Sr. Backend Engineer (TypeScript / Node.js)
Gain Fitness Backend Engineer Gain Fitness is looking for an ambitious, AI-savvy backend engineer who's excited about startups, fitness, and building intelligent software that helps people live better lives. Gain is a pioneer in workout apps and recently relaunched as an AI-first personal fitness assistant. Our platform serves many fitness personas and user needs through one app—weight loss, active aging, strength training, endurance athletes, HYROX competitors, recovery, yoga, posture correction, and more. Our vision: 1 fitness app to deliver 1,000 personalized experiences to 1,000 different users. You'll work directly with our founder and a small team of experienced product, design, and mobile engineers to build the intelligence behind the platform. What You'll Do:
- Build and maintain backend services using TypeScript and Node.js
- Own APIs, user accounts, authentication, and backend infrastructure
- Develop workout generation and personalization algorithms
- Translate exercise science into scalable software alongside fitness experts
- Expand intelligent recommendations across strength, recovery, yoga, and cardio
- Help build AI-powered systems that adapt workouts to each individual's goals, experience, equipment, and progress
Skills:
- Strong TypeScript and Node.js experience
- Experience designing production APIs and backend systems
- Strong software architecture and problem-solving skills
- Comfortable working with data models and recommendation algorithms
- AI-native mindset and curiosity about applying modern AI tools to software development
Bonus Points:
- Experience with recommendation engines or personalization systems
- Consumer mobile app or startup experience
- Interest in fitness, exercise science and/or health tech
We're a remote-first company with teammates across California, Vancouver, Thailand and Toronto. The team is small, senior, highly autonomous, and focused on building exceptional products. This is a flexible part-time role with hourly pay and meaningful equity for someone who wants to help build the future of personalized fitness. If you're excited about AI + fitness + startups, we'd love to hear from you.