Following ACL reconstruction, as per routine clinical practice, all participants will be referred to physiotherapy. Overall treatment aims, content and progressions of rehabilitation will follow current clinical guidelines. The choice of specific therapies, exercises and the number of treatment sessions needed to achieve the required treatment aims is at the clinical discretion of the treating physiotherapist.

We will use an 'all complaints' definition of injury - participants self-report any new knee problems that have occurred during the surveillance window.

The ACL-Return to sport after injury scale (ACL-RSI) is a 12-item scale designed to measure psychological readiness to return to sport after ACL injury. Scores range from 0 to 100; higher scores indicate greater psychological readiness to return to sport.

The 'future' domain of the knee self-efficacy scale comprises 4 questions to assess self-efficacy related to future knee function:

1. How certain are you that you can return to the same physical activity level as before the injury?
2. How certain are you that you would not suffer any new injuries to your knee?
3. How certain are you that your knee would not 'break'?
4. How certain are you that your knee will not get worse than before surgery?

Each question is scored on a 0-10 scale (higher scores represent greater self-efficacy). The domain score is the mean of responses to the four questions.

We will use three questions all measured on a 1-10 scale, where higher scores represent higher motivation:

1. How important is it for you to return to the same sport or recreation activity as before your knee injury?
2. Do you think it is possible for you to return to the same sport or recreation activity as before your knee injury?
3. How much time and effort are you willing to invest to return to the same sport or recreation activity as before your knee injury?

The ACL-QoL is a 30-item scale to measure knee-related quality of life following ACL injury. Scores range from 0 to 100; higher scores indicate greater knee-related quality of life.

The item wording for SANE is: "on a scale from 0-100, where 100 represents the best, how would you rate your knee today?" The IKDC subjective knee form is a 19-item condition-specific measure. The IKDC is scored out of 100 points, with a higher score indicating superior self-reported knee function.

We will use an isokinetic dynamometer to measure quadriceps and hamstrings concentric and eccentric peak torque.

We will measure the single hop for distance, triple hop for distance and side hop test for the affected and unaffected sides. The single hop for distance is the maximum distance the person can hop from a stationary starting position. The triple hop for distance is the maximum distance the person can hop with three successive hops from a stationary starting position. The side hop test is the number of hops the person can complete side-to-side over a 40cm strip of tape, in a 30 second period. If the person steps on the tape, the test must stop and be repeated.

Participants will self-report the number of supervised physiotherapy sessions, number of home-based exercise sessions and the number of gym-based exercise sessions completed in the previous 2 weeks.

We will count page views for different types of content (including Vimeo for video content, Soundcloud for audio content).

The Hospital Anxiety and Depression Scale (HADS) is a 14-item scale that measures psychological distress. Scores range from 0 to 21; higher scores indicate greater psychological distress.