The clinical features of PD include progressive postural instability, hypokinesia, tremor and rigidity. It is therefore common that people with PD experience reduced balance and gait function, symptoms which can have far-reaching negative effects on their health and quality of life. Injurious falls and fear of falling are especially prevalent among those with PD, a factor which may partly explain why this group are less physically active than older people of a similar age without the diagnosis. Balance and gait training, on the other hand, have been shown effective in PD and also appear to have potential neuro-protective properties. Research in the area of balance training in PD disease however is sparse and previous interventions have been criticized for applying training stimuli which lacked intensity and challenge.

The HiBalance program is based on scientifically well-established principles of exercise training and postural control as well as current research on training in PD. The program was developed to affect symptom-specific balance impairments in PD by targeting four main subsystems underlying balance control (stability limits, anticipatory postural adjustments, sensory integration and motor agility). The intervention consists of a 10-week progressive balance training program in group format (5-7 participants). Each training session is conducted by a minimum of 2 physiotherapist trainers, during twice weekly 1-hour sessions (20 hours in total). Additionally, a home-exercise program is carried out by the participants once a week during the trial period. The difficulty level of the group-based training is increased in three consecutive blocks. To ensure highly challenging exercises, each task is individually adjusted by altering the area of base of support, increasing movement speed/amplitude and/or restricting vision and varying the grade of multitasking. The program has previously been proven feasible \[Conradsson, 2012\] and effective \[Conradsson, 2015\] in improving balance and gait impairments in a randomized controlled hospital setting. In addition, favorable transfer effects were seen in relation to physical activity levels and the performance of activities of everyday life.

For research interventions to be adopted in real-life clinical settings however a level of adaptation is required. Best practice then involves evaluating the effectiveness of efficacious programs in clinical settings, in order to verify whether or not the adaptation has attenuated the effective core elements of the program itself.

The current study combines a clinical effectiveness trial together with implementation research. Use of a type-1 hybrid design will allow the effects of the HiBalance program in clinical settings to be tested while simultaneously gathering information on barriers and facilitators to the implementation process \[Curran, 2012\]. Use of the hybrid design also allows for constant monitoring of the process by which the intervention is applied, and therefore allow problems in early application to be identified and quickly altered so as to ensure better outcomes. A participatory approach will be adhered to whereby 'users' of the program (physiotherapist trainers) will be actively involved in all stages of the program adaptation, process and outcome evaluation. This approach is recommended in order to increase the relevance, acceptability and successful implementation of the program. The Consolidated Framework for Implementation Research (CFIR) will be used in the current study to guide the investigation of potential barriers to and facilitators of the implementation process \[Damschroder, 2009 \].