This is an excerpt from Yoga Anatomy-2nd Edition by Leslie Kaminoff & Amy Matthews.
When a muscle cell contracts, the molecules create and release bonds between the thick and thin filaments, which ratchet along each other and create a sliding movement that increases their overlap and draws the two ends of the myofibril toward each other. If enough myofibrils shorten, the whole muscle fiber slides shorter. As more and more muscle fibers contract, they attempt to shorten the entire muscle by sliding the attachment points at the two ends of the muscle toward each other.
Whether or not the entire muscle does actually shorten depends on outside factors, specifically how much resistance exists. If only a few filaments are sliding together inside the cells, they may not generate enough force to overcome the weight of whatever structure the muscle is attached to, such as the weight of the arm or the weight of the head. The weight of a body part is a product of the resistance created by gravity, which is a fundamental source of resistance for everything on this planet. We negotiate this force every time we lift an arm, stand up, roll over, or take a breath. Added resistance also comes from other forces, such as the weight of something being carried, an opposing muscle contraction, or even an emotional state (for example, tension, anger, or the effort not to cry will often create resistance, while relaxation, happiness, or relief will often decrease resistance).
Muscles do not contract in an all-or-nothing way. All of the fibers do not have to contract at the same time, meaning that a muscle can generate a precisely gradated amount of force, coordinated by the dialogue between the nervous system and the muscles. Because muscles work in this modulated way, they don't always end up shortening, even though the fibers might be actively contracting. A muscle may in fact be active and lengthening when the outside force is greater than the force that the muscle is exerting.
The words concentric, eccentric, and isometric are used to describe muscle actions (figure 4.4). These terms actually describe the effects of the relationship between the muscle and the resistance it meets.
Concentric Contraction The muscle fibers contract and generate more force than the resistance that is present so that the ends of the muscle slide toward each other and the muscle shortens.
Eccentric Contraction The muscle fibers contract and generate less force than the resistance that is present so that the ends of the muscle slide apart and the muscle actually lengthens. The muscle is active as it lengthens, so this is not the same as relaxing the muscle.
Isometric Contraction The muscle fibers contract and generate the same amount of force as the resistance that is present so that the ends of the muscle neither move apart nor move together and the length of the muscle does not change. Isometric contractions can be distinguished further: There is a difference in experience between intending to hold still against the resistance of something else trying to move you and intending to move but not being able to overcome the resistance to movement. There is also a difference in experience between maintaining an isometric contraction following a concentric contraction and maintaining an isometric contraction following an eccentric contraction.
A relaxed muscle generally means that there is not an intentional or voluntary contraction of the muscle fibers. If someone is conscious (even sleeping), however, there is always an underlying level of automatic activity in the muscle fibers to maintain the resting tone of the muscle. This resting tone keeps the muscles ready to respond and in postural muscles automatically adjusts for slight shifts in weight and balance when we sit, stand, and walk.
In the fields of fitness and movement training, the words lengthen and stretch are used in many different ways. It is important to understand that a muscle can lengthen and be active (an eccentric contraction), can lengthen and be inactive (a relaxed muscle), or can lengthen and gradually change from active to inactive or vice versa.
In any of these situations, the muscle lengthens because an outside force (such as the pull of gravity or the pull of another muscle) acts more strongly than the muscle being lengthened. Lengthening a muscle does not necessarily mean relaxing it.
The word stretch is sometimes used interchangeably with lengthen. If the term simply means to change the distance between the attachment points of the muscle in such a way that they move apart from each other, stretch is indeed interchangeable with lengthen.
If, however, stretch implies a particular quality of sensation in the muscle, then it is not interchangeable with lengthen. It is possible to lengthen a muscle without a stretching sensation—in fact, most of us do it all the time. Actions such as walking, talking, or picking up a cup all involve lengthening and shortening muscles, often without any particular muscular sensation at all.
Read more from Yoga Anatomy, Second Edition By Leslie Kaminoff, Amy Matthews.