Quadriceps muscle strength is one of the key determinants for patients to fulfill the Return-to-Play (RTP) criteria after an anterior cruciate ligament reconstruction (ACLR), in which the muscle size is directly linked to muscle strength. Quadriceps muscle atrophy is unavoidable after ACLR, but the rehabilitation program should increase quadriceps muscle mass. However, despite good rehabilitation compliance, some patient's progress is sub-par and fail to regain muscle mass. Quadriceps muscle atrophy can persist beyond the completion of the rehabilitation program in almost half the patients and the reason behind this is still unknown. This represents an area that requires significant investigation, as quadriceps muscle atrophy and weakness have been shown to be determinants of poor knee function, decreased performance in sports and increased risk of reinjury. Quadriceps muscle atrophy after ACLR is well documented. This can be due to a decreased ability to regain muscle mass with rehabilitation. Athletes are one of the high-risk groups for vitamin D insufficiencies. Vitamin D deficiency can potentially result in decreased hypertrophy when exercising the muscle, leading to a poorer outcome in rehabilitation. Vitamin D has long been recognized for its effect on musculoskeletal health. It can have a direct effect on muscle hypertrophy by acting on specific vitamin D receptors (VDRs) on myocytes, and sufficient or increased levels of vitamin D in patients have been found to correlate with an increase in the size, number, and strength of muscle fibres. Quadriceps muscle hypertrophy after ACLR is triggered by exercise training, facilitated by diet and a number of intrinsic factors. As the rehabilitation programs and diets are similar in patients with varying extents of quadriceps muscle atrophy, individual responses (intrinsic factors) to exercise training may account for the resulting persistent quadriceps muscle atrophy. In this study, the investigators hypothesize that the deficiency of vitamin D may contribute to persistent quadriceps atrophy and weakness. With a stringent double-blinded randomized-controlled-trial (RCT) research design, our proposal will then address the research questions: 'Does vitamin D supplements improve the vitamin D deficiency status in patients after ACL reconstruction?', and 'Does vitamin D supplements improve quadriceps muscle strength for patients after ACLR?'