Q: When I am doing certain poses in yoga my arms will begin to tremble. This is especially true with poses that are new to me and require me to hold my arms in an outward rotated position. Why does this happen?
A: What you are describing is a fairly common occurrence when a muscle is tasked with an activity it is not used to doing. If the shaking you are experiencing is relatively mild and goes away when you cease the exercise than there is no cause for concern.
Muscle fibers naturally shorten and then lengthen when we are doing exercise. This contraction and lengthening process is voluntary in that we are creating conscious actions and involuntary in that our nervous system exerts specific controls on exactly how much a muscle shortens or lengthens.
For any muscle contraction to occur there must be nerve function. Each muscle has several nerve structures called motor units that control a certain number of muscle fibers. The number of fibers per motor unit varies depending on the amount of fine control we have over that muscle. The muscles of our fingers have more motor units per number of fibers than the muscles in our back because we must have greater and more fine control over our hands than our back.
There is also a greater amount of area in our brain reserved for the function of certain body areas depending on how we use them. The motor area of our brain for the muscles of the hands is greater than for back muscles because our hand muscles are more neurologically ‘important’ than our back muscles. It is also true that muscles that we use more frequently will have larger neurological pathways established in our brain.
The shaking that you are experiencing is in those muscles that you use less frequently, thus your brain and body are less experienced in shortening and lengthening these fibers. Your neurological pathways are not as developed as they will be when you have done the activity many times. That inexperience likely contributes to the faulty muscle firing, the shaking that you feel.
Fatigue is also playing a role here. Muscles that get used infrequently have less endurance than muscles that are used all the time. Due to the natural effect of muscle disruption and building from resistance exercise, trained muscle will have larger muscle fibers than an untrained muscle. Trained muscle will experience a resistance activity as less intense than untrained muscle. A trained muscle will also have a greater blood supply due to the natural response of the body to support those areas we use more frequently.
Your current physical state plays a large role as well. Fatigue brought on by lack of sleep, improper or inadequate nutrition and inadequate hydration may also make us more likely to shake in an exercise. Prolonged physical exercise that brings on lactic acid build up will also contribute to exercise shaking.
If you begin to shake while doing a particular action, first of all check your alignment. If you are doing things properly then you are probably reaching the end of your endurance and need to back off the intensity a bit. If it has been several hours since you had a meal then you likely are due for some nutritional support. If it has been an hour since you had some fluid then improve your hydration. If you are suffering from lack of sleep then get a good nights rest.
Most shaking like what you are describing is relatively temporary and you will eventually train past this point. However, if the shaking is severe or lingers after exercise then check things out with your doctor. Some shaking can be a sign of an underlying medical condition.