Step-ups are a powerhouse functional exercise that promotes longevity by building critical lower-body strength and enhancing balance to prevent age-related falls, a leading cause of injury in older adults. By mimicking daily tasks like climbing stairs, this unilateral movement targets the glutes, quads, and hamstrings while forcing the core to stabilize the body, which helps correct muscle imbalances and protects joint health. Beyond physical strength, step-ups serve as a joint-friendly, weight-bearing activity that stimulates bone-forming cells to combat osteoporosis and maintain independence well into later years.

For overall leg mass, step-ups are generally more effective because they build quads, glutes, and hamstrings together, improving functional strength, balance, and coordination, whereas leg extensions isolate just the quadriceps but lack functional carryover and stability challenges, making step-ups a superior choice for holistic leg development, though extensions can target specific quad heads for definition. 

Why Step-Ups Win for Overall Mass

• Multi-Joint Movement: Step-ups engage your quadriceps, glutes, hamstrings, and calves, promoting more comprehensive muscle growth.
• Functional Strength: They mimic real-life movements, improving balance, stability, and neuromuscular efficiency, which benefits other lifts.
• Correct Imbalances: As a unilateral exercise (one leg at a time), they help address strength differences between legs.
• Progressive Overload: Easily add weight (dumbbells, kettlebells) for continuous challenge. 

When Leg Extensions Are Useful

• Isolation: They are excellent for isolating and focusing on the quadriceps, great for bodybuilders wanting definition.
• Rehab/Pre-Exhaust: Can be used to pre-fatigue quads before compound movements or for rehab. 

The Verdict

For longevity and building significant, functional leg mass, step-ups are the clear winner over leg extensions. They provide more muscle activation across the entire leg and build practical strength, making them a more complete exercise for overall leg development. Use leg extensions as a supplementary exercise for quad isolation if needed, but prioritize step-ups for mass and strength. 

For optimal results, integrating both types of exercises offers a balanced approach, combining the functional benefits of step-ups with the targeted strength and knee support of leg extensions.