Why are isotonic eccentric contractions passive?

Question

In an isotonic eccentric contraction, the muscle is lengthening because of gravity being pulled on it and more tension from the load, but since for a contraction to happen myosin heads must still pivot and pull on the thin filaments toward the middle of the sarcomere, doesn't that take energy and produce tension, just not enough to overcome the load? If that is the case, then are isotonic eccentric contractions passive or active? Isn't some tension still being produced, just not enough to overcome to load? Is there a such thing as a passive contraction, since for a contraction to happen, myosin heads must pivot? What about in an isometric contraction, where the muscle literally doesn't move at all? There is still tension being produced, just not enough to move the muscle? So is it active or passive?

Answer 1

In an isotonic eccentric contraction, the muscle is lengthening under tension, as the muscle insertion is pulled by a load away from the origin. Isotonic contraction of a muscle produces a movement at the joint of the muscle with the bone, the insertion end of the muscle moves relative to the point of origin of the muscle. Imagine you have a book in your hand that you were holding and reading. Now you lower your hand to put the book in your book bag on the floor. Your biceps muscle will lengthen to allow you to perform that movement and in the process the muscle will lengthen. In the opposite situation when you wish to take that book out of the bag and put on your desk, biceps will shorten to allow you to complete that movement and that would be a concentric isotonic contraction. In both cases muscle retains its normal tension, the tension does not change with contraction, only the length of the muscle changes with contraction to let you accomplish the movement. 

In case of isometric contraction, the muscle does not change length and does not produce movement, it works hard to push something and in the process its tension increases without change in length and any movement. Imagine, you are filling your backpack with all your clothes, you continue to push hard to put maximum amount of clothes in the pack, you push clothes with your hands. In the process, biceps produces additional force, increases tension without changing the length. that type of contraction is isometric contraction. 

 

I am not sure about use of words active and passive for muscle contractions. These are used for transport of molecules across a semipermeable cell membrane.

 

Any kind of muscle contraction would required ATP and myofilament movement with actin-myosin interactions, actin binding to myosin head in response to stimulus by calcium ions and neurotransmitters through synapse. The classical sliding filament theory of thick and thin filaments pulling towards middle of sarcomere remains true for individual muscle fibers but for isotonic and isometric contractions muscle contract relative to point of origin using prime mover and antagonistic movements of muscle fibers. 

 

Best,