Does anterior cruciate ligament (ACL) injury impair the molecular pathways responsible for building muscle?

A within-participant cross-sectional study will permit investigation of differences in exercise-induced molecular pathway within the same participant between his/her limbs (with/without a prior ACL injury) at rest and immediately following a single bout of strength/rehabilitation exercise. Demographic data will be collected prior to the observational testing sessions via online surveys. To assess a participant’s knee joint symptoms and function in daily living activities the International Knee Documentation Committee (IKDC) Subjective Knee Evaluation Form will be completed by all participants. To assess a participant’s psychological readiness to return to sport after ACL injury and reconstruction surgery the Anterior Cruciate Ligament Return to Sport After Injury (ACL-RSI) scale will be completed. Additionally, participants will rate their ability to partake in sport/activity both pre- and post-injury using the Tegner Scale. Details on the surgical approach used for ALC reconstruction, as well as any other co-morbidities, will also be ascertained from participants. Participants will be required to attend three observational sessions; a familiarisation (during which the leg press 1RM will be measured), a magnetic resonance imaging (MRI) scan, and the strength training session. All three sessions will occur within 7-14 days. Each session will take up to 2 hours to complete, the total observation period will not exceed 6 hours. Skeletal muscle protein content and activation of key strength training signalling pathways will be measured pre- and post-strength training session. Lower limb muscle morphology and leg press 1-repetition maximum (1RM) will be measured during the familiarisation session. All sessions will be conducted by highly trained researchers in this field. Unilateral leg press 1RM will be determined for both the uninjured and previously injured limb. Each limb will complete a warm up with a load that is equivalent to a 5 on a 1-10 rating of perceived exertion (RPE) scale, for between 3-5 repetitions. The load will incrementally increase (by between approximately 5-20kg) and the repetitions will decrease until the load that an individual cannot successfully lift for one repetition is found. The participant will be given one more opportunity to lift this load and if they fail to lift the load the highest load successfully lifted for one repetition will be considered the 1RM. Participants will be given a minimum of 2 mins between one repetition maximum efforts. During the strength training session, all participants will perform six sets of eight repetitions of a unilateral leg press exercise using a load equivalent to 80% 1RM for each leg (a total of 12 sets per participant). Each set will be completed one leg at a time, with limbs completing sets in an alternating order. The limb which completes the first set will be randomised between individuals using a coin flip. Each limb will be given a 90 second rest between sets. The duration of the strength training sessions will be approximately 30 minutes.