Inclusion Criteria:  Aged 18-40 with unilateral ACL injury Sporting injury with a Tegner score of 7 Pre-op serum vitamin D level <20 ng/ml 4 months post-ACLR with serum Vitamin D level remained <20ng/ml LSI for quadriceps strength <70% of contralateral leg at 4-month isokinetic assessment Both knees without history of injury/prior surgery  Exclusion Criteria:  Concomitant bone fracture, major meniscus injury or full-thickness chondral injuries requiring altered rehabilitation program post-operatively Pre-operative radiographic signs of arthritis Metal implants that would cause interference on MRI Non-HS graft for ACLR Patient non-compliant to the rehabilitation program Regular sunbed users