Hammer Curls vs. Bicep Curls: Muscle Recruitment, Biomechanics, and Training Implications

Why are hammer curls harder than bicep curls?

Hammer curls often feel more challenging than traditional bicep curls primarily due to a shift in muscle recruitment, specifically increased engagement of the brachialis and brachioradialis, coupled with a less mechanically advantageous leverage point for the primary elbow flexors in a neutral grip.

Understanding the Bicep Curl

The traditional bicep curl, performed with a supinated (palms up) grip, is the quintessential exercise for targeting the biceps brachii.

• Primary Muscles Targeted: The biceps brachii (both long and short heads) are the star of this exercise. The biceps have two main functions at the elbow: flexion and supination (rotating the forearm so the palm faces up).
• Grip and Forearm Position: The supinated grip maximizes the mechanical advantage of the biceps, allowing it to efficiently perform both its flexion and supination roles simultaneously. This grip places the muscle fibers of the biceps in an optimal position for contraction.
• Biomechanical Advantage: In a supinated grip, the biceps brachii can generate peak force effectively, leveraging its anatomical insertion points to create a powerful pull. The brachialis and brachioradialis also assist, but the biceps takes the lead.

Deconstructing the Hammer Curl

The hammer curl, characterized by a neutral (palms facing each other) grip, presents a distinct challenge to the elbow flexors.

• Primary Muscles Targeted: While the biceps brachii is still active, the hammer curl places a much greater emphasis on the brachialis and the brachioradialis. The brachialis, located beneath the biceps, is a pure elbow flexor, unaffected by forearm rotation. The brachioradialis, a prominent muscle in the forearm, is also a powerful elbow flexor, particularly with the forearm in a neutral position.
• Grip and Forearm Position: The neutral grip effectively minimizes the supination function of the biceps. This shifts the primary workload of elbow flexion more heavily onto the brachialis and brachioradialis.
• Biomechanical Challenge: The neutral grip changes the line of pull and the leverage for the forearm muscles, often making the lift feel heavier even with the same weight.

The Core Reasons for Increased Difficulty

Several interconnected factors contribute to the perceived greater difficulty of hammer curls:

• Muscle Recruitment Differences:
  • Brachialis Dominance: The brachialis muscle is often referred to as the "workhorse" of elbow flexion. In a hammer curl, with the biceps' supination role reduced, the brachialis is forced to take on a significantly larger share of the load. This muscle, while strong, doesn't possess the same mechanical leverage as the biceps in a supinated position.
  • Brachioradialis Engagement: The brachioradialis, a large muscle of the forearm, is optimally recruited for elbow flexion when the forearm is in a neutral position. It has a longer lever arm compared to the biceps and brachialis, meaning it has to work harder to produce the same amount of force for a given load, contributing to increased perceived effort.
  • Reduced Biceps Contribution: While still active, the biceps brachii's overall contribution to force production is diminished because its supination function is largely taken out of the movement. This forces the other, often smaller or less mechanically advantaged, muscles to compensate more.
• Leverage and Force Production: The neutral grip alters the biomechanical leverage for the elbow flexors. The muscles involved in a hammer curl may be operating at a less mechanically efficient angle compared to the biceps in a supinated curl, requiring more absolute force to move the same weight. This can make the weight feel heavier.
• Grip Strength and Forearm Engagement: The neutral grip often requires greater stabilization from the forearm muscles and can feel more demanding on grip strength throughout the movement, indirectly contributing to the overall perceived difficulty.

Practical Implications for Training

Understanding these differences is crucial for designing a comprehensive arm training program:

• Varying Stimulus: Both bicep curls and hammer curls are valuable exercises that provide unique stimuli to the arm muscles.
• Targeted Development:
  • Bicep Curls: Excellent for isolating and building the peak of the biceps brachii, emphasizing its full range of motion and supination function.
  • Hammer Curls: Superior for developing overall arm thickness, particularly targeting the brachialis (contributing to width under the biceps) and the brachioradialis (adding mass to the forearm). They also offer a functional carryover to activities requiring a neutral grip.
• Progressive Overload: Apply the principles of progressive overload (gradually increasing weight, reps, or sets) to both exercises to ensure continuous adaptation and strength gains across all elbow flexors.

Conclusion

The perceived difficulty of hammer curls over bicep curls is not a sign of weakness, but rather a testament to the intricate biomechanics of the human arm. By shifting the primary workload to the brachialis and brachioradialis and altering the leverage, hammer curls provide a unique and challenging stimulus essential for comprehensive arm development. Incorporating both variations into your routine ensures balanced strength, size, and functional capacity across all your elbow flexor muscles.