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First Thing You Need To Know


The nervous system
·         Central nervous system (CNS).
·         Peripheral nervous system (PNS).

Recruitment and the ‘all or none law’
According to the ‘all or none law’ the impulse sent down the neuron can’t gauge the intensity of the contraction it will stimulate. It will either activate a contraction or not.
To vary the intensity of a muscle contraction, different numbers of motor units must be stimulated at varying frequencies.
Motor unit recruitment is the process of stimulating all the muscle fibres connected to the motor neurons that the central nervous system wants to activate. Different numbers of motor units need to be recruited to achieve different levels of force.
Muscle fibres innervated by motor neurons contract with full force; the impulse sent down the neuron can’t gauge the intensity of the contraction it will stimulate – only whether or not it will activate a contraction. This is referred to as the ‘all or none law’.
So changing the amount of force or the duration you would like your muscles to contract depends on the number and frequency of motor units recruited or activated. As more units are recruited, and as the frequency of stimulation increases, muscle tension increases.
Many motor units can be stimulated at the same time to achieve a short, sharp, strong contraction or perhaps in an alternating sequence over a longer period of time for a less intense, longer contraction.
Which of the following would increase the intensity or duration of a muscular contraction?
Consider the options and select all that apply.
Recruit fewer motor unitsStimulate the motor neuron more frequentlyDecrease the frequency of motor neuron stimulationRecruit more motor units

Activation of more motor units produces a greater force. Larger motor units will contract along with the small motor units until all muscle fibres in a single muscle are activated and the maximum muscle force is produced.
Consecutive stimuli on the motor unit from the motor neuron produce a greater force than a single contraction. Decreasing the interval between the stimuli allows the muscle to produce a larger force with the same amount of motor units.
Act as sensors in the muscles and connective tissue of the limbs.
Provide feedback on the impact of the immediate environment on our musculoskeletal system.

Proprioceptors act as sensors in the muscles and connective tissue of the limbs. They provide feedback on the impact of the immediate environment on our musculoskeletal system, such as joint angle, muscle length and muscle tension. This sensory input is integrated to give information about the position of the limbs.
With this information we can respond to our environment with the most appropriate kind of muscular reaction, such as recruiting more motor units to move in a new direction or with a different amount of force.

Stretch reflex
Golgi tendon organs: tell us about muscle tension.
Muscle spindles: tell us about muscle length.
There are different types of proprioceptor that provide different sensory information for the nervous system to respond to.
We have Golgi tendon organs which tell us about muscle tension and we have muscle spindles which tell us about muscle length.
Muscle spindles sense changes in muscle length and respond by causing muscular contractions. If a muscle lengthens and activates the muscle spindle proprioceptor the nervous response is to contract that same muscle to shorten it back up. This response is called the ‘stretch reflex’.
When we move, we often take advantage of the stretch reflex by lengthening and reactively shortening our muscles in repetitive patterns; walking and running are perfect examples of this – where our muscles lengthen and shorten repetitively for extended periods of time.
Reciprocal inhibition
When a muscle contraction is activated the antagonist muscle group must be inhibited to allow for the contraction.

Opposing muscles groups work in synergy with each other. When a muscle contraction is activated, the antagonist muscle group must be inhibited to allow for the contraction to happen. This process is called reciprocal inhibition.

Neuromuscular adaptations to exercise
Cardiovascular training causes adaptations in the neuromuscular system in relation to type 1 muscle fibres. These changes include: increased size and number of mitochondria, increased oxygen delivery to muscle fibres, increased aerobic enzymes in the muscle tissue, a greater supply of glycogen and triglycerides for energy, and improved tissue tolerance and resistance to fatigue.
Resistance training and weighted workouts predominantly cause adaptations in the neuromuscular system in relation to type 2 muscle fibres. These changes include: increased thickness or diameter of recruited muscle fibres, increased force production capacity of the muscle fibres, decreased nervous inhibition, and increased tissue tolerance and resistance to fatigue under high-stress, anaerobic conditions.

There are a number of changes that occur within the neuromuscular system as a result of exercise and training. Adaptations in the neuromuscular system can have dramatic effects on the body and can be seen in improvements in speed and acceleration, coordination, strength, endurance and most aspects of fitness. Most types of training affect the nervous system and, as it’s the controlling communication network of the body, if you can develop your nervous system your muscular system will follow suit and overall performance will improve.
The endocrine system
Hormones: The chemicals released into the bloodstream to help control and manage the internal environment of the body.
Endocrine system: The technical name given to our hormonal controls.
Hormones are chemicals that are released into the bloodstream to help control and manage the internal environment of the body. The technical name given to our hormonal controls is the ‘endocrine system’. Hormones are released from various glands around the body known as the endocrine glands.
To make sense of this complex system to we need to look at how a hormone functions, look at the major endocrine glands and where they’re located, and then finally explore the key hormones that play a role in health and exercise.
How hormones work
Endocrine function:
1.   Endocrine gland receives stimulus.
2.   Gland releases chemical hormone.
3.   Hormone travels in bloodstream.
4.   Hormones received by target cell receptors.
5.   Cell stimulated to cause desired response.
6.   Feedback to the originating gland.

Endocrine function:
An endocrine gland receives a stimulus that initiates a specialised chemical called a hormone into the surrounding bloodstream.
The hormone is transported around the body within the bloodstream seeking out target cells.
Once the hormone reaches a target cell it docks within the cells receptor site initiating the desired response.
A feedback loop between the targeted tissue or organ and the originating endocrine gland help to reduce or stop hormone production.
It is not uncommon to find that there may be multiple hormones involved before the final cellular or glandular response is achieved. For example, the hypothalamus in the brain releases a hormone that stimulates the pituitary gland, which in turn releases a hormone that stimulates the adrenal gland, which then releases cortisol (a hormone that stimulates the body in many different ways to manage stress).
Feedback loop between the targeted tissue or organ and the originating endocrine gland help to reduce or stop hormone production.
Endocrine glands
The majority of the endocrine system and its related functions are governed by a number of specialised glands that are scattered throughout the body from the brain to the reproductive organs. These glands produce the hormones that help maintain the delicate internal environment of the body.

Pituitary gland: this is often considered the master gland, as many of the early endocrine outputs originate from here.
Thyroid gland: this is considered to be the master regulator of metabolism.
Adrenal glands: these are named after their location on top of the renals (or kidneys). These small glands help primarily control and manage the stress response in the body.
Pancreas: this sits below the stomach and helps in the control of carbohydrate metabolism.
Ovaries: these are located in the lower female abdomen on either side of the uterus. They are responsible for numerous functions of female sexuality.
Testes: these are located in the scrotum and are responsible for numerous functions of male sexuality.
The pancreas
Insulin control:
1.   Food ingested leads to rise in blood glucose.
2.   Pancreas releases insulin which drives glucose into the cells.
Glucagon control:
1.   Long periods between meals or prolonged exercise may lower blood glucose levels.
2.   Pancreas releases glucagon which causes liver to break down glycogen.
3.   Glucose from liver glycogen restores blood levels.

The pancreas produces two hormones called insulin and glucagon. They both help to regulate blood glucose.
Insulin: Rising blood glucose, as a result of food intake, signals to the body that the pancreas needs to release insulin. Insulin signals for cellular channels to open and allow the glucose to move out of the blood and into the cells for use. This results in a reduction of blood.
Glucagon: When an individual has gone for a long period without food or has engaged in prolonged activity, glucose levels may drop. The pancreas releases glucagon which stimulates the liver to break down glycogen into glucose. This release helps to restore flagging blood glucose back to more stable levels.
Insulin is responsible for driving glucose into adipose tissue and so plays a role in contributing to fat cell storage. Insulin is also responsible for driving proteins into cells. So protein-rich foods also stimulate an insulin response; though not as strongly as carbohydrate-rich foods.
Sex hormones
Ovaries: release oestrogen, which influences fat storage around the hips, buttocks and thighs.
Testes: release testosterone, which plays a part in stimulating the growth of muscle tissue.
The sexual glands in males and females release important hormones that bring about the physical changes that are typically witnessed during puberty. In females the ovaries release oestrogen and in males the testes release testosterone. These hormones also support health during adulthood. They govern vital functions related to growth, repair and storage in the body.
Testosterone in both males and females plays a part in stimulating the growth of muscle tissue, whereas oestrogen influences fat storage around the hips, buttocks and thighs. Both hormones also play a role in directing bone growth during puberty and in adulthood. The correct balance of sex hormones is necessary for health, as a teenager and as an adult.

Adrenal hormones

The adrenals are small glands that sit on top of the kidneys. They are divided into an inner and outer layer. The inner layer (or adrenal medulla) produces a category of hormones called the ‘catecholamines’. The outer layer (the adrenal cortex) produces a category of hormones called the ‘corticosteroids’.
The adrenals are well located on top of the kidneys, as they have a direct link to the major blood vessels of the body. This allows their valuable hormones to get out into circulation and into effect very quickly.
Adrenaline is the most well known hormone in the catecholamine category. Adrenaline and noradrenaline are released from the medulla when life necessitates a typical ‘fight-or-flight’ response; they are the hormones of action as they drive heart rate, blood flow, and breathing and alertness levels upwards preparing the body very rapidly for activity.
The primary corticosteroid released from the adrenal cortex is called cortisol. Cortisol helps provide needed reserves in the body to manage stress, whether mental, physical or emotional. Cortisol is primarily catabolic and helps break down carbohydrates and fats to provide energy for the body during stressful periods. It offers support during short-term bouts of stress. Long-term chronic stress and the resulting excess cortisol can lead to deterioration in health by unbalancing the endocrine system.
Aldosterone is another of the corticosteroids released from the adrenal cortex. It is important in maintaining the water balance in the bloodstream, as it helps regulate the sodium/potassium balance in the blood. These minerals play a major role in water balance.
Growth and thyroid hormones
Growth hormone:
·         Released directly from the pituitary.
·         Stimulates bone growth, protein synthesis and fat burning.
Thyroid hormones:
·         Released from the thyroid gland after stimulus from the pituitary.
·         Controls cellular energy production, body temperature and metabolic rate.


The remaining two hormones we’re going to look at are primarily driven by the pituitary gland. They are the ‘growth hormone’ and the ‘thyroid hormones’.

Growth hormone is released directly from the pituitary gland and stimulates growth in all areas of the body. In particular, growth hormone drives bone growth during puberty, but it also stimulates protein synthesis in muscle tissue and helps break down and release fat tissue from storage sites around the body for oxidation.

The thyroid hormones are not released directly from the pituitary, but from the thyroid gland located in the upper chest. But, it is secretions from the pituitary that stimulate the thyroid gland to release important thyroid hormones. They help to regulate several processes; including the use of oxygen in producing cellular energy, the maintenance of body temperature and overall metabolic rate.

Both growth and thyroid hormones have a very broad influence on the way the body operates and maintains good health. So it’s vital that they are kept at the correct levels in the body.

Glands and hormones
Pancreas: The pancreas releases glucagon when blood sugar (glucose) levels fall too low.
Adrenal cortex: Cortisol increases blood sugar and aids fat, protein and carbohydrate metabolism.
Pituitary gland: Growth hormone stimulates growth in all areas of the body.
Adrenal medulla: Adrenaline regulates heart rate and is crucial for the fight-or-flight response.

FYI : Cortisol helps break down and release glucose and stored fats for use under stressful conditions such as exercise. Glucagon helps release stored glucose from the liver if blood glucose drops too low. Low blood glucose can often occur during exercise, especially if adequate food has not been ingested prior to the activity. The other hormones may all justifiably have some involvement around exercise, but they are not directly responsible for the release of stored energy.

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