Why do I sometimes tremble while doing static exercises like planks? It sometimes doesn't happen even when I push myself, and sometimes happens from the start before I start experiencing discomfort.Static exercises (isometric) require the muscles to hold a fixed position and this requires your nerve impulses to continuously fire the motor units to trigger muscle contractions to maintain position. Sometimes the nerve impulses are uneven and go on/off which feels like shaking.
It is a form of fatigue but is typically not dangerous and holding through shaking can lead to fitness gains. It is important to distinguish between shaking due to nerves and shaking due to weakness and fatigue. With the former you can hold through it, with the latter you should stop the exercise.
It can happen anytime but more likely to happen as you get fatigued. Sometimes it can happen just because your mind-nerve-muscle connection is not working as usual.
As you train more then the shaking should be reduced or the time and intensity for shaking to start should be increased.
As with any exercise take care to warm-up and stay hydrated.
I remember some of my first Hatha Yoga sessions I was shaking like a leaf even though I had great fitness level in my legs. The reason is that I was trained in only dynamic movement and not in static holds which is vastly different.
Dear Willow: Why Do My Muscles Shake In Yoga?