Background and Aims
Physical activity (PA) during cancer treatment has many health benefits. For adolescents and young adults (AYA), treatment is generally intense, and occurs during a developmental time of declining PA. To design effective PA interventions, a greater understanding of the biopsychosocial processes underpinning AYA PA behavior is required. We aimed to assess the demographic, medical, and psychosocial predictors of PA among AYA with cancer using the Multi-Process Action Control (M-PAC) framework.

Methods
Participants were AYA ages 14-24 receiving cancer treatment [N=80, Mage(SD)=17.6(2.2), 55% male, 56.3% non-Hispanic white, 51.2% leukemia/lymphoma]. Participants completed a survey containing demographic and M-PAC measures of reflective (instrumental and affective attitudes, perceived opportunity, perceived capability), regulatory (behavioral regulation), and reflexive (habit, identity) processes. Participants wore an accelerometer on their non-dominant wrist (ActiGraph GT9X) for 14 days to determine daily moderate-to-vigorous PA (MVPA). Significant (p<.10) demographic (race/ethnicity), medical (diagnosis type, treatment modalities), and M-PAC correlates of MVPA were then entered into a hierarchical linear regression controlling for age and sex.

Results
Univariate analysis found that race/ethnicity (r=.34), number of treatment modalities (r=-.36), PA opportunities (r=.19), perceived capabilities (r=.20), habit (r=.21), and behavioral regulation (r=.29) were correlated with MVPA (p<.10). The overall linear regression model explained 38% of the variance in MVPA (R2=.38, p=<.001). Significant (p<0.05) predictors of MVPA included age (B=8.2), non-Hispanic, White race/ethnicity (B=27.0) and number of treatment modalities (B=-65.5). M-PAC constructs explained an additional 11% of the variance in MVPA (R2 change=0.11, p=.03), with only behavioral regulation approaching significance in the full model (B=11.7, p=.08).

Discussion
Younger, racially and ethnically diverse AYA, and those receiving multiple treatment modalities are at risk for lower PA during cancer therapy, suggesting more targeted PA support is needed for these groups. Encouraging self-regulatory behaviors, including planning and scheduling PA into daily routines, could be a promising strategy for increasing MVPA in this population. Interventions designed to enhance perceived PA capabilities, increase PA opportunities and support PA habit formation may also be beneficial.