Force output during stretch
During "concentric" contractions, force and shortening velocity are inversely proportional. This is because a muscle that is contracting rapidly will have fewer crossbridges attached at any one time due to the rapid cycling. Eccentric contractions, on the other hand, are the opposite: force increases as lengthening velocity increases. This property of eccentric contractions makes them more efficient than concentric contractions, at the cost of increased risk of muscle damage (structural proteins can be overstretched and damaged).
Eccentric contractions start off similar to concentric contractions: the myosin heads bind to actin. The next step is a bit different, as the myosin head moves in the opposite direction: instead of bending inwards and pulling the actin filament with it, the myosin head stretches outwards. As this happens, the crossbridges become more strained, causing an increase in force production. Eventually, beyond a "transition point" which occurs at a certain length, the crossbridges detach and then reattach. The force at this "transition point" depends on velocity: if you stretch a muscle too slowly, then some of the crossbridges might have already detached by the time you get to the "transition point" length.
All of this is a bit confusing I know, so I'm going to try and explain it again with a graph.
Prior to the transition point, the myosin heads are pushing outwards, creating more strain and thus more force. This occurs until the muscle reaches a certain length. This certain length is the same for a given muscle regardless of the velocity of stretching. (However, the force at the transition point will be greater if the velocity is greater.) At the transition point, the muscle is so long that the crossbridges can no longer remain attached, so they detach and re-attach at a lower strain, which is why force does not increase as steeply after the transition point as before it.
One point of interest is that slower muscles, like the soleus, tend to have a greater force at the transition point compared to faster muscles. How is this so, given that I just said that stretching a muscle quickly creates greater force? Well, recall what I said about crossbridges detaching before the transition point is reached. Faster muscles have a faster crossbridge cycle, so more of their myosin heads will detach before the transition point as compared to the soleus muscle.
Residual force enhancement (RFE)
After a muscle has been stretched, the isometric force that it can produce at this longer length is greater than the isometric force that it could produce at the shorter length. This sustained increase in force is also known as residual force enhancement, or RFE, and increases with the amount of stretch. One of the weird things about RFE is that it is completely independent of cross-bridge cycling: it still occurs even when an inhibitor of crossbridge cycling, like blebbistatin, is given. Also, RFE is greater for fast muscles compared to slow muscles (which is the opposite of the peak force produced by stretch, as mentioned above). How is this possible? Well...
Titin's role in passive force and mechanotransduction
Titin, as you might recall from my first skeletal muscle post, connects the M line to the Z line. There are different isoforms of titin, which are expressed in different types of muscles. Fast muscles tend to express a shorter, stiffer form of titin, whereas slow muscles tend to express a longer, more elastic form of titin. As titin is considered to be the origin of passive force in muscle, it's quite possible that proteins such as titin are responsible for RFE.
More fun facts about titin! Titin can actually become stiffer when calcium is around, as calcium allows it to interact more with the thin filament. This might also contribute to RFE, and would explain why RFE only occurs during activity and not at rest. Another fun fact about titin is that it might contribute in mechanotransduction pathways, but that's probably going to be covered in more detail in a later lecture.
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