Why complexity matters in physiotherapy research

We talk of complexity throughout physiotherapy and rehabilitation - complex interventions, complex patients and complex conditions are all too familiar phrases in both research and practice [1]. But what does complexity mean? It may be helpful to start with a distinction – a complicated system (such as a space rocket) comprises many intricate, multi-faceted elements which behave linearly and as we would predict. Each of these constituent parts have a clearly defined and constant relationship with each other and the external context and so can be reliably replicated ad infinitum [2].