Hi
I was looking up comparisons between human and ape anatomy and strength. The insertion of a chimp’s biceps farther away from the joint is given as one reason why they are 3-5 times stronger than humans. Why does the distance from the joint affect strength. Please explain how it compares to levers. I understand when I lean down on a lever I can raise a load. I’m not sure how a muscle works in the same way. I look forward to your feedback.
Sorry, my heading was meant to read as follows.
Why does a muscle insertion farther away from a joint as in a chimp’s make a chimp stronger than humans?
From here: Lever - Wikipedia we learn that skeletal muscles in general are examples of class 3 levers. That is to say the fulcrum is at one end,the load at the other end, and the force is applied in the middle. And in the case of most skeletal muscles the force is applied pretty close to the fulcrum end, not the load end.
Here’s an easy to visualize example: Bend your elbow at about 90 degrees across your chest and then think about your upper arm muscles pulling to bend your arm more. Move your fist up towards your throat and watch what happens around your elbow joint. Use your other hand to feel what’s moving in the elbow area, and what’s pulling on what.
You’ll find that the upper arm muscle is attached to your lower arm bone just an inch or so from the joint. But your hand is 14 or 16 inches away. So the lever provides about 15:1 anti-leverage. That is, the muscle must pull 15 times harder for 1/15th the distance to move the hand.
Now imagine we rebuilt your elbow joint so the upper arm muscle attached to the lower arm bone 2 inches away from the joint. It’s just a small change in the shape of the meat around the elbow. Now the anti-leverage is 15:2 = 7ish:1.
With no change in the muscle you just got twice as strong. Because the leverage is twice as good (or 1/2 as bad) as the original design. The tradeoff is now the muscle has to contract twice the distance to move the hand the same distance.
All else equal that might slow you down or reduce the range of motion available. But in a successful animal all else won’t be equal. Its muscles are “designed” to match its skeleton to provide the speed, range, and precision of motion required for its lifestyle.
Play with a door. Try to open and close it by holding it close to the hinge. Now open and close it from other end where the knob is. So much easier, right? More leverage. The hinge is the joint.
Chimps have stronger muscles in a biochemical sense. Swinging from trees is hard work. Even the strongest human could not do what chimps do. The are deceptively strong animals.
Thanks LSL Guy for clearing that up. Thank you all
And here is where what seems straightforward read briefly starts to become more complicated as one thinks about it more. (Thought he explanation of muscles as levers is of course spot on.)
Especially given that any muscle exerts its greatest force at the midpoint of its contraction and is fairly weak at near full extension or contraction. The power a muscle can exert is contingent upon the length it can contract, the speed that it can contract, and how many motor units of what size are recruited in a manner maximally coordinated for the specific task.
Which joints are under discussion and which attachments are supposed to be different where? Elbow flexion apparently.
Is there any actual citation available that the biceps and/or brachialis muscle attachments on the forearm are actually appreciably different between the species?
I cannot find any. Looking what I can find is this (2007)
That “muscles are inserted differently” bit is pretty much completely urban legend, repeated so often that it is accepted by many on-line without any basis in fact.
I don’t have a specific cite for this, but I have read some scientific articles on this subject in the past, so I’ll jump in.
If the levers and muscle insertions aren’t that different between the species - what is different is the amount of muscle fibres activated per contraction. For example - measure the muscle fibres activated in say, the biceps or lats when a chimp is pulling himself up on a bar, and it will be fairly close to 100%. The chimp will be using most of his available physical resources to perform the movement. Hence, it will be easy.
If we get an average fit human to do the same movement, only around 25% of their muscle fibres will be activated. It becomes quite hard work.
From what I recall, the reason for this is the human brain is protecting the body from physical injury. But we can learn how to activate more fibres through training and practice, but not as much as a chimp (unless maybe when on certain psychotic drugs).
It is said that an average gym goer, can activate 25% of their muscle fibres per contraction, an elite power athlete can activate around 35%, and an Olympic level gymnast can activate as much as 50%. Try and do an iron cross on the rings, and you’ll see how much muscle strength is required round the joints to hold the body still with horizontal arms.
Things like testosterone/steroids and adrenaline can also cause the body to recruit more fibres per contraction.
On the flip side, we can probably recruit more fibres in our quads/hamstrings than a chimp, as we use these limbs when moving 95% of the time, as opposed to a chimp who will use their arms far more than us.
And there we go. Thanks for the wake-up call.
I was focusing on explaining how it would work mechanically, just assuming it was true physiologically since I’d heard it so often.
Yes, exactly, and likely more. We’ve had threads on this several times before. (Though not sure about the percent that “it is said.” Part of what enables the gymnast’s feat, for example, is not using all the muscle at once and having some motor units go off duty as they fatigue and less used units come on replacing them seamlessly and evenly.)
Human arm muscles are evolved for graded responses, i.e. greater control, and speed over power. Many smaller motor units that can learn many patterns of firing. Comes in handy with say throwing a spear and chipping a flint and more complex tasks.
Chimpanzees indeed have fewer much larger motor units controlled by fewer motor neurons, and fire much more muscle mass all at the same time. Adapted to do great at one armed pull-ups. Not throwing a well aimed spear quickly. Not at mastering complex pattern of movement.
Also much more of their body muscle mass is upper body and the fibers are longer and more densely packed.
But that was still a very good explanation of how class 3 levers work!
Thanks guitario and DSeid for all that additional clarification. I’ve searched quite a bit online and your posts do help a lot.