A volunteer-supported walking programme to improve physical function in older people (the POWER Study): study protocol for a randomised controlled trial - Grede, Nina; Rieckert, Anja; Muth, Julia; Steinbuck, Joana; Weissbach, Sabine; Schneider, Annika; Weber-Schicker, Beate; Freiberger, Ellen; Jegan, Nikita; Donner-Banzhoff, Norbert; Sönnichsen, Andreas (Friedrich-Alexander-Universität Erlangen-Nürnberg, 2021)