If you have limited time to train, should you do squats or leg extensions? The research strongly favors compound movements — but isolation work still has its place.
Defining the Terms
Compound exercises involve multiple joints and muscle groups: squats, deadlifts, bench press, rows, pull-ups, overhead press. Isolation exercises target a single muscle group through a single joint: bicep curls, leg extensions, lateral raises, tricep pushdowns.
Hypertrophy: Comparable Results, Different Time Investment
Gentil et al. (2015), published in the Journal of Strength and Conditioning Research, compared elbow flexor (bicep) hypertrophy between two groups over 10 weeks: one performed lat pulldowns only (compound), the other performed lat pulldowns plus bicep curls (compound + isolation). Result: both groups achieved statistically identical bicep growth. The isolation work provided zero additional hypertrophy benefit.
de França et al. (2015), in the Journal of Sports Science & Medicine, found similar results: adding isolation exercises for the biceps and triceps to a compound-only program did not increase arm muscle thickness over 8 weeks in trained subjects.
Paoli et al. (2017), in Frontiers in Physiology, studied trained men performing either multi-joint (compound) or single-joint (isolation) exercises. Muscle growth was equivalent, but the compound group completed their training in significantly less time — roughly 35-40 minutes vs. 55-60 minutes for equivalent volume.
Strength: Compound Wins Clearly
For pure strength development, the advantage of compounds is more pronounced. Suchomel et al. (2018), in a comprehensive review in Sports Medicine, concluded that multi-joint exercises produce greater improvements in maximal strength, power output, and athletic performance metrics than single-joint exercises. The neuromuscular coordination demands of compound lifts — recruiting multiple muscle groups in a coordinated pattern — transfer more directly to sport and functional movements.
When Isolation Makes Sense
- Rehabilitation: Targeting a specific weak link without loading other structures
- Addressing imbalances: If one side is measurably weaker (unilateral isolation work)
- Bodybuilding specificity: Bringing up a lagging muscle group for aesthetic purposes
- After compounds: As finisher work when primary movements are done
Practical Programming
For a time-efficient, evidence-based program, build around 4-5 compound movements:
- Squat pattern: Back squat, front squat, or goblet squat
- Hinge pattern: Deadlift, Romanian deadlift, or hip thrust
- Horizontal push: Bench press or push-up variations
- Horizontal pull: Barbell row or cable row
- Vertical push/pull: Overhead press and pull-ups/chin-ups
If time permits, add 1-2 isolation exercises at the end targeting personal weaknesses. But never sacrifice compound work to fit in isolation exercises.
Key Takeaway
References
- Gentil, P. et al. (2015). Effect of adding single-joint exercises to a multi-joint exercise resistance-training program. Journal of Strength and Conditioning Research, 27(4), 927-932.
- de França, H.S. et al. (2015). The effects of adding single-joint exercises to a multi-joint exercise program on upper body muscle size and strength. Journal of Sports Science & Medicine, 14(1), 210-217.
- Paoli, A. et al. (2017). Resistance training with single vs. multi-joint exercises at equal total load volume. Frontiers in Physiology, 8, 1105.
- Suchomel, T.J. et al. (2018). The importance of muscular strength: training considerations. Sports Medicine, 48(4), 765-785.