A total of 78 people with medial knee OA ( mean SD age 62.1 6.9 years , mean SD body mass index 29.4 4.0 kg/m ( 2 ) , and radiographic disease severity 19 % mild , 49 % moderate , and 32 % severe ) who completed a 12-week physiotherapist-supervised exercise trial were randomly allocated to 2 30-minute physiotherapy booster sessions ( delivered by 8 physiotherapists in private clinics ) or no booster sessions for the subsequent 24 weeks .