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?'

Inclusion Criteria:

1. Aged 18-40 with unilateral ACL injury
2. Sporting injury with a Tegner score of 7
3. Pre-op serum vitamin D level <20 ng/ml
4. 4 months post-ACLR with serum Vitamin D level remained <20ng/ml
5. LSI for quadriceps strength <70% of contralateral leg at 4-month isokinetic assessment
6. Both knees without history of injury/prior surgery

Exclusion Criteria:

1. Concomitant bone fracture, major meniscus injury or full-thickness chondral injuries requiring altered rehabilitation program post-operatively
2. Pre-operative radiographic signs of arthritis
3. Metal implants that would cause interference on MRI
4. Non-HS graft for ACLR
5. Patient non-compliant to the rehabilitation program
6. Regular sunbed users

All IPD that underlie results in a publication

starting 6 months after publication

Journal reviewers

It is double blinded, subject will receive either the supplement or the placebo with same appearance and package. The lot number of the bottle will be used for randomization. And the investigators will ask the manufactory about it at the end of the study.