One of the problems with scaling up creatures without increasing their proportions is that their muscles won't be strong enough to support their weight due to the square-cube law, which I'm sure you're all familiar with. Muscles produce 20-135 (see page 8) or 30-40 newtons per square centimeter, or 4-8 kilograms force per square centimeter (see page 2); this is (unfortunately for worldbuilding purposes) a limit.
One solution is to add more muscles in proportion to body mass. However, I don't like that solution; if I'm making a giant cat, I'd rather have a cat-shaped giant cat and not an elephant-shaped giant cat.
Another solution is to make the creature less dense. However, I don't like that solution either; it doesn't work particularly well when you're working with really giant creatures who'd still be incredibly massive even if filled with helium and structural voids.
I believe my solution is better than those two (of course; it's mine, after all). My solution is to make the muscles themselves stronger, and then to put said super-muscles into super-large-but-still-proportionate creatures in worldbuilding settings so they can exert enough force to function. To explain how I wish to implement this, a brief crash course in how muscles work is necessary.
This is a not-to-scale cross-section of the smallest functional unit of a muscle cell - i.e. the point at which smaller structures are no longer identifiable as being part of a muscle, simply as being proteins.
Relevant things here are:
The thick filament: the pile-shaped thing on the bottom. All it's made of, functionally speaking, is myosin.
Tropomyosin (green wavy lines): bonded to the actin, these are to troponin as a zip line is to a person riding it. As far as I know, it doesn't really serve a purpose other than being structural material, although there are some theories that tropomyosin is what's responsible for myosin attaching to actin.
Actin (unlabeled blue circles): what the myosin sticks to when given a chance. Bonded to the tropomyosin.
Troponin (red circles): stops the myosin from sticking to the actin unless bribed with calcium (more on that later). While it's labeled "troponin" here, as if it were one protein, it's actually a protein complex made of troponin C, troponin I, and troponin T (more on that later too).
I would go over the entirety of the process by which a muscle contracts, but much of that'd be irrelevant for the purposes of this question. All you need to know is that:
I'm not going over why all the Ca2+ gets pumped into the cytoplasm; that's more related to the nervous system, and not relevant to this question.
Those Ca2+ ions stick to the troponin C in the muscles - remember that bit about troponin being "bribed" with calcium? This is that bit. Troponin C - again, remember that bit about troponin being made out of separate sub-parts? - is connected to troponin I. Troponin I covers the slot where myosin can attach to actin, and, when Ca2+ ions stick to troponin C, the entire piece of troponin gets twisted - C, I, T, and all. That twisting pulls the tropinin I away from the slot where myosin can attach to actin (troponin T is just there to connect itself and the other 2 troponins to the tropomyosin, and serves a minimal part in this process).
Once the troponin I has been pulled away from the actin, the myosin takes advantage of the troponin I's absence and attaches to the actin, producing force and moving the muscle.
I'm not going through how the muscle un-contracts; what's relevant to this question is that all force exerted by muscles is caused by myosin attaching to actin.
This process is visualized in a semi-comprehensible and again not-to-scale fashion below:
What I wish to do is to attach more myosin heads (the club-shaped or boxing glove-shaped things) to the end of the first myosin head, as well as to add more myosin-to-actin attachment points to the thin filament so that those extra myosin heads have something to stick to. This is visualized in a semi-comprehensible, yet-again not to scale, poorly-Photoshopped fashion below:
Why would I do this? Well, it stands to reason that, since myosin heads attaching to attachment points on actin are what makes a muscle exert force, more myosin heads attaching to more attachment points on actin will make the muscle exert more force.
Now, of course, the extra force this generates will necessitate mechanically stronger muscles, but I believe spider silk or a similarly strong protein-based material can cover for that, and I've already worked around any heat-generation problems this might cause. I haven't troubleshot the metabolism-related issues (energy, ATP, and oxygen supply to these super-muscles) yet, and those will likely be insurmountable. I also have to deal with the bones of a super-large creature, which is also not quite done yet.
However, I'm not really here to get a solution to those problems; I'm here to ask whether or not something like Image 3 is possible via any means - for instance, and I'm just spitballing here:
- grafting more myosin heads onto pre-existing ones to increase the number of myosin/actin pairings
- having the body fabricate a new myosin-esque protein with multiple heads to increase the number of whatever-this-new-protein-is/actin pairings
- modifying myosin II so it grows multiple myosin heads, thereby increasing the number of myosin/actin pairings
- altering the structure of actin so it has more places for myosin to attach
- altering the structure of troponin T, so it can attach to two troponin C/troponin I pairs, as opposed to one, letting it cover two myosin/actin attachment spots at once, rather than one
- simply adding more troponin - again, so that more myosin/actin attachment spots can be covered while inactive
The things on this list are just ideas, not questions I'm asking about - I'm aware of the single-question policy - but I'd really like to get more attachment points on these muscles, these might be ways to do that, and throwing my brainstorming at other people might help them come up with a solution.