In general, baseline vitamin D levels in blood, measured as 25(OH)D, is anticipated to be a determinant of the efficacy of the strength training intervention. In response to vitamin D3 supplementation, individuals with low baseline levels of 25(OH)D are expected to display more pronounced changes in biological active vitamin D, leading to more pronounced changes in functional and biological outcome measures in response to strength training. In contrast, supplementation may not lead to further elevation of blood 25(OH)D levels in individuals with high baseline levels, essentially meaning that vitamin D3 ingestion will be leveled out by or exceeded by the elimination of vitamin D derivatives. In these individuals, vitamin D3 ingestion will not have an additive effect on functional and biological outcome measures in response to strength training. To assess individual variation in vitamin D responses, data on functional and biological variables will be divided into quartiles based on baseline 25(OH)D-levels, whereupon comparisons will be made between low-end and high-end quartiles. Individual variation in responses to vitamin D supplementation and strength training will also be assessed using a mixed model approach.