The impact of hamstring muscle length during isometric strengthening exercise on muscle structure and function adaptations

1. Name: i) Short-muscle-length (70° knee flexion) intervention group (SML)during isometric strengthening exercise and; ii)Long-muscle-length (20° knee flexion) group intervention (LML) during isometric strengthening exercise 2. Why: In running-based sports, hamstring strain injuries (HSIs) are one of the leading causes of time lost from competition for athletes. HSI also has a high reoccurrence rate, with the risk of sustaining a HSI only increasing with a history of previous HSIs. Training interventions that load the hamstrings while they lengthen, known as eccentric strengthening exercises, have demonstrated the ability to reduce HSI incidence by modifying the hamstring internal structure (known as muscle architecture) and strength of the hamstring muscle. Eccentric strengthening exercises are believed to be effective as they occur where the muscle is being stretched, mimicking the hamstring position where most HSI occur. As such, hamstring eccentric strengthening protocols have been recommended and attempted to be adopted by many professional sporting organisations. However, a common adverse side effect of eccentric strengthening exercise is muscle soreness, stiffness and tenderness, which often persists for 24-72 hours post-training. As a result, compliance with eccentric strengthening exercise programs can be poor. Isometric strengthening exercises, where the muscle is loaded but does not change in length, is associated with less muscle soreness and has been proposed as a possible alternative to eccentric strengthening exercise due to the ability to still train the hamstrings in lengthened muscle positions. However, there is minimal evidence surrounding the impact of isometric strengthening exercises on hamstring muscle structure and strength. Thus, the effect of isometric strengthening exercises at lengthened muscle positions on markers of HSI risk is unknown. This proposed project will determine if an isometric strengthening exercise intervention can lead to favourable adaptations in the structure and function of the hamstrings. 3. What/ Procedures: Experimental Protocol: Longitudinal cohort training study. Participants will complete a 6-week intervention, comprised of either a short-length (70° knee flexion; 0° being full knee extension) or long-length (20° knee flexion) isometric strengthening exercise. Participants will be familiarised with all testing/training methodologies included in the project. After a minimum of seven days, lower limb strength and ultrasound-derived measures will then be assessed in a separate session (“pre-intervention testing”). Three to five days after these assessments, participants will commence their isometric training intervention. At the halfway point of the intervention, biceps femoris long head (BFlh) fascicle length will be re-assessed to track the rate of change across the two intervention groups. All measures will be re-assessed following the completion of the training intervention (“post-intervention testing”) and after a subsequent 28-day detraining period (“de-training testing”). Intervention: 30 participants will undertake a six-week intervention comprised of either a short-length (70° knee flexion) or long-length (20° knee flexion) isometric strengthening exercise using an isokinetic dynamometer (n = 15/ group). Each participant will train one leg only, with the other leg serving as a within-participant control leg. The training leg will be randomised, allowing for the contralateral leg to act as a control comparison as it will not receive any training stimulus but will be tested alongside the trained limb at pre-, post- and mid-intervention and de-training time points. Data collection techniques: Hamstring strength: Isokinetic dynamometry will be used to assess the lower limb strength of both legs (training and control) at all time points (pre-, post- and mid-intervention and de-training). BFlh architecture: A passive 2D ultrasound scan of the participants’ BFlh will provide a measurement of fascicle length. BFlh stiffness: Passive shear wave ultrasound scans of participant’s BFlh will provide a measurement of muscle stiffness. Muscle soreness: Approximately 24 hours following each training session, participants will be contacted by the research team to rate their perceived posterior thigh soreness from 0-10 using a Visual Analogue Scale (VAS), where 0 = no soreness whatsoever, and 10 = unbearable soreness. 4. WHO The investigation will be conducted by Miss Abigail Paduch (Bachelor of High-Performance Sport), who is undertaking her Honour’s research project in sports science and injury prevention. The project will be supervised by A/Prof David Opar, Dr Robert Crowther and Mr Declan Carmichael from the School of Behavioural and Health Sciences at the Australian Catholic University (Melbourne Campus). A/Prof David Opar is the Director of ACU’s Sports Performance, Recovery, Injury and New Technologies (SPRINT) Research Centre. As the lead for SPRINT’s Injury Research Program, A/Prof Opar has led the way in hamstring injury research. Professional sporting teams and rehabilitation protocols around the world have adopted findings from his research. Dr Robert Crowther is a Senior Lecturer in ACU’s Exercise Science department. Dr Crowther is a well-published human movement specialist, completing his Doctor of Philosophy in exercise therapy, biomechanics, and motor control. Mr Declan Carmichael is a research assistant within the SPRINT Research Centre. Mr Carmichael brings experience from his own research in hamstring isometric and eccentric training interventions and has experience in using ultrasound to non-invasively assess muscle structure and function. An internal reliability study on the ultrasound device will be carried out by Miss Abigail Paduch and Mr Declan Carmichael to ensure valid and reliable data collection. How/ Mode of Delivery: Individual, face-to-face 5. WHERE: All sessions will be conducted on-site at Australian Catholic University, Melbourne Campus (115 Victoria Parade, Fitzroy VIC 3065) in the laboratory on Lv 4 of building 420. This is to ensure we have access to the isokinetic dynamometer and ultrasound machine. 6. WHEN & HOW MUCH: Wk 1: 1 session. 2 sets of 5 repetitions, 5s isometric maximal efforts with 1.5 min rest between sets. Wk 2: 2 sessions. 2 sets of 4 repetitions, 5s isometric maximal efforts with 1.5 min rest between sets. Wk 3: 2 sessions. 2 sets of 6 repetitions, 5s isometric maximal efforts with 1.5 min rest between sets. Wk 4: 2 sessions. 3 sets of 5 repetitions, 5s isometric maximal efforts with 1.5 min rest between sets. Wk 5: 2 sessions. 2 sets of 5 repetitions, 5s isometric maximal efforts with 1.5 min rest between sets. Wk 6: 2 sessions. 3 sets of 6 repetitions, 5s isometric maximal efforts with 1.5 min rest between sets. Session compliance will be reported as a percentage of overall sessions completed and/or participant drop out. Tailoring/ Modifiation & How Well: N/A