In an ageing population, the need for interventions to help older people remain healthy, active and independent for as long as possible, increases. Although several studies suggest a beneficial effect of vitamin D3 on maintaining or improving muscle strength and physical functioning, particularly in vulnerable populations, results are contradicting. Randomized, placebo-controlled trials are needed to further establish the effect of vitamin D on muscle strength in the frail elderly population.The primary aim of this study is to determine the effect of daily supplementation with two different forms of vitamin D on muscle strength in frail elderly people over a period of 24 weeks.