50 COPD patients and 50 healthy subjects will be allocated into two strata and separately randomized into two equally sized supplementation groups; (1) vitamin D3 and (2) placebo. The planned 50:50 ratio between COPD patients and healthy individuals may change, depending on the access to COPD patients. All subjects will perform lower-limb strength-training protocols in a contralateral manner: (leg 1) high-resistance (10 RM) and (leg 2) low-resistance (30 RM). Such a one-limb-at-a-time protocol ensures training that is unconfined by the cardiorespiratory limitations inherent to these patients, and allow comparison of the two training modalities in a manner unconfined by individual variation in exercise adaptability. A pilot study investigating the possible central pulmonary capacity limitation to two-legged strength training exercise in COPD patients will be performed. In this pilot study, we will compare exercise performance involving large and small muscle mass. In addition, all subjects will perform a selection of bilateral upper body exercises (10 RM), ensuring adequate hormonal responses and compliance to the study. The study is likely to revitalize guidelines for rehabilitation of COPD patients, and to provide vital information regarding the role of vitamin D in adaptations to strength training.