Evidence from randomized controlled trials shows that exercise during cancer treatment benefits physical fitness, fatigue and quality of life. Since the effect of exercise on clinical outcome is currently unknown, exercise is not included as integral part of standard cancer care. Moreover, evidence regarding the optimal exercise prescription in terms of type and dose is lacking. To maintain quality of life in patients receiving palliative treatment with chemotherapy, toxicity-induced modifications in the prescribed chemotherapy dose are common. Such modifications - occurring in 40% of patients with metastatic colorectal cancer - may reduce benefit of treatment. The investigators hypothesize that exercise prevents chemotherapy dose modifications by reducing toxicity and enhancing psychological strength. Additionally, based on studies in rodents and preliminary data in patients with cancer, the researchers hypothesize that exercise has beneficial effects on the functionality of the natural killer cells, which play an important role in the innate immune defense against cancer. Both, fewer dose modifications and improved immune function may improve progression-free survival. This study is a three-armed trial comparing resistance exercise, aerobic interval exercise and usual care in patients with metastatic colorectal cancer to select the optimal exercise prescription for preventing chemotherapy dose modifications. The trial will use a Bayesian adaptive multi-arm multi-stage design with several interim analyses after which an ineffective study arm can be dropped early. This novel design makes the trial more efficient and reduces patients' exposure to suboptimal study arms. Evidence regarding the exercise effects on i) clinical outcome, ii) the optimal exercise prescription, and iii) the underlying mechanisms, elucidates the potential of exercise to boost benefit from chemotherapy treatment. This evidence provides leads to improve progression-free survival and quality of life of patients suffering from one of the leading causes of cancer death worldwide.