This study is examining the post-operative rehabilitation of anterior cruciate ligament (ACL) reconstruction patients within the National Health Service (NHS). It will compare the effectiveness of two resistance training protocols, light load blood flow restriction (BFR) training and heavy load training, on improving multiple aspects important to the rehabilitation process in this patient demographic, whilst also examining the feasibility of this novel training modality. It is hypothesised that light load BFR training will be equally effective as heavy load training, but more tolerable for patients.

Light load BFR training is an effective rehabilitation tool for improving strength in individuals suffering clinical muscle weakness (Hughes et al. 2017). This is a single blinded, randomised controlled clinical trial, comprised of a two (group) by five (time) between-subjects repeated measures design. This study will place within the NHS at University College London Hospital (UCLH). Eligible patients from the orthopaedic department theatre lists of four orthopaedic surgeons at UCLH will be contacted prior to surgery via phone call and sent the patient information sheet and schematic outlining the study. Patient clinical history and documentation will be screened initially by the principle researcher followed by screening over the phone upon contact. Patients who are willing to participate will then be invited to the hospital for a full health screening appointment following provision of written informed consent, in compliance with the Declaration of Helsinki, 7th version, October 2013.

Baseline assessment of all outcome measures will take place at the pre-surgery screening appointment. Patient will then undergo their surgery. Following two weeks of recovery, repeat assessment of outcome measures will take place once patients achieve the following criteria for beginning strength rehabilitation, as developed by Cavanaugh and Powers (2017): 1) Able to unilaterally weight bear without pain; 2) Able to perform a single leg raise without quadriceps lag; 3) Have a range of motion (ROM) of 0-90 degrees; and 4) Have minimal swelling. Patients will then be allocated to one f the two training intervention groups and undergo eight weeks of twice weekly unilateral leg press training. Training sessions include five minutes of light cycling at a free cadence on a stationary ergometer for both groups, followed by the respective leg press protocols. Repeat assessment of outcome measures will take place mid-training (between weeks four and five) and post-training. Ratings of perceived pain and exertion, blood pressure and any adverse events will be recorded at all training and assessment sessions. All outcomes will also be assessed at 3 and 6 month follow up assessments. Pain will also be assessed 24 hours after every training session. During the study, all participants will see their assigned physiotherapist fortnightly (as per standard hospital protocol) and receive individualised programmes focussing on improving ROM, balance and minimising swelling.

BFR will be achieved using the Delfi Easy-fit variable contour tourniquet cuff (11.5 cm x 86 cm x 5 mm) connected to the automatic personalised tourniquet system (PTS) device (Delfi Medical, Vancouver, British Columbia, Canada). The Delfi PTS for BFR is equipped with the capability of automatically measuring limb occlusive pressure (LOP) and calculating the personalised tourniquet pressure, comprised of a dual-purpose personalised tourniquet cuff and a personalised tourniquet instrument containing LOP calculation sensors and software. The PTS system connected to the tourniquet cuff increases the cuff pressure in stepwise increments, analyses the pneumatic pressure pulsations in the cuff bladder by the arterial pressure pulsations at each cuff pressure increment, and uses these characteristics to determine LOP.

The primary outcome measure of muscle strength was used for calculating the required sample size. Based on previous data regarding muscle strength increases in light load BFR resistance training studies in clinical populations, to achieve a power of 80% at an alpha level 0.05, a total of 28 participants is required to detect a meaningful effect. Based on hospital record of rehabilitation programmes, a drop-out rate of 10% can be expected during the course of rehabilitation programmes. Therefore, a total of 30 participants will be recruited.

All data will be stored on the NHS password protected server at UCLH as per hospital and NHS ethical requirements. Data from drop-outs will not be included in the final analysis. Descriptive statistics (mean ± standard deviation) will be used to summarise adherence rates, exercise session attendance, training volume any adverse events. All statistical analysis will be performed with IBM Statistical Package for the Social Sciences (SPSS) Version 22.0 (IBM Corp, Chicago Illinois, United States of America). All data will be presented as means ± standard deviation with 95% confidence intervals unless stated otherwise. Normal distribution of data will be assessed using Shapiro-Wilks test (p>0.05). Ten repetition maximum strength, muscle size measures, self-reported physical function measures and functional performance will each be analysed using 2 x 5 (rehabilitation intervention x TP) repeated measures analysis of variance (ANOVA)s. Isokinetic strength and knee ligament laxity will be assessed using 2 x 3 (rehabilitation intervention x TP) repeated measures ANOVAs. Perceptual responses for ratings of perceived exertion and pain will be assessed using 2 x 16 (rehabilitation intervention x exercise session) repeated measures ANOVAs. For any statistically significant two-way interaction, one-way repeated measures ANOVAs with Bonferroni correction will be used to examine simple main effects. Non-normally distributed data will be analysed using the non-parametric Friedman test. Alpha significance will be set a priori p<0.05.

Inclusion Criteria:

1. Registered patient of University College London Hospitals
2. Over 18 years of age
3. Present with a unilateral ACL tear

5. Scheduled for reconstructive surgery 6. Have the mental capacity to make informed decisions

Exclusion Criteria:

1. Multiple ligamentous ruptures or trauma
2. Rheumatoid arthritis or significant comorbidities
3. Presence or history of cardiovascular, pulmonary, respiratory of neurological disease
4. Intraarticular injections into the knee in the preceding 6 months
5. History of deep vein thrombosis or vascular pathology in any lower limb
6. Current use of anticoagulant medications or other medications that may affect blood flow or fluid exchange
7. Hypertension (>140/80 mmHg)
8. Inability to follow instructions for rehabilitation classes (e.g. advanced dementia)
9. Scheduling for post-operative leg bracing

The assessor of all outcome measures will be masked as to which group patients have been assigned. Patient information will be coded using a numerical system by and independent member of the research team during the course of the study and data analysis.