In Hong Kong, over 3000 Anterior Cruciate Ligament Reconstructions (ACLR) are performed each year in order to restore knee function after an ACL injury. The ultimate goal of ACLR is to fulfil the return-to-play (RTP) criteria. Despite successful surgery and a demanding rehabilitation process, some athletes still fail to comply to RTP. For those who achieve RTP, 23% of those who return to their sports would suffer a second ACL injury. Quadriceps muscle strength is one of the key determinants for a patient's successful return- to-play after ACLR. 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. Therefore, there is need to find a more effective way to increase quadriceps strength. There are emerging evidences showing that pulsed electromagnetic field (PEMF) can modulate mitochondrial activities for muscle gain. PEMF exposure on top of regular exercise training may promote muscle regeneration and tissue healing. This study aims to conduct a double-blinded, randomized controlled trial to investigate the effects of PEMF treatment during the late postoperative period on quadriceps muscle strength in ACL injured patient. Muscle endurance could only be investigated in late postoperative period. The investigators hypothesize that PEMF treatment is effective to reduce muscle weakness and promote gain in quadriceps muscle strength in ACLR patients. Based on the aim of this study, adult patients (aged 18-30) with a unilateral ACL injury, total quadriceps muscle volume is equal or morn than 7% deficit on involved leg compared with uninvolved leg, sporting injury with a Tegner score of 7+, both knees without a history of injury/prior surgery will be recruited. To estimate the improvement of patients, Isokinetic muscle assessment, ultrasound imaging and MRI for quadriceps muscle thickness, self-reported outcomes with questionnaires, KT-1000 for knee laxity and biomechanical analysis, Xtreme CT for Bone mineral density will be performed. To investigate the mechanism of PEMF therapy on increasing quadriceps strength, samples of blood serum will be draw before and after intervention.