Physical inactivity remains a critical global health issue, yet scalable strategies for sustained motivation are scarce. Conversational agents designed as simulated exercising peers (SEPs) represent a promising alternative, but their long-term impact is unclear. We report a six-month randomized controlled trial (N=280) comparing individuals exercising alone, with a human peer, or with a large language model-driven SEP. Results revealed a partnership paradox: human peers evoked stronger social presence, while AI peers provided steadier encouragement and more reliable working alliances. Humans motivated through authentic comparison and accountability, whereas AI peers fostered consistent, low-stakes support. These complementary strengths suggest that AI agents should not mimic human authenticity but augment it with reliability. Our findings advance human-agent interaction research and point to hybrid designs where human presence and AI consistency jointly sustain physical activity.

翻译：缺乏身体活动仍然是一个严峻的全球健康问题，然而可扩展的、能维持长期动机的策略却十分稀缺。作为模拟锻炼伙伴（SEPs）设计的对话代理代表了一种有前景的替代方案，但其长期影响尚不明确。我们报告了一项为期六个月的随机对照试验（N=280），比较了个人独自锻炼、与人类同伴一起锻炼以及与大型语言模型驱动的SEP一起锻炼的情况。结果揭示了一个伙伴关系悖论：人类同伴引发了更强的社会临场感，而AI同伴则提供了更稳定的鼓励和更可靠的工作联盟。人类通过真实的比较和问责来激励，而AI同伴则提供了一贯的、低风险的支持。这些互补的优势表明，AI代理不应模仿人类的真实性，而应以其可靠性来增强它。我们的研究发现推进了人机交互研究，并指向一种混合设计，即人类临场感和AI一致性共同维持身体活动。