DMC Blog

Stretching: An In Depth Look

Stretching is a component of physical fitness that is largely neglected by many. Although it’s importance can’t be overstated enough as it is one of the most significant components in any training programme.

Stretching is the important link between the sedentary life and active life. It keeps the muscles supple, prepares the body for movement and helps make the daily transition from inactivity to vigorous activity without any undue strain. It is especially important for those involved in a specific sport or particular exercise, such as jogging or weight lifting.

These activities can increase tightness and inflexibility both in the lower and upper legs, lower back, hip flexors and buttocks. Stretching before and after training will maintain and possibly increase flexibility, whilst reducing the risk of common injuries such as shin splints and Achilles tendonitis mainly associated with runners.

Stretching makes the body feel good and relaxed when done correctly. It is peaceful, relaxing and non-competitive. The subtle, calming feelings of stretching will allow the mind to get in touch with the muscles.
Stretching should be specific to the individuals sport, muscular structure, flexibility and varying tension levels. The key to stretching is regularity and relaxation.

The goal is to reduce the tension within the muscles and thereby increasing the range of movement at a joint or group of joints. Stretching lengthens muscles and tendons, making them more pliable. Joint mobility, flexibility and range of motion. are increased, which will in turn reduce the risk of injury. It is the muscles inability to stretch that leads to loss of flexibility rather than an actual shortening of the muscle fibres.

Stretching

Increased flexibility through stretching is created by increasing the relaxation capacity of the fibres, rather than making them permanently longer. In conclusion stretching and relaxing combinations improve and increase flexibility.

Advantages of stretching:

  1. An increased range of movement at joints.
  2. Increased flexibility of connective tissues.
  3. Increased elasticity and length of muscles.
  4. Reduced tension and increased relaxation in the muscles.
  5. Increased circulation to the muscles.
  6. Improved balance and co-ordination between muscle groups.
  7. Improved posture.
  8. Improved mechanical efficiency, speed and skill.
  9. Improved technique and performance due to relaxation of the antagonistic muscles allowing the prime mover to maximise performance.
  10. If performed after eccentric work, a reduction of muscles soreness.

Disadvantages of stretching:

  1. Damage to muscles by causing micro-tears of muscle fibres, caused by sudden stretch of cold muscles or ballistics.
  2. Damage to joints caused by poor technique.
  3. Over-stretching of ligaments caused by poor technique or forcing at the end of the range.
  4. Straining other body areas due to incorrect positioning.
  5. Raising blood pressure caused by incorrect breathing.

Most people need to stretch prior to and after exercising for prevention of muscle tearing, connective tissue damage and poor performance. If the muscles and connective tissues are more pliable at the beginning of training then they will be able to perform the necessary movements without any problems, but if the muscles are tight, then it makes it harder for the body to perform certain acts which could result in tearing.

Stretching after exercise will help increase performance, but will also prevent blood pulling and stiffness of joints and muscles.

Why do we warm up?

“Exercise places additional stress on the body. As the muscles contract, circulation increases, heart rate rises and breathing rate increases as more oxygen is needed for the working muscles. Blood flow is diverted from vital organs to supply the demand. More fuel is needed for the working muscles and various metabolic processes that provide this additional energy. The by-product of these additional reactions is heat, which causes a rise of the core body and muscle temperature. As muscles contract they demand more energy in the form of ATP and require additional oxygen delivery. Greater blood supply to these muscles is provided as a result of the increased heart rate, rising stroke volume and therefore, higher cardiac output. Once again, these factors contribute to the increase in muscle temperature. Muscles can contract faster and more effective when warmer and the blood supply can release oxygen at a faster rate.”

(Dalgliesh; J; Dollery; S (2001)

Overview of a warm up:

  1. Minimising injury
  2. Maintaining performance
  3. Preparing the mind and body

Types of warm-ups:

Whatever physical activity or sport it is, there is always a warm-up that is specific to that sport, for example, a martial arts individual would not do the exact same warm-up as a footballer. This is due to the purpose of warm-ups from beginners level to advanced sports people are sports specific, which means that they are specific to that sport, as they incorporate movements and exercises needed and performed
within that activity. An example of this would be a footballer performing a lot of mobility work on their abductors and adductors before competing, whereas a hurdler would incorporate a lot of leg raising movements with simulated hurdling movements.

Whatever physical activity or sport there is, there is always a warm-up that is specific to that sport, for example, a martial arts individual would not do the exact same warm-up as a footballer. This is due to the purpose of warm-ups from beginners level to advanced sports people are sports specific,
which means that they are specific to that sport, as they incorporate movements and exercises needed and performed within that activity. An example of this would be a footballer performing a lot of mobility work on their abductors and adductors before competing, whereas a hurdler would incorporate a lot of leg raising movements with simulated hurdling movements.

What are the physiological responses of a warm-up?

  1. Increased heart rate and blood flow-> The release of adrenaline will increase heart rate and dilate (open) capillaries, which in turn enable greater amounts and an increased speed of oxygen delivery to the muscles and the heart.
  2. Increased muscle temperatures associated with exercise will facilitate enzyme activity. This increases muscle metabolism and therefore ensures a readily available supply of energy.
  3. Increased temperature also leads to decreased viscosity within the muscle. This enables greater extensibility and elasticity of muscle fibres, which ultimately leads to increased speed and force of contraction and reduces the risk of sprain and tears.

Neuromuscular responses -> Warm-ups also make us more alert, due to an increase in the speed of nerve impulses imduction to the muscles.
Joint lubrication -> Increased production of synovial fluid ensures efficient movement at the joints.

Relate your muscles to a piece of chewing gum, when it comes out the packet and is stretched it tears, but when stretched after being chewed and warmed-up, it can go to 5 times its original length.

Different stretching techniques

Static stretching – What is it?

Static stretching involves stretching to the furthest point and holding the stretch. The splits are an example of static stretching. This method of stretching is not only the safest, but also has been test proven for centuries by practitioners of hatha yoga as a way of increasing flexibility.

Advantages

  1. Is simple to learn and easy to execute.
  2. Requires little expenditure of energy.
  3. Allows adequate time to reset the sensitivity of the stretch reflex.
  4. Permits semi-permanent change in length.
  5. Can reduce muscular relaxation via firing of the GTOs if the stretch is sufficiently intense.

Disadvantages

  1. Does little to enhance co-ordination.
  2. Does not offer optimal specificity in training.
  3. Potentially impairing effect on muscle performance.
  4. Has a negative effect on active force production.

Who would use it?

Most people of all fitness levels would use this type of stretching due to lack of difficulty and muscular advantages. This type of stretching would be best suited for people just starting physical activity, but should be performed within the cool-down of a workout, as the stretches keep the body static and relaxed, which will gradually bring the heart rate down after a warm-up.

Alternatives

  1. Static-Active stretching
  2. P.N.F

Dynamic stretches – What is it?

Dynamic stretches involve putting the muscle through its full range of motion at speed, for example, doing a bicep curl fully up and down at speed would be a dynamic stretch. Kicking an imaginary football is a dynamic stretch for the hamstrings and adductors. Twisting side to side is a dynamic stretch for the trunk. This type of stretching should not get confused with ballistic stretching. Dynamic stretching is useful before starting an aerobic workout and particularly for martial arts.

Advantages

  1. Keeps body temperature elevated.
  2. Lubricates joints.
  3. Lengthens the muscles.
  4. Allows for a fluent R.O.M.

Disadvantages

  1. Needs to be trained to an individual.
  2. Sometimes allows for strict movements.
  3. Can cause muscle tears if not adequately warmed-up.

Who would use it?

Dynamic stretches would be used by sports people mainly, as it is the best type of stretch after a warm-up. This is only just coming out in the fitness industry and athletes are now being told to perform these stretches instead, as athletes in football, martial arts and racket sports are now performing these type of stretches as part of there warm-up. These are the best warm-up stretches due to there advantages. Most importantly they do not take long and allow the athlete to get straight into more detailed parts of training / competing.

Alternatives

1. Ballistics (only for martial arts, dance and gymnastics)


Ballistics – What is it?

Ballistic stretching is done using rapid bouncing movements,
to force the target muscle to elongate. This type of stretching may evoke a strong stretch reflex and leave the muscle shorter than its pre stretching length. Ballistic stretching has twice the tension in the target muscle, compared to a static stretch. This increases the likelihood of tearing a muscle, because the rapid bouncing does not allow enough time for the inverse stretch reflex to be engaged and relax the muscle.

Advantages

  1. Increases range of motion in sport specific people (e.g. dancing, gymnastics, martial arts).

Disadvantages

  1. A quick or rapid stretch does not allow significant time for the tissues to adapt, resulting in strain.
  2. A sudden jerk applied to a muscle will initiate the stretch reflex and muscle tension will increase. Further pulling
  3. against the tension may result in microscopic tears of the myofibrils. Healing will result in formation fibrous tissue, which impairs the function of the muscle.
  4. A quick stretch does not allow for neurological adaptation. Research has shown that the tension generated by a slow stretch. Therefore the tensile resistance to fast stretching is much greater.

Who would use it?

Most sports people used to use ballistic stretches and in some gyms, beginners and more experienced people use it, whereas they shouldn’t. It is mainly martial artists, dancers and gymnasts that should use this type of stretching, as they are top athletes and it is specific to their sport for them to do this type of stretching. This type of stretching will allow them to reach the very peak of dynamic flexibility.

Alternatives

  1. Dynamic stretching

Static-Active stretching – What is it?

Active stretching is accomplished using your own muscles and without any assistance from an external force. Active stretching can be divided into two major classes: free active and resistive. Free active stretching occurs when muscles produce movement without application of additional external resistance.

An example of free active stretching is standing upright and slowly lifting one leg to a 100degree angle. In resistive active stretching, the athlete uses voluntary muscle contractions to move against an applied resistance. Using the previous example, a manual resistance or weight can be applied to the leg being lifted. Active stretching is preferred when the weakness of those muscles producing the movement (agonists) restricts flexibility.

Advantages

  1. Develops active (and potentially dynamic) flexibility, which has been found to have a higher correlation with sport achievement than does passive flexibility.
  2. Easier to work into a routine as it does not require a partner or other equipment.

Disadvantages

  1. May initiate the stretch reflex.
  2. May be ineffective in the presence of certain dysfunctions and injuries such as severe sprains, inflammation, or fractures.

Who would use it?

This type of stretching would be used by most people of all fitness levels, but especially people starting out. It is very easy for people to do and doesn’t need the use of any people or equipment. It is very easy for people to do and doesn’t need the use of any people or equipment. It has an easy structure to follow, but must be done at the end of the training session.

Alternatives

  1. P.N.F.
  2. Static-Passive stretching

Static-Passive stretching – What is it?

Passive stretching is a technique in which you are relaxed and make no contribution to range of motion. Instead, an external force is created by a manual or mechanical outside agent. Passive stretching is preferred when the elasticity of the muscles and connective tissues to be stretched (antagonists) restricts flexibility and for muscles or tissues undergoing rehabilitation.

Advantages

  1. Effective when the agonist (the primary muscle responsible for the movement) is too weak to respond.
  2. Effective when attempts to inhibit the tight muscles are unsuccessful.
  3. It is preferred when the elasticity of the muscles to be stretched (antagonists) restricts flexibility.
  4. It allows stretching beyond one’s active range of motion.
  5. It provides a reserve for increasing the joint’s active mobility.
  6. Direction, duration and intensity can be measured when more advanced stretching machines and modalities are used in rehabilitative therapy.
  7. It can promote team camaraderie when athletes stretch their partners.

Disadvantages

  1. Greater risk of injury if partner applies the external force incorrectly.
  2. May initiate the stretch reflex if the stretch is too rapid.
  3. Likelihood of injury increases with greater differences between the ranges of active and passive flexibility.

Who would use it?

This would be used mainly by people in martial arts, dance and gymnastics that want to increase passive flexibility. It is also used in team sports, as the athletes will work in pairs to help increase passive flexibility.


P.N.F. – What is it?

P.N.F. is another technique that can be used to improve the flexibility of a muscle or muscle group. P.N.F. was originally designed as a rehabilitation technique. Several methods of P.N.F. have been developed and names and descriptions vary according to the source. It has been defined as the most effective way to increase static-passive flexibility. It combines passive stretching with isometric stretching in order to achieve maximum static flexibility.

  • First the muscle group passively stretched.
  • Then it is isometrically contracted against a resistance while in the stretched position.
  • There is now a short relaxation phase before the muscle is passively stretched again through the resulting increased range of motion.

The times for each stretch of the sections of the stretch vary from researcher to researcher, but it is generally agreed that the muscle should be relaxed for 20 seconds between each full P.N.F. stretch. The isometric contraction is said to increase tension in the muscle, fatiguing the fast twitch fibres and inhibiting the nerve endings that act to try to prevent the muscle stretching further. P.N.F. can be performed without a partner, but it is generally thought tom be more effective with a partner to provide the resistance.

P.N.F.->PROPRIOCEPTIVE. NEUROMUSCULAR . FACILITATION.

Advantages

  1. Increases static flexibility.
  2. Relieves delayed onset muscle soreness (DOMS).
  3. Returns muscle to normal length.

Disadvantages

  1. Needs a partner.
  2. Takes time.

Who would use it?

All sports people would use this, as it helps people of all fitness levels. It is especially good for people in re-habilitation as it doesn’t allow for the muscle to be stretched too far and still maintains flexibility.

Alternatives

  1. Static
  2. Static-Active
  3. Static-Passive

Plyometrics – What is it?

Plyometrics is a type of dynamic action resistance training based on the theory that use of the stretch reflex during jumping will recruit additional motor units. The faster a muscle is stretched or lengthened, the greater its concentric force after the stretch.

An example would be:

  1. Jump from an 18 inch box to the ground.
  2. Land with knees partly bent.
  3. Then rebound upward by a forceful contraction of your knee extensor muscles.

The above exercise shows that the muscles can briefly store the tension developed by rapid stretching so that they possess a sort of potential elastic energy. For example, after stretching an elastic band, it has the potential to go back to its original length. Plyometric loads both the elastic and the contractile components of the muscle and makes use of the stretch reflex in order to increase power.

Advantages

  1. Increases power within the muscles.
  2. Can increase performance.
  3. Keeps heart rate high, body and muscles warm.

Disadvantages

  1. Stretch reflex will kick in straight away.
  2. Can tear muscles if not adequately warm.

Who would use it?

This type of exercise would be used by sports people, where power is important to there performance, for example, basketball players, martial arts and possibly racket sports. This type of exercise should ideally be used as a type of sports performance training, not as a type of stretching. As it allows for the stretch reflex to kick in quickly and can tear if not adequately warm. Its main purpose is to shorten the gap between strength and speed, and build power, which is a combination of both.

Alternatives

  1. Dynamic stretches.
  2. Ballistic stretches (only if sport specific).

How the CNS effects the muscles during stretching

How muscles work

Anatomical factors
Brain-nerve link
The process by which muscles contract or relaxes under the control of the central nervous system (CNS).

Stretch reflex

Stretching should be slow or static. Sudden or painful movements will elicit the STRETCH REFLEX causing the muscles to contract. This response is called the STRETCH REFLEX or MYOTATIC REFLEX. This action is mediated through spindle cells in the belly of the muscle. When muscle lengthens too quickly the spindle cells are activated which cause muscle to contract. Therefore stopping overstretching of the muscle. This reflex is basically a protection mechanism. It operates to control the speed of movement at a particular joint to prevent injury to the joint and to the muscles which are stretched.

Golgi Tendon Organs (GTOs)

If the muscle stretch is over 6 seconds, it responds to change in length and tension. Impulses from GTOs are able to over ride impulses from muscle spindles.

Inverse myotatic/stretch reflex

Perhaps when you have experienced a sudden and involuntary relaxation to your muscles when stretching. This is due to the inverse myotatic reflex. The GTOs were thought to be solely responsible for this reflex, however it is believed that the GTOs along with other receptors are involved in this reflex. The GTOs thought to operate in the following manner.

When the intensity of a muscular contraction or stretch on a tendon exceeds a certain critical point, an immediate reflex occurs to inhibit the muscular contraction. As a result, the muscle immediately relaxes and the excess tension is removed.

This reaction is possible only because the impulses of the GTOs are powerful enough to over ride the excitatory impulses of the muscle spindles. This relaxation in protective mechanism- a safety device to prevent tendons and muscles from being injured by tearing away from their attachments.

Reciprocal innervation

Muscles operate in pairs. When one muscle receives an impulse to contract, the other relaxes. For example, flexing the elbow involves contraction of the biceps and lengthening of the triceps. If we did not have reciprocal innervation the muscles would be pulling against each other and no movement would occur.

Muscle spindle basics

Muscle spindles are found within the belly of muscles and run in parallel with the main muscle fibers. the spindle senses muscle length and changes in length. It has sensory nerve terminals, who’s discharge rate increases as the sensory ending is stretched. The nerve terminal is known as the ANNULOSPIRAL ending, so named because it is composed of a set of rings in a spiral configuration. These terminals are wrapped around specialised muscle fibers that belong to the muscle spindle (INTRAFUSAL FIBRES) and are quite separate from the fibers that make up the bulk of the muscle (EXTRAFUSAL FIBRES).

There are two main types of intrafusal fibre, NUCLEAR BAG and NUCLEAR CHAIN. For now we still stay with the nuclear bag fibres, so called because there is a bunch of about 100 nuclei in the central (or optional) region underlying the sensory nerve ending. Another important feature, a motor supply to the intrafusal muscle. In this region, on either side of the central area the intrafusal fibres are able to contract if their
motor supply is active. The motor comes via efferent fibres that usually fall into the gamma classification of diameters. They are often referred to as FUSIMOTOR fibres. Two things can in princaple, cause the annulospiral ending to be stretched and so increase its discharge;

  1. A stretch of the muscle as a whole will stretch the spindles within it, and thus the sensory endings.
  2. Fusimotor activity will cause contraction of the intrafusal fibres below the fusimotor nerve terminals either side of the central region. This will result in stretch of the central sensory region.

Nuclear chain fibre also have annulospiral sensory endings in the central region (the nuclei are in line). It is a shared branch of the axon that supplied the central area of the nuclear chain fibre. This sensory nerve is of group 1a, the fastest found in the body. There are sensory endings that are more closely associated with the chain fibres. They fall into the slower group 2, division of the sensory nerves are referred as SECONDARY endings in contrast to the centrally located PRIMARY endings.

The two types of intrafusal fibre (bag and chain) have different mechanical properties, and respond differently to their largely seperate fusimotor fibres. They also differ in respect to their sensory endings. Consequently, the information relayed to the CNS by the spindle via group 1a and 2 sensory endings is different.


Factors affecting flexibility

Pregnancy

Pregnancy causes the body to release the hormone called relaxin, which works to relax all the body’s ligaments, creating high susceptibility to sprains and strains. So for example, the women would be able to stretch a lot further into a stretch due to this, but this may be avoided, as they can tear and when the baby has been born, the ligaments will all be returned to normal, which can cause problems.

Osteoporosis

Common in approximately 30% of post-menopausal women, osteoporosis is decreased bone mass that may lead to compression of vertebral bodies and bone fracture. Postural problems occur. People with osteoporosis should avoid twisting and turning quickly due to the decreased bone mass. Due to fast movements affecting osteoporosis, ballistic and dynamic stretches should be avoided.

Sex / gender

“It has been suggested that females are more flexible than males, although the evidence in inconclusive. Females on the whole have lighter and smaller bones, which may influence flexibility. In the main they have shorter leg length and lower centre of gravity, making certain movements easier. They are designed for flexibility of the pelvic region to facilitate childbirth.”

(Rosser; M(1995))

Body temperature

“The elevation of body temperature increases flexibility. Therefore, warm-up exercises performed before flexibility training. Pre-heating with hot-packs, heat-packs, hot baths or showers, diathermy or massage will increases the effect. These methods may be used before warm-up but not instead of exercises. The heat reduces viscosity and relaxes tissues, which thus offers less resistance to movement. Heat also increases the extensibility of connective and muscle tissue surrounding the joint.”

(Rosser; M(1995))

Anatomical factors

Bone shape

Range of movement is dependent on bone structure. Bones are designed to create movement at a joint. Depending on which bones meet at a particular joint determines the range of movement created by that joint, for example, the ball and socket joint of the shoulder with a shallow socket permits a great variety of range of movement. Hinge joint: when the arm is fully extended, the elbow locks against the humerus, this is because the humerus meets with the radius and the ulna at the elbow joint, preventing excessive movement, which may cause injury-OLECRANON PROCESS.

Contour and size

There are situations where bone-to-bone contact stops any further movement at a joint. The joint surfaces and how they accept each other determines the range of movement at that particular joint. If the ball and socket joint, the cup like socket of the shoulder allows the spherical head of the humerus to fit comfortably to aid movement. The ball and socket is the most moveable point in the body. In contrast, in straightening the elbow, the point of the elbow comes into contact with the back of the humerus.

Joint capsule

The joint capsule is designed to prevent unnatural movement. The ligamentous sleeve that envelopes the two bone ends of a joint is called the joint capsule. The capsule is lined with a synovial membrane and lubricated by synovial fluid. The synovial fluid is a significant factor in determining the flexibility of the joint, when the synovial fluid warms, for example, during the warm-up phase before exercise it thins and the mobility at the joint capsule increases.

Ligaments

These are strong bonds of tissue with limited elasticity, as they connect bone to bone. Designed to limit movement, although they have the ability to stretch a little further after a thorough warm-up. Its main function is to bind bones closely together while allowing limited range of movement in a given direction. They are extra strong where stress and strain are most likely to occur, thus protecting the joint from damage. Together with the joint capsule, the ligaments contribute about 47% of total resistance to movement-extremely significant in determining range of movement.

Tendons

Joins muscle to bone, cartilage and ligament. Fairly rigid, inelastic structure made up almost entirely of a protein called collagen. An abnormal stretch is especially significant and if the stretch continues, it may result in injury. Like ligaments, tendons have a poor blood supply, thus the healing process is prolonged.

Connective tissue

This is tissue that connects muscle-to-muscle or muscle fibre to other muscle fibres. Found between muscle fibres and bundles of muscle. Adds strength and resistance to a muscle.

Fascia

Skin is separated from muscle and bone by 2 layers of fibrous tissue:
(1) SUPERFICIAL FASCIA->Fat filled fibrous mesh connecting skin to deep fascia.
(2) DEEP FASCIA->Dense, elastic tissue fat covers the muscle. When a stretch occurs, it is the tissue that stretches.

Viscosity of connective tissue

Therefore the viscosity (thick & sticky) of connective tissue is the resistance of connective tissue to movement or stretching. Viscosity is mainly caused by collagen in the connective tissue. Collagen is very strong, but not very flexible.

The effect of age on flexibility

Flexibility can be developed at any age, given the appropriate training; however, the rate of development may not be the same at every age for all athletes. Generally, research indicates that small children are quite supple and that during the school years-flexibility until about puberty, then increases throughout adolescence. After adolescence, however, flexibility tends to level off and then decrease. Although flexibility decreases with age, the loss appears to be minimized in those who remain active. Research has found that maturational age is measured by sexual maturity, rather than by chronological age, was better correlated with strength and flexibility in the lower extremity. However, many parents, coaches and adolescent athletes fail ‘to consider developmental variation (which) often leads to inappropriate performance expectations and an unsatisfactory sports experience’.

(Pratt(1989))

A primary factor responsible for flexibility decreasing with age is certain changes in the connective tissues of the body. Surprisingly, it has been suggested that exercise delays the loss of flexibility due to dehydration within the connective tissues. This is based on the theory that stretching simulates the production or retention of lubricants between the connective tissue fibres, thus preventing adhesions.

Other physical changes that occur with ageing and affect flexibility include:

  • Increased calcium deposits.
  • Increased dehydration in connective tissues.
  • Increased adhesions and cross-links in the connective tissues.
  • An actual change in the chemical structure of the tissues.
  • The replacement of muscle fibres with fatty and fibrous (collagen) fibres.

This has been my in depth look in to stretching. As you can see there is a lot of thought and science that can go into the process of preparing and recovering from an exercise session.

Over the years I have generally found that you have to find out what best works for you, even if evidence says otherwise at that current moment of time, and using the method(s) that give you the biggest return without causing any harm to the body.

If you have any questions about stretching, or are looking to begin a programme with us, get in touch.

“If it doesn’t challenge you, it won’t change you”,

Pablo

Reference Materials

Dalgliesh; J; Dollery; S (2001): The Health & Fitness handbook; Pearson Education Limited.
Rosser; M (1995): Body fitness & exercise; Hodder & Stoughton.
Delavier; F (2001): Strength training anatomy; Human kinetics.

https://content.byui.edu/file/a236934c-3c60-4fe9-90aa-d343b3e3a640/1/module9/readings/somatic_reflexes.html

https://wikigait.wordpress.com/tag/inverse-myotatic-reflex/

Catalyst university YouTube channel

http://www.axishospital.co/blog/osteoporosis/

https://teachmeanatomy.info/the-basics/joints-basic/synovial-joint/

https://www.britannica.com/science/ligament

https://telmagrant.com/10-facts-tendons/

https://courses.lumenlearning.com/nemcc-ap/chapter/connective-tissue-supports-and-protects/

https://deeprecovery.com/understanding-fascia/

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