercise.
Being the male sex hormone, testosterone is produced in significantly greater amounts in men, which contributes to the differences in body weight and composition between men and women. That’s also why women need not worry about bulking up like men when they work on increasing their muscle mass!
While increasing the size of our muscle cells, technically called ‘hypertrophy, is the main driver of increased strength, there are other ways through which our body gets stronger as well.
Considering our skeletal muscles are under the control of our nervous system, it makes sense that our strength can also be controlled via neural changes.
Neural adaptations to strength training
The more we perform strength-training exercises, the more our body ‘learns’ the movements. It’s something referred to as ‘muscle memory’.
We know that our muscles are controlled by the nervous system. This happens through the neurons coming from our spinal cord, which connect to our muscle fibres and stimulate them to move.
One neuron makes contact with an average of 150 skeletal muscle ﬁbres. Together, the neuron and its connected muscle fibres are called ‘a motor unit’. All the muscle fibres of a motor unit always contract in unison.
When we perform strength-training exercises, our body improves its ability to simultaneously stimulate an increasing number of motor units in the muscles being used. Simultaneous contracting more motor units means more power strokes generated by the given muscle.
Another way we grow stronger through exercise is via the tissue that connects our muscles to our bones. It stems from a law that outlines how our soft tissue responds to stress.
The integrity of our skeletal muscles is maintained by a network of connective tissues that surrounds them.
Certain connective tissue envelopes an individual muscle fibre, while another type wraps around a fascicle, i.e., a group of fibres, and yet another connective tissue surrounds these groups of fascicles. When our muscles grow, they all respond to the stress by stretching out to provide more support.
Davis’s Law states that the connective tissue will adapt to both mechanical strain and to the lack of it. When put under tension, the cells within these will “feel” the stretch and respond by making new material. (The opposite is also true, where they will shorten during periods of inactivity.) As more connective tissue is made, the support provided increases accordingly.
The final effect of our strength-training is on our bones and cartilage.
Enhancing bone strength
The idea of enhancing our bone strength through exercise is based on Wolff’s law, which states that the bones of a healthy person will adapt to the loads they face.
When we exercise, the pressure on our bones increases the activity of cells called osteoblasts, which are designed to form new materials for the bones (including collagen and a form of calcium called ‘hydroxyapatite’), making them denser and stronger.
Periods of inactivity, on the other hand, eliminate stress on the bones. For instance, astronauts in space face no pressure on their bones through gravity and can end up losing as much as 1% of their bone mass every month they’re in space! Prolonged bedrest or long periods of inactivity can have a similar effect.
Although the effects of exercise are better understood on the bones, rather than on the cartilages, we do know that joint motion and pressure are important for the structure and function of the cartilage, and that inactivity leads to its degradation
Factors that affect strength gain from exercise
When the body is stimulated to repair and strengthen its structures, it requires the raw materials to do, specifically:
Consuming protein allows the body to produce muscle proteins and increase our muscle mass. The amino acids that have specifically been found to be involved in this process are:
– Branched Chain Amino Acids (BCAAs): these play a role in the production of energy during exercise, and set off muscle protein synthesis.
– Creatine: This also plays a role in energy production, and may even improve the functioning of our muscle cells.
Muscle growth can occur only if the amount of muscle protein created exceeds the amount that’s broken down, which is why we need a positive muscle protein balance. It’s also important to keep an eye on our protein intake because amino acids even make other structural proteins, like collagen in our bones and connective tissue.
B] Carbohydrates and fat
These are recognised as the main materials needed to power prolonged muscle contractions during endurance exercises. However, the kind of carbohydrates and fats we eat also makes a difference. For instance, polyunsaturated fatty acids (PUFAs) such as omega-3 fatty acids help resolve inflammation and help keep muscle tissues healthy. Excess refined carbohydrates, on the other hand, have been found to compromise the way our bones absorb and retain calcium. Adding dietary fibre to one’s diet has been found to increase one’s calcium absorption.
Here are examples of how vitamins and minerals from the food we eat play their own roles:
B-vitamins are all about energy production, building muscle and forming the oxygen carrying red blood cells.
Vitamin C helps synthesise collagen (and is a potent antioxidant).
Deficiencies of zinc, magnesium and potassium inhibit a muscle’s growth and protein synthesis.
Calcium, phosphorus, magnesium, fluoride, manganese and vitamin K help form and strengthen our bones.
Not only does vitamin D help build our bones, but it also affects our muscle’s ability to contract. As we age, the number of vitamin D receptors in our muscle fibre decreases- this is believed to be a contributing factor to the reduced muscle strength of the elderly.
Lean muscle mass generally contributes up to 50% of a young adult’s total body weight, but declines to 25% during the ages of 75-80. This degenerative loss of muscle is called sarcopenia, and is accompanied by the loss of muscle strength. There is also a 1% decline in our BMD (bone mineral density) every year after we hit 30 years, and the ability of our body to make collagen also declines with age.
3] Hormonal balance
Both muscle protein synthesis and muscle hypertrophy are regulated by our hormones, which guide the growth of muscle tissue and even dictate how energy is used for contractions – without which force cannot be generated.
Muscle atrophy (the opposite of hypertrophy) can be caused when a person has lower levels of testosterone, oestrogen and growth hormone – or excessive levels of certain steroid hormones like glucocorticoids, and thyroid hormones.
Insulin, another hormone, facilitates the entry of glucose into our muscles, and seems to limit muscle protein breakdown after we exercise.
Genetics may predispose an individual’s ability to gain muscle and bone tissue in response to an exercise regimen.
Our body needs to repair the exercise-induced damage to our muscles and other tissue of the body, which can be compromised by inadequate sleep- as can our hormonal balance. Sleep can also affect our body’s ability to push itself, and even the perception of pain, all of which have a direct effect on exercise.
A note on exercise intensity and sustainability
After reading about the effects of exercise on our body, it may sound like we need to keep exercising as much as we can to maintain our strength- but there’s a limit to how much our body can take.
When the load we put on our body exceeds its ability to adapt, injuries can occur- and you should know that muscular tissue has a relatively poor ability to renew its lost cells.
Even though we have satellite cells to replace damage, they do not differentiate and divide rapidly enough to replace extensively damaged muscle ﬁbres. That’s when our muscles make fibrous scar tissue, which is less resilient and lacks mechanical and functional integrity.
Although bone tissue repairs are resilient, bone remodelling takes approximately 3 to 6 months to complete, depending on the bone. The inactivity during an injury would lead to the atrophy of muscle, bone loss, and the shortening of connective tissue. That’s why it’s best to keep up an exercise regimen that we can sustain without pushing ourselves to the point of injury.