The muscle belly is not the total muscle; its tendons are in a sense a part of the muscle. In fact, the tendon is called the “stroma”, or supporting framework, and the muscle belly is called the “parenchyma”, or the functional tissue of an organ as distinguished from its connective or supporting tissue. Together the stroma and the parenchyma make up a muscle, or muscle-tendon unit, which is responsible for the movement of the skeletal system and the maintenance of joint integrity.The latter responsibility is usually attributed to ligaments, but ligaments are actually back-up to muscle-tendon units; if they were tight enough to keep movable skeletal joints “in joint”, we wouldn’t be able to move, because ligaments lack elasticity. Their primary function is to keep a dislocated joint from flying off into space, and come into play only after the muscle-tendon unit has surpassed its limit.

We tend to group ligaments and tendons are together, and think of them of as closer relatives than muscles and tendons; after all, ligaments and tendons are both connective tissue, made from sub-units of parallel collagen fibers. However muscles and tendons, the “muscle-tendon unit”, would more appropriately be grouped together. There are two main differences between tendons and ligaments. The first difference is in their structure.

Structurally, both ligaments and tendons have sub-units of parallel collagen fibers; collagen fibers are inherently elastic because they are parallel fibers of helical crystals, which look like a coiled spring.  But while tendon fibers remain in strictly parallel strands, ligament fibers are arranged in parallel layers that are criss-crossed one on top of the other. This criss-cross layering neutralizes the inherent elasticity of the collagen. The only elasticity in ligaments is white elastic fibers between each layer of the ligament, to allow some movement between the layers. But this doesn’t make the ligament itself elastic.

Ligament composition – bundles of white fibrous tissue placed parallel but closely interlaced with one another, presents a white, shining silvery aspect. Ligaments are strong, tough, and not able to extend, and are therefore not involved in stretching. Ligaments do not become involved in maintaining the integrity of movable joints unless the muscle-tendon units are breached. Because they act as backup to the muscle-tendon unit, elasticity is not a desirable quality. During normal range of motion, ligaments remain lax. So even though they may have some white elastin between the layers, the overall ligament is non-elastic. The elastin between the layers simply allows some change of motion, or direction. If they were all bound together, it would be an almost rigid structure.

So again, while ligament fibers are criss-cross and overlapped, tendon fibers are parallel and twisting. The reason the tendon is twisting, by the way, is because each one of its collagen fibers is twisting, so when you put them together they tend to twist, just like the DNA helix. When you put DNA molecules together, each one twists the next, and that in turn twists the next one, and then finally by the third twist you’ve got a chromosome. In the same way, the inherent twisting of the collagen fiber creates the twisting in the tendon.

[The human body does have four examples of truly elastic ligaments, however. They are:

1. Ligamentum flavum, made from yellow elastic fibers rather than white elastic fibers, coloring the ligament yellow. It runs through the entire spinal column, all the way from top to bottom. Because its job is to protect the spinal cord from the spinal column, it doesn’t hold anything in place or stop any joints from dislocating. It is a soft elastic curtain which gives with all the movements of the spinal column.

2. Broad ligament of the uterus holds the uterus in place, and of course has to stretch when the uterus expands, which is why it’s elastic.

3.  The vocal chords. We have two sets of vocal chords: vocal ligaments and vocal muscles. We get falsetto by relaxing the muscles so that nothing but the ligament is vibrating. The chords stretch to get a feel for pitch change.

4. The suspensory ligaments in the lens of the eye. They have to be elastic, because when the eye muscles want to change the diameter of the lens, the ligaments have to stretch.

The second difference between tendons and ligaments is that while ligaments are totally passive structure, tendons are active structure [this includes retinaculae*]. This is because the helical crystals in tendons are piezoelectric.

‘Piezoelectricity’ has to do with electricity, or electric polarity produced in certain non-conducting crystals when subjected to pressure or strain; piezoelectricity turns rapidly pulsing electrical currents into rapidly pulsing mechanical action; piezoelectric crystals expand and contract when injected with electricity, and they also generate electricity when smashed. But for our purposes we are mainly interested in the fact that when electricity is introduced into the helical coil spring of the collagen tendon fiber, that fiber tenses up. The piezoelectricity doesn’t actually contract the tendon so much as it prevents it from stretching. If tendons did not stiffen during muscle contraction, they would just stretch out and counteract the contracting action of the muscle belly, and we’d wind up with zero. In order to achieve shortening of the muscle-tendon unit, the co-laterals nerves must excite the tendon to stiffen at the same time the muscle belly contracts.

And that’s where the electricity comes from: motor nerves that innervate muscle fibers give off co-laterals (little parallel sidetrack nerves), some of which go to the muscle spindles and some to these tendons of the muscles they’re innervating. While muscle fibers twitch on and off in a fraction of a second, a stiffened tendon – because it’s a piezoelectric contraction – takes longer to let go. So tendons can’t be used as muscles; they are slower contracting/relaxing, they don’t contract as much as they stiffen, and because they contract to a lesser degree, they don’t provide much movement. However, if they did not stiffen, they would stretch. Some articles state that tendons do stretch, and “undergo permanent elongation and/or loss of elasticity”. However tendons don’t elongate, nor do they lose elasticity. If they did, we would constantly and easily disjoint. What really happens is a muscle becomes sick, seizes up permanently (becomes hypertonic), puts more stress on the tendons, and the tendons become extra-fibrotic. We said tendons are active rather than passive.

Remember, each collagen fiber is a coiled spring. Say there are a limited number of coiled springs (tendon fibers) operating just fine within their limits of stretch under certain stress; then eventually additional stress (increased muscle contraction) overstretches the springs. The tendons have the ability to call for more springs. This spreads that stress over a larger number of springs, until the springs are back within the range of their allowed limits. The additional springs – extra fibrosity – make it harder to move and stretch the tendon, which is the only real loss of elasticity. But the tendon is still elastic, it just takes more force to make it happen. More fibers equals less stretch. It’s the inherent coiled spring nature of the collagen fibers.

That’s why, when muscle tone increases, the stretch reflex mechanism decreases; when muscles can’t stretch much to begin with, they have less need to resist force. And a muscle that won’t stretch very much indicates fibrotic tendons – additional springs to hold additional stress.

Nevertheless, repeated or intense stretching will activate the stretch reflex mechanism and drive the muscle crazy. That’s why the derrières of ballet dancers bulge out like basketballs. When a dancer has a leg up on the barre, she is stretching the bellies of her muscles, throwing them into spasm, or contraction. She’s done nothing to her tendons except possibly thickened them with more coils, but those shortened muscle bellies have lost their range of motion. Now even somebody with an extremely shortened muscle belly can temporarily extend the tendon, so ballet theaters have a barre in the back of the stage; as the dancers dance and use their muscles, the co-lateral nerves excite the tendons to stiffen and shorten. Then between their onstage bits, they run backstage to stretch at the barre again, but are stretching out only their tendons, which soon shorten up again once they resume dancing.

*Retinaculae, like tendons, have piezoelectric innervation and can become spastic, even though they have no muscle attached to them. For example, the muscles that pass through the ankle (that that retinaculum is there to retain), cause the retinaculum to tense up so it won’t stretch during activity.  This is not related to a propensity to sprain the ankle joint, which occurs simply because the ankle is a very small joint at the bottom of a larger mass.

When you flatten out a tendon, it’s called an “aponeurosis”, but it’s still a tendon. When you roll an aponeurosis into a tube, now it’s a “myofascia”, but it’s the same stuff. Then, when you separate an aponeurosis from a muscle altogether, it’s called a “retinaculum”. It’s all the same composition – tendonous material.

Another example of this name-changing in anatomy is when blood plasma diffuses through the capillaries, it’s called “interstitial fluid”. Then when some of it doesn’t get picked up at the end of the capillaries and is left to go back through the lymph ducts, it’s called “lymph fluid”, but it’s the same stuff.  And when it comes back through the subclavian vein and dumps back into the vena cava, then it’s “blood plasma” again, but it has never really changed.