The shakes usually hit when you do isometric exercises (planks, pull ups, push ups) where you stop and hold. To keep your muscles contracted your motor neurons fire on and off, however some shut down from fatigue and the rest have to work harder. It means your reaching your neuromuscular threshold, your body’s point of exhaustion. Keep going with the training, your muscles will adapt and the shaking will subside. Only hold the position until your form starts going.