Ankle ligamentous sprain is the most common sport-related injury. Repeated ankle sprains, without adequate treatment, may lead to ankle instability or even osteoarthritis. Besides the research on effective treatment, prevention of ankle sprain injury is also important. Since 2014, I have been working on a series of studies on the injury mechanism of ankle sprain, and an innovative design of a wearable anti-sprain system for preventing the injury. This is a inter-disciplinary research involving a lot of biomechanics investigation in an orthopaedic sport medicine topic .
Epidemiology: Firstly, a review of the sport injury pattern reported by 227 studies published between 1977 and 2005 was conducted . The ankle joint was the second common injured body sites (after the knee) in these 70 sports, with inversion sprain being the most common diagnosis accounting for 84% of all ankle injuries. The incidence was highest in rugby, football, volleyball, handball and basketball. An epidemiology study in 2008 on the audit of the attendance to accident and emergency department was conducted to reveal that 14% of the sport-related attendances were on the ankle joint, with 81% being ligamentous sprains and 10% being fracture, sustained mainly in basketball (37%) and football (29%) sports . Although we now have ‘better’ sports shoes  when compared to 30 years ago, the incidence of ankle sprain injury is high. This means that there is still a need to do something on the prevention of ankle sprain injury.
Aetiology: There are two major aetiologies which cause an ankle inversion sprain injury . Firstly, an incorrect landing posture with a slightly inverted or supinated ankle joint would cause a ground reaction force acting on the lateral foot edge to point medially and not pass through the ankle joint centre, thus creating a vigorous inward twisting torque, subsequent excessive inversion, and finally high ligament strains which tear the lateral ligaments. Secondly, the reaction of the peroneal muscles, which function to resist ankle inversion, is too slow (60-90ms) to catch up to accommodate the sudden explosive inversion which happened within 50ms after a foot strike. The stability of a joint depends on both the contraction of the muscles and the contribution from the ligaments, therefore, when the muscles are inactive, the joint stability would rely mainly on the ligaments. Since the ligaments possess viscoelastic property, a sudden explosive stretch would tear them.
Mechanism: There are many ways to further study the injury mechanism, and the most direct one is to study the real injury incidents. In 2007, I was fortunate enough to collect video and plantar pressure data when a subject sustained a Grade I ligamentous sprain injury while performing a cutting motion in a laboratory during a sport shoe stability experiment. The comprehensive biomechanics data were later reported in the first ever ankle sprain case report with biomechanically presented mechanism . The data suggested that the peak inversion has reached 48 degrees, and such a range of inversion is often regarded as normal from the literature. Moreover, a dorsiflexed instead of plantarflexed ankle orientation was observed, and this was not in agreement with the supination mechanism clinically suggested. The peak inversion velocity reached 638 deg/s, and this was the first time such a value was reported. I and my other co-investigators could not compare to the literature but we believed that such a vigorous twist should have had injured the ankle ligament, which was diagnosed immediately by an orthopaedic surgeon right after the incident. It was exciting to have these data, and there is also a need to have data from more injury cases before we could draw conclusion. However, one could not just wait for another accident to happen, and therefore we must plan for another research methodology.
A new method for investigation: Since 2007 I developed a model-based image-matching motion analysis technique to study ankle sprain mechanism , and utilised the method to study two injury cases during the 2008 Beijing Olympics . I continued to analyse five cases from tennis competitions , and the data from these reports suggested that the ankle joints were all in an inverted and internally rotated orientation, plus either a dorsiflexed or plantarflexed ankle orientation. Nevertheless, all data showed high inversion velocities (>500 deg/s) which were all greater than that collected during common sporting activities . I further used the profile of ankle kinematics data to drive a computational foot and ankle model  to simulate the injury, and the result suggested that an inversion moment of 23Nm and an internal rotation moment of 11Nm were presented, resulting in a 15-20% strain at the anterior talofibular ligament . Another geometric analysis showed that an inversion ankle sprain with an internal rotation mechanism would cause injury to the anterior talofibular ligament .
An innovative invention for prevention: There are many modalities for injury prevention and I started with the shoes , as I believed that the design of the construct of shoes or inserts may change the foot and ankle biomechanics, hopefully in a desired way [15,16]. Based on the two identified aetiologies, a monitor system was designed to detect incorrect landing posture, and a corrective system was designed to overcome the delayed peroneal muscle reaction.
For the monitor system, the first attempt was to monitor the ankle joint supination torque. In a biomechanics laboratory, such is often determined by inverse dynamic calculation with kinematics, kinetics and anthropometry data. I first devised a method to estimate complete ground reaction force from pressure insoles , and further calculated the ankle joint supination torque . Although we reported that a 23Nm inversion moment was observed during an ankle sprain injury , there was a need to determine the threshold which the ligament started to tear. For this purpose, a cadaveric biomechanics study was conducted , but the attempt was suspended after all because the twisting speed on the cadaveric specimen had not been concerned, and the device utilising pressure sensors for estimation would record no data when a foot rolls over the lateral edge to sustain an ankle sprain injury.
The second attempt was to use motion sensors to identify injury-like and normal motion. For this purpose, a mechanical sprain simulator was built [20,21], and a total of 300 injury-like and 300 normal motions were collected by eight motion sensors attached to the dorsal foot. A support vector machine model was used to identify these motions, and the method reached an accuracy of 91.3% . However, the method was also suspended as the identification could only be done post data collection, but not in a real-time manner. The final attempt was to use a uni-axial gyrometer attached to the heel counter and to identify the injury hazard by the inversion velocity . The threshold was based on the data from the reported injury incidents [6,8,9] and the normal motions we collected .
For the corrective system, the current attempt is to deliver myoelectric stimulation to the peroneal muscles to initiate very fast ankle eversion or pronation within 25ms to resist the injury mechanism which happens within 50ms after the foot strike . The system has been evaluated to be effective in a biomechanics laboratory, and was granted patents in several countries since 2012. Because of the position of the peroneal muscle belly, the stimulation has to be delivered at the upper shank for the best effect , and the system is currently carried by a pair of wireless wearable parts, or a sport legging covering the whole shank. The device only gives mechanical support by assisting the muscle reaction to generate muscular forces when in need, and allows the athlete to have an agile ankle joint for most of the time for the best performance.
In order to optimise the corrective system, we are currently building a computational biomechanics model to mimic the injury scenario and to determine the optimal setting for the best effect. The computational model will also be used in other ankle related orthopaedic research topics, such as the effect of surgical treatment on osteoarthritis.
On-going work: The intelligent anti-sprain system is being revised for better accuracy in identifying injury risk, and for better agility, comfort, durability and user setting interface when it is being carried in a pair of sport legging. Our team will evaluate, firstly, if the intelligent sprain-free sport shoe is biomechanically effective in laboratory trials on a mechanical sprain simulation device, and finally, if our invention is clinical effective in a large-scale epidemiology study. We hope that the device could be commercialised to a innovative sport apparel which is affordable and useful to runners and other athletes.
On the other hand, we are also trying to adopt the same mechanism in training runners with laterally-shifted foot pressure, which is a confirmed injury risk factor, to bring the pressure back to the mid-line in order to reduce the risk of ankle sprain injury. We are also developing another wearable ankle brace to try to tackle another common chronic injury to the ankle joint, which is the impingement at the anterior aspect. This injury is often called ‘Footballer’s ankle’.
 Chan KM*, Fong DTP, Hong Y, Yung PSH, Lui PPY (2008). Orthopaedic sport biomechanics – a new paradigm. Clinical Biomechanics, 23(1 Supp), S21-30.
 Fong DTP, Hong Y, Chan LK, Yung PSH, Chan KM* (2007). A systematic review on ankle injury and ankle sprain in sports. Sports Medicine, 37(1), 73-94.
 Fong DTP, Man CY, Yung PSH, Cheung SY, Chan KM* (2008). Sport-related ankle injuries attending an accident and emergency department. Injury, 39(10), 1222-1227.
 Fong DTP, Hong Y*, Li JX (2007). Cushioning and lateral stability functions of cloth sport shoe. Sports Biomechanics, 6(3), 407-417.
 Fong DTP, Chan YY, Mok KM, Yung PSH, Chan KM* (2009). Understanding acute ankle ligamentous sprain injury in sports. Sports Medicine, Arthroscopy, Rehabilitation, Therapy and Technology, 1, 14.
 Fong DTP, Hong Y*, Shima Y, Krosshaug T, Yung PSH, Chan KM (2009). Biomechanics of supination ankle sprain – a case report of an accidental injury event in laboratory. American Journal of Sports Medicine, 37(4), 822-827.
 Mok KM, Fong DTP*, Krosshaug T, Hung ASL, Yung PSH, Chan KM (2011). An ankle joint model-based image-matching motion analysis technique. Gait and Posture, 34(1), 71-75.
 Mok KM, Fong DTP*, Krosshaug T, Hung ASL, Engebretsen L, Yung PSH, Chan KM (2011). Kinematics analysis of ankle inversion ligamentous sprain injuries in sports – two cases during the 2008 Beijing Olympics. American Journal of Sports Medicine, 39(7), 1548-1552.
 Fong DTP*, Ha SCW, Mok KM, Chan CWL, Chan KM (2012). Kinematics analysis of ankle inversion ligamentous sprain injuries in sports – five cases from televised tennis competitions. American Journal of Sports Medicine, 40(11), 2627-2632.
 Chu VWS, Fong DTP*, Chan YY, Yung PSH, Fung KY, Chan KM (2010). Differentiation of ankle sprain motion and common sporting motion by ankle inversion velocity. Journal of Biomechanics, 43(10), 2035-2038.
 Fong DTP*, Wei F (2012). The use of model matching video analysis and computational simulation to study ankle sprain injury mechanism. International Journal of Advanced Robotic Systems, 9, 97.
 Wei F, Fong DTP*, Chan KM, Haut R (2015). Estimation of ligament strains and joint moments in the ankle during a supination sprain injury. Computer Methods in Biomechanics and Biomedical Engineering, 18(3), 243-248.
 Panagiotakis E, Mok KM, Fong DTP, Bull AMJ* (in press). Biomechanical analysis of ankle ligamentous sprain injury cases from televised basketball games: understanding when, how and why ligament failure occurs. Journal of Science and Medicine in Sport..
 Fong DTP (2012). An intelligent sport shoe to prevent ankle inversion sprain injury. Journal of Foot and Ankle Research, 5(Supp 1), K6.
 Fong DTP, Lam MH, Lao MLM, Chan CWN, Yung PSH, Fung KY, Lui PPY, Chan KM* (2008). Effect of medial arch-heel support in inserts on reducing ankle eversion: a biomechanics study. Journal of Orthopaedic Surgery and Research, 3, 7.
 Fong DTP*, Pang KY, Chung MML, Hung ASL, Chan KM (2012). Evaluation of combined prescription of rocker sole shoes and custom-made foot orthoses for the treatment of plantar fasciitis. Clinical Biomechanics, 27(10), 1072-1077.
 Fong DTP, Chan YY, Hong Y, Yung PSH, Fung KY, Chan KM* (2008). Estimating the complete ground reaction force with pressure insoles in walking. Journal of Biomechanics, 41(11), 2597-2601.
 Fong DTP, Chan YY, Hong Y, Yung PSH, Fung KY, Chan KM* (2008). A three-pressure-sensor (3PS) system for monitoring ankle supination torque during sport motions. Journal of Biomechanics, 41(11), 2562-2566.
 Fong DTP*, Chung MML, Chan YY, Chan KM (2012). A mechanical jig for measuring ankle supination and pronation torque in vitro and in vivo. Medical Engineering and Physics, 34(6), 791-794.
 Chan YY, Fong DTP, Yung PSH, Fung KY, Chan KM* (2008). A mechanical supination sprain simulator for studying ankle supination sprain kinematics. Journal of Biomechanics, 41(11), 2571-2574.
 Ha SCH, Fong DTP, Chan KM* (2015). Review of ankle inversion sprain simulators in the biomechanics laboratory. Asia-Pacific Journal of Sports Medicine, Arthroscopy, Rehabilitation and Technology, 2, 114-121.
 Chan YY, Fong DTP*, Chung MML, Li WJ, Liao WH, Yung PSH, Chan KM (2010). Identification of ankle sprain motion from common sporting activities by dorsal foot kinematics data. Journal of Biomechanics, 43(10), 1965-1969.
 Fong DTP*, Chan YY (2010). The use of wearable inertial motion sensors in human lower limb biomechanics studies – a systematic review. Sensors, 10(12), 11556-11565.
 Fong DTP*, Chu VWS, Chan KM (2012). Myoelectric stimulation on peroneal muscles resists simulated ankle sprain motion. Journal of Biomechanics, 45(11), 2055-2057.
 Fong DTP*, Wang D, Chu VWS, Chan KM (2013). Myoelectric stimulation on peroneal muscles with electrodes of the muscle belly size attached to the upper shank gives the best effect in resisting simulated ankle sprain motion. Journal of Biomechanics, 46(6), 1088-1091.