Background <p>Physical activity levels in individuals with a degenerative meniscus lesion and the impact of different treatment approaches remain unclear. The purpose was to compare sensor-based measurements of physical activity and self-reported physical activity at 10-year follow-up of a randomised controlled trial between individuals with meniscal symptoms allocated to (1) exercise therapy or (2) knee arthroscopy combined with exercise therapy, and to analyse correlations between physical activity and a) patient-reported outcomes, b) functional performance.</p> Methods <p>Patients aged 45 to 64 years with meniscal symptoms were randomised to exercise therapy (non-surgery group), or knee arthroscopy combined with exercise therapy (surgery group). The physical activity of 62 patients (30 in non-surgery group and 32 in surgery group) was assessed using an activPAL accelerometer for 7 days and the Physical Activity Scale at the 10-year follow-up. Patients answered the Knee injury and Osteoarthritis Outcome Score (KOOS) and EuroQol quality of Life assessment. Functional performance was assessed with the 30-second chair stand test. Intention-to-treat was the primary analytic approach.</p> Results <p>Accelerometer-assessed physical activity did not differ between treatment groups. High to moderate physical activity was more frequently reported in the non-surgery group compared to the surgery group (66% vs. 34%, <i>p</i> = .015) (Intention-to-treat). As-treated analysis showed no group differences. Accelerometer measured steps per day and self-reported physical activity correlated positively with the 30-second chair stand test (<i>r</i> = .336, <i>p</i> = .008; <i>r</i> = .423, <i>p</i> &lt; .001). Self-reported physical activity correlated positively with KOOS_PAIN, KOOS_ADL and KOOS_SPORT <i>r</i> = .259 − .375, <i>p</i> ≤ .044).</p> Conclusions <p>Sensor-based physical activity levels did not differ between the treatment groups. However, higher self-reported physical activity was associated with less pain and better function in daily activities as well as in sports and recreational pursuits. Furthermore, participants who walked more steps per day and reported higher levels of physical activity demonstrated better functional performance. These findings highlight the importance of promoting physical activity in this population to enhance knee health and overall quality of life.</p> Trial registration <p>Clinical Trials NCT01288768, retrospectively registered 2011-02-01.</p>