An activated muscle can generate tension without always shortening. The mechanical outcome depends on the relation between muscle force and the external load.

Three useful contraction descriptions are isometric, concentric and eccentric.

Isometric contraction: force without overall shortening

In an isometric contraction, muscle length remains approximately constant while active tension changes.

If you hold a weight stationary, actin–myosin cross-bridges still cycle and generate force even though the joint and overall muscle length change little.

Thus

$$\text{muscle velocity}\approx0$$

but

$$\text{muscle force}>0.$$

'Isometric' describes the macroscopic length condition; it does not mean that molecular cross-bridges are frozen.

Concentric contraction: active shortening

In a concentric contraction, the activated muscle shortens while producing force. This occurs when its shortening tendency is sufficient to move the external load in the shortening direction.

For example, during the upward phase of a biceps curl, the elbow flexor muscle can shorten while lifting the weight.

Eccentric contraction: active lengthening

In an eccentric contraction, the muscle is active and generating tension while an external load lengthens it.

During a controlled lowering phase of the same curl, elbow flexors can remain active while lengthening to resist gravity.

Eccentric contraction is therefore not muscle relaxation. Cross-bridges bear force while the muscle length increases.

'Isotonic' is not synonymous with 'concentric'

Historically, isotonic means contraction under approximately constant tension or load. A shortening contraction performed against a nearly constant load may be approximately isotonic, but real muscle force often changes during movement.

It is usually clearer to distinguish contractions by what happens to length:

isometric  → length nearly constant
concentric → active shortening
eccentric  → active lengthening

These categories provide the mechanical language needed to analyze how muscle force depends on starting length, shortening velocity and external load.