Why do muscles shake when tired? Doesn’t that take even more energy?


Why do muscles shake when tired? Doesn’t that take even more energy?

In: Biology

Think of your muscles like a shitload of pulleys inside you. When you’re tired, some of the pulleys stop pulling as hard as the others, or fail to pull at all. The ones that do still work don’t move and shake at the same time.

You’re made up of cells. In order for cells to do their thing they need to be within a certain temperature range. Shivering does take energy but it’s converting it to thermal energy to keep your internal temperature high enough to keep the cells alive.

The basic mechanism that causes muscles to contract is very similar to a person reaching out their hand, grabbing on to a rope, then hauling on that rope.

If you want to picture how a muscle contracts, imagine a bunch of boats lined up front-to-end in a lake, with a group of people in the front and back of each boat, and each boat connected to the one before (and after it) with a rope. The ropes aren’t tied to anything, the people in the boats are holding on to them like they were going to play tug-of-war. (the people in the front of one boat are paired up with the people in the back of the one ahead, and so on)

Suddenly someone gives an order, everyone pulls smoothly (nobody screws around and plays actual tug-of-war, they’re cooperating) and the whole line of boats gets shorter – no one boat has enough rope to make a big difference, but if you have 10 boats with 20 feet of rope between each, and they all pull, the first and last boat will end up 180 feet closer together. If nobody screws up.

Now, imagine that everyone is tired after doing this for a very long time, and someone asks them to do it again. Some people are distracted and miss the orders, others are so tired their grip slips and they barely manage to grab the rope again, in one of the boats someone starts throwing up, etc., and they just barely manage to pull the boats together, but it’s really slow and sloppy and uneven. That’s the “shaking.”

You’re thinking about it the wrong way. It’s not that you’re shaking because all of your muscles are suddenly pushing/pulling really hard, you’re shaking because some of your muscles have stopped while the rest kept trying.

That “shaking” is not being able to pull 100% of the time and instead firing only part of the time, as much as it can.

In the spirit of ELI5: It’s like when you’re drinking through a straw and get to the bottom of the cup. You go from “nice smooth silent drinking” (100% “on”) to a spluttering on-and-off “shaky” flow. That flow is NOT drawing more liquid than the smooth drinking was, it’s sputtering out to nothing.

Muscles contract when a certain neurotransmitter (acetylcholine) activates the muscular cross bridge. Briefly, acetylcholine allows calcium to enter the muscle cell, alter the ‘shape’ of the muscle cell, and something called a power stroke occurs. This leads to muscle contraction.
When you’ve completed a very hard workout, the buildup of acetylcholine is too much for its enemy (acetylcholine esterase) and you end up with an altered signal within the muscle, leading to extra twitches or even muscle cramps.

PS: just because I’ve seen it mentioned a few times in the comments, it’s not lactic acid. Low pH leads to pyruvate turning into lactate, but that is to maintain functional pH levels. Lactate is actually incredibly beneficial to exercise, not only serving as a pH buffer, but as another substrate to be turned into glucose via the Cori Cycle. Also, lactic acid is typically fully restored to baseline within 30 minutes of ceasing exercise.

Muscle contraction takes energy. When muscle runs out of energy, muscle stops contracting.
Muscles dont all contract at the exact same time. When some bits of muscle fail because no energy, others still have some energy, so they alternate failing and recovering and contracting again.

You feel this when you do a long plank exercise.