Muscle Contractions

The termination of muscle contraction is followed by muscle relaxation, which is a return of the muscle fibers to their low tension-generating state.

Muscle contractions can be described based on two variables: 

• length
• tension

THREE TYPES OF MUSCLE CONTRACTION:

Isometric

Isotonic

• Concentric
• Eccentric

Isokinetic

Isometric Contraction

An isometric contraction is a muscle contraction without motion. Isometric contractions are used to stabilize a joint, such as when a weight is held, there is no movement in the joints of the hand, but the muscles are contracting to provide a force sufficient enough to keep a steady hold on the dumbbell.

Because isometric exercises are done in one position without movement, they'll improve strength in only one particular position. 

Isometric exercises are much easier on the joints in both the short term and the long term. They still cause the muscle fibres to fire, without the additional stress on the joints. 

Isometric exercises are often used in rehab routines for individuals that have had joint problems or issues.  

Isotonic Contraction

In an isotonic contraction, tension remains the same, whilst the muscle's length changes.

This can occur only when a muscle's maximal force of contraction exceeds the total load on the muscle. Isotonic muscle contractions can be either concentric (muscle shortens) or eccentric (muscle lengthens). 

 There are two types of isotonic contractions: 

Concentric Contractions

A concentric contraction is a type of muscle contraction in which the muscles shorten while generating force. This is typical of muscles that contract due to the sliding filament mechanism, and it occurs throughout the muscle. Such contractions also alter the angle of the joints to which the muscles are attached, as they are stimulated to contract according to the sliding filament mechanism.

This occurs throughout the length of the muscle, generating force at the musculo-tendinous junction; causing the muscle to shorten and the angle of the joint to change. For instance, a concentric contraction of the biceps would cause the arm to bend at the elbow as the hand moves from near to the leg to close to the shoulder (a biceps curl). A concentric contraction of the triceps would change the angle of the joint in the opposite direction, straightening the arm and moving the hand toward the leg.

Eccentric Contractions

An eccentric contraction results in the elongation of a muscle. Such contractions decelerate the muscle joints (acting as "brakes" to concentric contractions) and can alter the position of the load force. These contractions can be both voluntary and involuntary. During an eccentric contraction, the muscle elongates while under tension due to an opposing force which is greater than the force generated by the muscle. Rather than working to pull a joint in the direction of the muscle contraction, the muscle acts to decelerate the joint at the end of a movement or otherwise control the repositioning of a load.

This can occur involuntarily (when attempting to move a weight too heavy for the muscle to lift) or voluntarily (when the muscle is "smoothing out" a movement). Over the short-term, strength training involving both eccentric and concentric contractions appear to increase muscular strength more than training with concentric contractions alone.

Muscle soreness, once thought to be due to lactic acid accumulation, has more recently been attributed to small tearing of the muscles fibers caused by eccentric contraction. 

Isokinetic Contraction

When the velocity of the muscle contraction remains constant while the length of the muscle changes. The force exerted by the muscle is not fixed, and can vary depending on the position of the joint in its range of motion and the participation effort of the subject.    Isokinetic muscle loading can be either concentric or eccentric. 

In an isokinetic concentric contraction, the muscle shortens while under load. In an isokinetic eccentric contraction, the muscle lengthens while under load.

Isokinetic training is a type if exercise training that uses a special machine that produce a constant speed no matter how much effort you expend.

These machines control the pace of an exercise by fluctuating resistance throughout your range of motion. Your speed remains consistent despite how much force you exert. 

Isokinetic exercises are mostly used for recovery and physical therapy

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Delayed Onset Muscle Soreness

DOMS is caused by ultrastructural disruptions of myofilaments, particularly with the z-disk and with damage to the muscle's connective tissues. 

Muscle biopsies taken a day after hard exercise often show bleeding of the z-band filaments that hold muscle fibers together. The pain then is thought to be largely due to this damage to the connective tissue, which in turn increases sensitivity of the muscle's nociceptors (pain receptors); this then causes pain with stretching and the like, basically when using the muscles.

The delayed effect is thought to be as a result of the fact that the inflammatory process that ends up making the nociceptors more sensitive takes some time to happen.