Physiology

Abduction is caused by muscles moving a limb away from the imaginary centre line which splits the body into halves (left and right or the anteroposterior or sagittal plane) - think of abduct or to take away as in kidnap.
Adduction brings a limb back in to the centre. Raising an arm or leg to the side involves abduction and returning the limb inwards is adduction, a controlled movement against the effects of gravity. Both abduction and adduction can take place vertically or horizontally (movement of the arms at the shoulder either forwards or backwards).
Agonist or prime mover muscle contracts and moves a limb either in flexion or extension.
Antagonist muscles move in the opposite direction to agonists and are usually attached to the same joint so if the agonist flexes a limb then the antagonist will extend the same limb and vice versa.
Circumduction is the movement of a bone the end of which describes a circle is called (ulna or radius bones of the forearm below the elbow).
Depression is lowering a limb against gravity.
Dorsiflexion is raising the toes and the foot towards the tibia (shin bone).
Elevation is the opposite of depression.
Eversion is the movement of the sole of the foot outward at the ankle.
Extension occurs when straightening a joint or increasing the angle between two bones. The flexion examples are simply reversed.
Flexion is bending and causes the angle between two bones to be decreased: at the trunk by bending forwards or sideways (lateral flexion), bending the leg or arm, moving the thigh forwards at the hip and movement of the arm or the shoulder forwards.
Inversion is the movement of the sole of the foot inward at the ankle.
Plantarflexion is the opposite of dorsiflexion, pointing the toes away from the tibia.
Pronation is the rotation of the forearm and hand to the palms-down position..
Rotation is the movement of a part around an axis.
Supination is the rotation of the forearm and hand to the palms-up position.
Synergist muscles work with the prime mover and are normally associated muscles which stabilise or protect a joint.

It is important to recognise the difference between muscle, tendon and ligament. Muscles within the body can be stretched by more than 30% of their normal length within the body. Both ligaments and tendons are made from collagen and are only slightly elastic. Collagen is very strong and a 1 mm diameter cord can support a 10kg weight. A tendon is a structure that attaches a muscle to a bone in a moving joint and are white, glistening, fibrous cords and vary in length and thickness. They are sometimes round and sometimes flattened and have considerable strength, but cannot contract and have little elasticity. Any extension beyond 4% of their length causes irreversible deformation.

The fibres are arranged into wavy bundles which allows motion until the slack of the bundles is taken up. Tendons are sparingly supplied with blood vessels, the smaller ones having none. Nerves are present only in the larger tendons like the Achilles tendon.

Ligaments connect bone with bone in moving joints and are similar in appearance to tendons being white, shining silvery fibrous tissue and are placed parallel with, or closely interlaced with, one another. A ligament is pliable and very flexible and offers perfect freedom of movement and is strong, tough and only slightly elastic so does not yield under the most severely applied force. It is well adapted to serve as the connecting medium between the bones, but any lengthening of more than 6% will cause a tear.

When children stretch, the muscles contract less powerfully and the ligament can lengthen a little. An age related increase in the rigidity of the collagen fibres makes movements which elongate ligaments more hazardous. On the other hand some ligaments consist entirely of yellow elastic tissue which connect adjacent arches of the vertebrae. In these cases the elasticity of the ligament is a substitute for muscular power. Reference to elasticity in this context does not mean stretching capacity. Stretching is about increasing the length permanently. Elasticity simply means an elastic property, being able to return to the same length.

Connective tissue forms the sheath around muscle fibres. It also stretches and grows as the muscle lengthens. Anabolic steroid abuse can cause the muscle growth to be faster than the connective tissue modification with a resultant tearing of the muscle.

There is a balance of power between extended and flexed muscles. The biceps/triceps and quadriceps/hamstrings are all agonist/antagonist muscle groups. When the hand is moved upwards towards the shoulder the biceps are the agonists and the triceps are the antagonists. Lowering the arm reverses the role in this case, the triceps being the prime movers and the biceps becoming the antagonists.

Around the body many smaller synergist muscles and are involved in stabilising, protecting and allowing other minor adjustments to the position of limbs. Stretching does not only involve the major muscles of the body but these smaller ones too. Neglect them at your peril - minor strains and tears although not necessarily debilitating are very uncomfortable and will impair your training. Any imbalance (10% or more) in strength, length or flexibility between agonist/antagonist pairs will almost certainly result in injury. Remember that to maximise the speed of movement of a limb then a powerful contraction of the agonist must be accompanied by almost complete extension of the antagonist. If the antagonist is too short or not flexible enough then it will almost certainly tear.