BackgroundPhysical activity (PA) counselling research has mainly focused on identifying which behavior change techniques (BCTs) are delivered by a counsellor. Less is known about how BCTs are received by clients. State Space Grids (SSG) is a dynamic system method that can be used to study counsellor-client interactions by examining frequencies, durations and sequences of BCT delivery and receipt. This study demonstrates how to use SSG to characterize counsellor-client interactions during a PA behavioural support intervention for adults with disabilities. Methods This secondary data analysis study used data from adults with spinal cord injury (age: 45.79±13.63; females: n=5; males: n=9) who received PA counselling. Transcripts of 30 audio-recorded counselling sessions (total duration: ~8.3 hours) were double-coded for BCT delivery and receipt statements using a reliable coding method (>84% agreement) and analyzed using SSG methods. Results The SSG analyses revealed that frequencies, durations, and sequences of BCT delivery and receipt varied largely within and between dyads. Across all sessions, the counsellor and client spent on average 32-34% of their time on talking about BCTs related to goals and planning, ~29% of their time talking about other BCTs (e.g., self-belief, support strategies), and the remaining 27-29% of their time talking about other topics (not BCT-specific). Conclusion This study showed how dynamic system methods can be used to characterize counsellor-client interactions and illustrate the variability of how BCTs are delivered by a counsellor and received by clients in a PA behavioural support intervention. We demonstrated that insights into frequencies, durations and sequences of BCT delivery and receipt can help advance our understanding of PA behavioural support for adults with and without disabilities.