MRI scans of patients with vitamin D deficiency and muscle fatigue, showed that "those with very low vitamin D levels improved their muscle efficiency significantly when their vitamin D levels were improved."
Vitamin D Boosts Energy Levels from Within Cells: Study (Science World Report)
A latest study conducted by researchers at the Newcastle University suggests that vitamin D is important for improving the functions of our muscle, which thereby enhance the function of the mitochondria (power house of the cell), leading to a boost in energy levels.