The goal of this clinical trial is to compare two models of delivery of guided exercise in patients with exercise intolerance after mild head injury.

The main question it aims to answer is:

• Is a program that includes elements of in-house exercise and follow-up sessions, and repeated treadmill testing, superior to a program with telephone-based follow-up only?

Participants will undergo a treadmill test to determine eligibility for the study, and to determine at what intensity level their symptoms worsen (symptom threshold). Thereafter they will exercise 15-20 minutes, 3-5 times per week at 80-90% of the heart rate that was found to be the symptom threshold. One group will receive face-to-face folllow-up and repeated testing, one group will receive telephone-based follow-up only .

Researchers will compare these two groups to see if closer follow-up is superior when it comes to recovery from exercise intolerance after 12 weeks of exercise.

Patients with self-reported symptom exacerbation in relation to physical activity will be screened for inclusion with Buffalo Concussion Treadmill test (BCTT), an incremental treadmill exercise test according to a standard Balke protocol, to the first sign of symptom exacerbation or submaximal exertion. Heart rate and perceived exertion (Borg scale) are measured every minute. Patients who demonstrate symptom exacerbation according to predefined criteria will receive an individually tailored program for workouts with self-selected aerobic activites, of 15-20 minutes duration, 4-5 times per week at 85-90% (measured with HR monitor) of the HR at test termination, and they are randomised to either:

A. Every week the first three weeks, participants will exercise one individual session at the hospital, and the BCTT will be performed every 3rd week at the hospital to be able to shape the intervention.

B. The participants will only exercise at home, and will be contacted by telephone/video call, every week during the first three weeks, thereafter less often, up to every third week.

After 12 weeks, there will be a repeated BCTT, performed by a blinded assessor.

Patient reported outcomes are collected using electronic questionnaires before and after the intervention and 6 months after inclusion.

Inclusion Criteria:

• Age 18-65 years
• Having sustained a minimal head injury or mild traumatic brain injury at least 2 weeks ago.
• At least one postconcussion symptoms, of at least moderate degree, occurring within the first week after the head injury.
• Intolerance for physical activity (self-reported and measured; including symptom exacerbation later on the test day).
• Capable of giving informed consent.

Exclusion Criteria:

• More than 2 years since last injury.
• The symptoms are better explained by other conditions.
• Severe communication problems, typically due to poor knowledge of Norwegian.
• Severe psychiatric, neurological, somatic, or substance abuse disorders that will make it problematic to function in a group and/or will complicate follow-up and outcome assessment.
• Safety concerns according to the study medical checklist.

The treadmill test after completion of intervention is performed by an assessor that is blinded for group membership.