Training splits define how the body's muscle groups are distributed across the training week — including full-body training (all major muscle groups each session, 2–4 days/week, optimal for beginners and time-limited individuals), upper-lower splits (alternating upper and lower body, 4 days/week, allowing higher frequency per muscle group than full-body while managing volume per session), push-pull-legs splits (grouping muscles by movement pattern, 3–6 days/week), and body-part specialization splits (1 muscle group per session, optimal only for advanced trainees with high volume requirements).
Role
Training split design is the organizational decision that determines whether the total weekly training stimulus is distributed optimally for the frequency and volume that the evidence supports — yet most recreational exercisers adopt their training split from gym culture tradition (the 'bro split' of one muscle group per day) rather than from evidence about optimal training frequency for their goals and experience level. The beginner or intermediate trainee who does a body-part split (chest Monday, back Tuesday, shoulders Wednesday) is training each muscle group once weekly while the evidence supports twice-weekly training for superior hypertrophy outcomes — a systematic under-frequency error producing inferior results from equivalent weekly training time.