Why Do Muscles Shake? - Exploring the Physics Behind Shaking Muscles

[MattRob]

So, here's an interesting question.

I was at the shooting range today, and I noticed that when trying to aim, and when my dad was trying to aim, there was some shaking, making it just a little bit difficult. It seems hard to have a muscle evenly acting for a few seconds - simply put, there was a little bit of shaking/trembling. You can also see it when weight-lifters are straining to lift a very heavy object.

Now, I'm not looking for "low blood sugar" or "arthritis" or such as to "what causes shaking", but the root fundamental cause.

One thought that crossed my mind is perhaps the signal from the neurons is a bit noisy - but that doesn't really make sense because you can feel shaking, which means you're receiving a sensory signal that's not as noisy as the signal that's triggering the muscles to contract.

So all the way down to the physics-level, why is it that muscles shake?

Another thread I looked up on spasms said that they occur because of buildup of ions that can cause nerves to fire without receiving a command from the brain (I assume as a small batch of ions at one end of a synaptic gap disperse over time until they bridge a gap and allow a neuron to fire?). Perhaps it's similar to this somehow?

 

[berkeman]

Now, I'm not looking for "low blood sugar" or "arthritis" or such as to "what causes shaking", but the root fundamental cause.

Why not? Can you do a comprehensive search and list all of the reasons that this can happen? Google is your friend.

For me at the range, it's too much coffee beforehand, or not having slowed my CV system down enough yet. What were your vital signs when you were shaking at the range? HR, RR...

 

[BillTre]

I assume as a small batch of ions at one end of a synaptic gap disperse over time until they bridge a gap and allow a neuron to fire?

This is wrong.
A build up of ions would depolarize the neuron being affected by the ion build-up. The neuron would then fire off an action potential, which would result in calcium ion influx at the pre-synaptic terminal which would cause synaptic vesicle release, which would bind post-synaptic receptors, which can the depolarize the post-synaptic membrane.

Like @berkeman said, there are a lot of potential causes for shaking.
Another possible reason could be a poorly tuned feedback loop from a muscle or tendon sensor to the spinal cord and then back out to the muscle.

 

[rootone]

"Adrenaline", is a natural hormone produced by your body and can do this while at the same time making you very concentrated and alert.
It's mostly produced in 'fight or flight' situations.
Technically it's called Epinephrine.
https://en.wikipedia.org/wiki/Epinephrine
https://en.wikipedia.org/wiki/Adrenal_gland
https://en.wikipedia.org/wiki/Sympathetic_nervous_system

 

[Fra]

Check this out, you might find more in the ilnkts

The influence of muscle tremor on shooting performance.
"...The cause of the rhythmic acceleration at 7–11 Hz is not entirely settled. In part, the acceleration may result from the repetitive force impulses generated by pulsatile motor unit firing. Even if motor unit activity is entirely unsynchronized, there will always be some tendency for the force modulation to reflect the activity of the largest (that is, the most recently recruited) motor units. The characteristic firing frequency of motor units when first recruited is in this frequency band, and when the frequency becomes higher, ‘grouping’ into action potential doublets or triplets may keep the principal fluctuation frequency lower (Elble & Randall, 1976; Christakos et al. 2009). Random forcing input from motor units may excite internal resonances of the spring/mass, muscle–tendon–limb system, which will cause an oscillation of the limb close to its natural frequency. In part, the rhythmic activity may also result from partly synchronized motor unit firing so that there is an increased likelihood of firing in active motor units at particular intervals. Such rhythmic modulation may in principle be a consequence of central drive or peripheral feedback from the moving limb. There is evidence that both of these mechanisms may operate even in apparently isometric conditions (Christakos et al. 2006)"

-- https://www.ncbi.nlm.nih.gov/pubmed/19923157

/Fredrik

 

[PeroK]

For some serious muscle trembling see this, for example:



It's also called "Elvis Leg"!

 

[MattRob]

Lots of good answers here, and thanks for every one.

@BillTre I think what I said came from a misunderstanding of what someone said in a thread about twitching, where a response said it had something to do with an ion buildup. Very vague, I know, wish I could find that post again, but good to know the buildup must be somewhere else, if it isn't related to the underlying mechanisms behind twitching at all.

But since the firing of muscles is related to building up ions, it seems like a possible, if naive, guess as to a possible cause of the random forces that are probably the cause of the tremors (described below).

Check this out, you might find more in the [links]

The influence of muscle tremor on shooting performance.
"...The cause of the rhythmic acceleration at 7–11 Hz is not entirely settled. In part, the acceleration may result from the repetitive force impulses generated by pulsatile motor unit firing. Even if motor unit activity is entirely unsynchronized, there will always be some tendency for the force modulation to reflect the activity of the largest (that is, the most recently recruited) motor units. The characteristic firing frequency of motor units when first recruited is in this frequency band, and when the frequency becomes higher, ‘grouping’ into action potential doublets or triplets may keep the principal fluctuation frequency lower (Elble & Randall, 1976; Christakos et al. 2009). Random forcing input from motor units may excite internal resonances of the spring/mass, muscle–tendon–limb system, which will cause an oscillation of the limb close to its natural frequency. In part, the rhythmic activity may also result from partly synchronized motor unit firing so that there is an increased likelihood of firing in active motor units at particular intervals. Such rhythmic modulation may in principle be a consequence of central drive or peripheral feedback from the moving limb. There is evidence that both of these mechanisms may operate even in apparently isometric conditions (Christakos et al. 2006)"

-- https://www.ncbi.nlm.nih.gov/pubmed/19923157

/Fredrik

7-11 Hz sounds about right for exactly what I'm talking about. I think this is probably the most on-target response. That paper was fantastic.

An immediate question might be if these proposed mechanisms would be reduced by β-blockers, since later the paper goes on to say they and β2-blockers were observed to reduce the tremors. It's rather surprising, then, that apparently β2-blockers haven't been tested in shooting performance.

Also, I thought this line was kind of funny,
"Increased tremor is obvious to speaker and audience when a nervous lecturer uses a laser pointer."

I guess keeping "cool" is a strong countermeasure to the tremors - psychologically and physiologically.

Though with all this to say on the practical end, while the understanding of underlying mechanisms does satiate some curiosity, It'll be nice to see less uncertainty around it in the future.

 

[WWGD]

Why not? Can you do a comprehensive search and list all of the reasons that this can happen? Google is your friend.

For me at the range, it's too much coffee beforehand, or not having slowed my CV system down enough yet. What were your vital signs when you were shaking at the range? HR, RR...

CV system? Central...? system?

 

[BillTre]

CV = cardio-vascular (heart and circulation)

 

[gleem]

Have you looked into the importance of Ca(2+) ions and various vitamin deficiencies on muscular contraction. As we age the mechanism for muscular contraction using these substances are affected to various degrees sometimes to a debilitating degree depending on the severity of the deficiencies. Of course there are also possible neurological issues contributing to tremors like Parkinson's disease.

 

[Dr_Zinj]

"Adrenaline", is a natural hormone produced by your body and can do this while at the same time making you very concentrated and alert.
It's mostly produced in 'fight or flight' situations.
Technically it's called Epinephrine.
https://en.wikipedia.org/wiki/Epinephrine
https://en.wikipedia.org/wiki/Adrenal_gland
https://en.wikipedia.org/wiki/Sympathetic_nervous_system

I get it, and see it, all the time in both fencing and martial arts competitions. Hands shake with a good half inch range tremor from the adrenaline rush. Oddly enough, I don't see it making a significant impact on accuracy in martial arts like Tae Kwon Do or Karate, but there is a variable amount of negative impact on accuracy of touches in fencing that appear to be mitigated by the experience of the fencer.

 

[Fervent Freyja]

Giving your father's age, health, and fitness would help. Any current life stressors or a family history of neurological difficulties would as well. The type of rifle, like with a large bullet size, could make lots of people anxious...

If I were you, I'd watch him more closely with everything he does. If then you find anything else troubling, take it to a specialist!

 

[berkeman]

Giving your father's age, health, and fitness would help. Any current life stressors or a family history of neurological difficulties would as well. The type of rifle, like with a large bullet size, could make lots of people anxious...

If I were you, I'd watch him more closely with everything he does. If then you find anything else troubling, take it to a specialist!

I believe he said in the OP that he noticed that both his dad and he were shaking, and just had the general question. A little bit of shaking at the range is normal (and as you say, can be a little bit more when shooting larger calibers).