Smart PA

The ‘When’ and ‘How’ of Precision Physical Activity Promotion

Many people aim to become more physically active but struggle to follow through on their intentions in everyday life. This is often because contextual factors (e.g., stress, lack of time, weather) interfere with the goal of being more active. To support people in achieving their long-term activity goals, (digital) interventions need to take these contextual influences into account. The SmartPA project therefore investigated how situational, motivational, and emotional factors at the micro-timescale (hours to days) influence whether intentions to be physically active are successfully enacted.

Methodological approach

Using intensive longitudinal methods such as Ecological Momentary Assessment (EMA), data on intentions, self-efficacy, planning, affect, and contextual factors related to physical activity were collected several times per day. This approach was applied first in observational studies and later in intervention studies, including a Micro-randomized Trial (MRT) and a Randomized Controlled Trial (RCT).

Key findings

Study 1: Momentary physical activity intentions increased the likelihood of being active within 4 hours by ~28-fold, though one-quarter of intentions still remained unfulfilled.
Study 2: Positive, energetic affective states and low contextual barriers facilitated the formation of such intentions but did not directly increase activity.
Study 3: A mobile intervention with reinforced implementation intentions (concrete action plans) led to ~18 minutes more daily reported moderate-to-vigorous physical activity compared to an active self-monitoring control group.

These results highlight that both reflective processes (intention, planning, self-efficacy) and automatic processes (affect, contextual cues) play a role in translating intentions into behavior in everyday life.

Relevance for digital interventions

Based on the findings, the concept of “Transitional Intentions” – short-term, context-related derivatives of long-term intentions – was introduced as a central construct for precision health behavior promotion. Building on this, a micro-dynamic model of physical activity was developed, extending socio-cognitive and dual-process theories with a temporally dynamic perspective.

This model provides concrete guidance for the design of Just-in-Time Adaptive Interventions (JITAIs), which can support users precisely in moments when barriers are low or intentions require reinforcement. In addition, the project was the first to evaluate the use of Generative AI for creating JITAIs, showing that users found AI-generated interventions more acceptable than those authored by healthcare professionals.

Publications related to the project

Haag, D., Carrozzo, E., Pannicke, B., Niebauer, J., & Blechert, J. (2023). Within-person association of volitional factors and physical activity: Insights from an EMA study. Psychology of Sport and Exercise, 68, 102445.
Haag, D., Smeddinck, J. D., Vogelsang, A., & Blechert, J. (2025). Contextual and affective precursors of physical activity intention and enactment examined through EMA. Psychology of Sport and Exercise, 77, 102796.
Haag, D., Aulbach, M., Islambouli, R., Smeddinck, J., Thürmer, J., & Blechert, J. (2025). Turning Goals into Action: How Reinforced Implementation Intentions Can Boost Physical Activity. [OSF Preprint]
Haag, D., Kumar, D., Gruber, S., Hofer, D. P., Sareban, M., Treff, G., Niebauer, J., Bull, C. N., Schmidt, A., & Smeddinck, J. D. (2025). The Last JITAI? Exploring Large Language Models for Issuing JITAIs. CHI 2025.