It is easy to a visualize a passive force contribution during a concentric contraction when an eccentric contraction immediately precedes a concentric contraction as often occurs during animal/human locomotion. I would think this passive force would be measurable during a contraction when the active force production is zero (i.e., as in when a muscle shortens at its maximal shortening velocity).

Thank you prof. Gordon L Warren for your answer. I fully agree with you about the possibility of passive force during stretch-shortening cycle. I was referring to a "pure" concentric contraction: from rest (zero force) to the maximum concentric force in the shortest time possible; a sort of "explosive" concentric contraction.

Presuming that at rest, you also mean at a resting length, there would be no elastic storage in non-contractile tissues. Without elastic energy contributions, the only contribution to force from non-contractile tissues would be force transmission from other tissues or realignment of force.

There is an interesting hypothesis that suggests part of the constituent forces that make up the resultant force generated by muscle fibres is a hydrostatic force. If true, this has potential to explain the pressure/tightness felt when a muscle is in a contracted state, potentially helping to control the size and shape of a contracted muscle.

The hypothesis could be tested using ultrasound imaging and computer modelling, though it would be difficult to get direct measures without surgical intervention - which of course would influence the normal muscle function.

I'm not sure if that helps, but perhaps is at least a couple of different ways to look at the question posed.

Thank you Dan Robbins. I really appreciate your answer. I will use those ways to interpret my data

Best

Andrea

When thinking in terms of contractile element (CE, muscle fibres) and serial elastic (SE, tendon) interaction, when there is a potential in the SE (i.e. the tendon is stretched and so produces force) there is too a passive force of the whole muscle, despite the CE operates concentrically. Roberts and colleges name this power attenuation:Article Contribution of elastic tissues to the mechanics and energet...

I think that what matters is the contraction range, viz. the muscle-tendon unit length range over which contraction occurs. When this is known, passive force should be deduced from the passive muscle-tendon unit F-L relationship. Am I wrong?

There are many conditions, a muscle-tendon-complex can operate on. The force-length relation is only one description, let's say first order. There is the force-velocity relation, too, let's say second order. On top of that, passive tissue like tendon (in serial to fibres) and in parallel to the fibres (titin) can be charged, for example in wallaby hopping (Biewener) or some pre charged motions like catapulting a tongue (de Groot, J. H. & van Leeuwen, J. L. Evidence for an elastic projection mechanism in the chameleon tongue Proceedings. Biological sciences / The Royal Society, 2004, 271, 761-70). In summary, external conditions are of interest and the possible charge (stretch) of passive tissue.