Isometric contraction: no apparent change in muscle length, the muscle develops tension but does not shorten or lengthen.
Isotonic contraction: no apparent change in muscle tone, the muscle fiber shortens as thin filaments slide over thick filaments.
Tension and Load
Tension is the force the contracting muscle exerts on an object; load is the force the object (its weight) exerts on the muscle, opposing that tension.
To lift an object, muscle tension must become greater than the opposing load.
Isometric Contraction
Example: a person trying to lift a heavy object; recorded experimentally using an isometric lever, with both ends of the stimulated muscle held at fixed points.
Step 1 Cross bridges bind actin and attempt to pull the thin filaments but cannot drag them, since the load is greater than the tension the fiber exerts.
This exerts a force on the thin filament, the isometric tension.
Step 2 If contraction continues, cycling cross bridges repeatedly bind the same actin molecules without producing shortening.
Displacement is zero, so work done (force multiplied by displacement) is nil, even though force is generated and energy is spent: no external work is done.