Sr. Backend Engineer (TypeScript / Node.js)
Build and maintain backend services using TypeScript and Node.js. Develop workout generation and personalization algorithms while translating exercise science into scalable software.
- Remote
- California, United States, Ottawa, Ontario, Canada, Thailand, Toronto, Ontario, Canada, Vancouver, British Columbia, Canada
- Posted Jul 3, 2026
- 1 position
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Job summary
Remote • Consumer Fitness • AI • Mobile Apps 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. https://www.gainfitness.com 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 About the Team 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.
What you’ll do
Build and maintain backend services using TypeScript and Node.js. Develop workout generation and personalization algorithms while translating exercise science into scalable software.
Requirements
Strong experience in TypeScript and Node.js is required, along with a solid understanding of software architecture and problem-solving skills. An AI-native mindset and curiosity about applying modern AI tools to software development are also essential.
Listed skills
- TypeScriptPreferred
Other relevant skills
Identified from the job description. Confirm important requirements above.
- TypeScript
- Node.js
- API Design
- Backend Systems
- Software Architecture
- Problem-Solving
- Data Models
- Recommendation Algorithms
- AI
- Exercise Science
- Workout Generation
- Personalization Algorithms
- Fitness
- Health Tech
Job areas
- Technology
- Software
- Sports & Recreation
Additional details
- Minimum experience
- 5+ years
- Posting language
- English
- Working hours
- 40 hours per week
- Location requirements
- Country, United States
- Application method
- Direct apply is available