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Other questions want to know how to train for isometric strength. I want to know what is physiologically different about a person who can hold static weights vs. a person who can lift heavy weights in a full range of motion.

While doing one type of exercise clearly helps in the other, there are people who are clearly stronger in one over the other.

Arm wrestlers tell me that their incredible static strength comes from stronger tendons. This doesn't make sense to me, since tendons are only a part of the chain. I believe there must be something different in the musculature, but aside from bigger muscles I have no idea how. One of the answers to this quetion says

Isometric contraction work outs major downfall is they only train the range of motion you are using (90º for example). This means if you wanted to have the same "grocery lifting" strength anywhere else in your range of motion you would have to move to that angle and train that portion (45º). You would then be less strong at all other angles than 45º and 90º.

So somehow the muscle is stronger at different levels of contraction. Clearly something more is happening here than just bulkier muscles, or else different angles wouldn't affect it. What is happening, then?

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  • I assumed this was a basic question but maybe not? I've searched online (with admittedly poor research skills) and found nothing. Is the answer to this even known to medical science? Apr 24 '20 at 13:09
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    It's still under study. I was looking through some of the results available on google scholar, and some say isometric strength enhances isotonic, others say no difference, etc. I don't think there is a definitive answer yet.
    – JohnP
    Apr 27 '20 at 14:25
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The difference between the two scenarios you describe is the result of a number of phenomena.

First, isometric contractions are inherently easier, and isometric contractions are therefore inherently stronger. By definition, a concentric isotonic contraction is required to exceed the opposing force, while an isometric contraction is required only to meet the opposing force.

However, it is equally likely that you are observing differences in the individuals' physiology and training. Athletes who have a predominance of Type I (slow oxidative) muscle fibres have far greater resistance to fatigue than those with a predominance of Type II (fast glycolytic). This distinction is believed to be entirely genetic. Strength-endurance training can alter Type II fibres to become intermediate (Type II A and AB), but we are otherwise slaves to our genetics in this regard.

We do, however, alter our strength characteristics markedly through training, most notably (in this context) through two mechanisms: hypertrophy and neuro-muscular (motor unit) development. The former involves the thickening of muscle fibres, and it occurs in both/all muscle fibre types, albeit to a greater degree in Type II fibres. The latter involves the development of motor neuron-muscle fibre bundles that influence all of the characteristics of muscular output—control, strength, speed, power, and endurance.

Finally, skeletal geometry also makes a huge difference to the ease or difficulty by which an individual can apply force to a load. Shorter bio-mechanical levers combined with variations in muscle tendon attachment points, which alter the mechanical advantage of the lever give, with all other things being equal, shorter and stockier individuals a huge strength advantage. Essentially, such individuals can exert greater strength with the same muscular tension.

I hope that helps.

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Muscle fibers are composed of two major filaments. A thick filament composed of the protein myosin and a thin filament composed of the protein actin. The myosin thick filament slide inside a "tube" of many thin actin filaments. The myosin filament has many heads that are uniformly distributed along its length. Muscle force is produced by the interaction between these heads and binding sites that are uniformly distributed along the actin filament. This muscle force causes the tube to contract. enter image description here

The actin tube can both be extended and compressed in the length direction. The myosin filament can only be compressed in the length direction. Max force is produced when the length of the actin tube equals the uncompressed length of the myosin filament. As the length of the actin tube increases from this the number of binding sites that the heads can interact with decreases (linearly). However when the length of the actin tube decreases from full myosin length the force produced also decreases. This is probably caused by the lateral distance between the heads and bridges increasing eg. by the tube bulging. The result is a force length graph where force produced decreases away from the full myosin length: enter image description here

References

Length dependence of active force production in skeletal muscle. Journal of Applied Physiology.

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  • Like @T0r0nt0 's answer, this explains how muscle contractions work, but doesn't say anything about static vs dynamic strength. Apr 26 '20 at 13:01
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The best way I can explain how a muscle contract is if you think of muscle kinda like a line of people pulling a rope. Picture a bunch of people pulling a rope without the people moving. Each person is in line, and they pulls the rope closer, and the rope moves along from person to person. That's kinda how muscles contract. When your muscle is fully stretched out, this is equivalent to say the end of the rope only being in reach of the first person. He picks it up and starts pulling it until eventually the person behind can also grab and start pulling as well. As more and more people pull on the rope at the same time, the strength at which the rope is being pulled increases. Same thing with muscle. There's actually an optimal length at which the muscle is the strongest.

I don't know if that analogy makes sense, but if you wanna read about it yourself, look up the sliding filament theory and maybe it will make more sense to you.

Also, another factor to take into account is fatigue ability of muscles. Certain muscles are more prone to fatigue then others. So they won't be able to maintain a constant tension as long as other muscles.

Edit:

So to answer your question, on a physiological level, the muscular contraction is the same no matter what type of contraction is happening (whether its isometric, isotonic, eccentric or concentric). whenever you lift weights, your muscles are generating just enough force to overcome gravity and not more.

your question is more of a biomechanics question, not a physiological one. Theres a tradeoff between speed and force. What that means is that if your muscles are contracting at a high speed, the force that the muscle is able to produce is low. if your muscle is contracting at a low speed, the force being produced is fast. so in an isometric contraction (i.e one in which muscle stays in constant length, meaning the object is not moving) you will be able to generate more force than during a concentric contraction.

Does this help answer your question?

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  • This is a great explanation of how muscle fibers work -- however my question was about isometric strength vs isotonic strength. I don't think this answers the question Apr 26 '20 at 12:59
  • Kindly see my edits to the original answer and hopefully this helps to answer your question
    – T0r0nt0
    Apr 26 '20 at 14:55
  • Okay I see what you are saying. I may need to edit my original post for clarity. I'm asking about what makes one person stronger than another person in different lifts... Apr 26 '20 at 15:34
  • For instance, suppose person A can curl a 50 pound dumbbell and can isometrically hold a 75 pound dumbbell. Now person B can only curl a 40 pound dumbbell, but can hold 90 pounds. What makes them different, on a muscular level? Apr 26 '20 at 15:37
  • I am speculating but my guess would be that this has to do with the relative composition of each persons muscle. Skeletal muscle is actually composed of smaller units that are called muscle fibers. a bundle of different muscle fibers makes up the muscles we are familiar like biceps or triceps. Now these fibers are different from each other, metabolically and functionally. they are normally characterized based on their metabolic properties as well as their speed of contraction. so on one end we have slow oxidative muscles (known as Type I) and on other end we have fast glycolytic(Type II)
    – T0r0nt0
    Apr 26 '20 at 15:58

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