JM Press vs. Skull Crushers: Differences, Similarities, and Optimal Use

Is a JM press the same as skull crushers?

While both the JM press and skull crusher are highly effective triceps exercises, they are distinct movements with differences in grip, bar path, joint involvement, and overall biomechanics, leading to variations in muscle activation and training stimulus.

Understanding the Skull Crusher (Lying Triceps Extension)

The skull crusher, also known as the lying triceps extension, is primarily an isolation exercise designed to target the triceps brachii muscle. It is a fundamental movement for developing triceps mass and strength.

• Primary Muscle Targeted: The triceps brachii, with all three heads (long, lateral, and medial) engaged, though the long head often receives significant emphasis due to the arm position.
• Movement Pattern: It is almost exclusively an elbow extension movement. The shoulders remain relatively static, acting mainly as a pivot point.
• Bar Path: While lying supine on a bench, the weight (barbell, EZ bar, or dumbbells) is lowered from an extended arm position above the chest, by flexing the elbows, towards the forehead or just behind the head. The elbows point towards the ceiling throughout the movement.
• Joint Involvement: Primarily an elbow joint isolation exercise, minimizing involvement from other joints.
• Benefits:
  • Excellent for direct triceps hypertrophy and isolation.
  • Can help improve the "lockout" phase of pressing movements.
  • Relatively straightforward to learn the basic mechanics.
  • Allows for a significant stretch on the triceps, particularly the long head.

Understanding the JM Press

The JM press is a unique and specialized triceps exercise, often favored by powerlifters for its effectiveness in building lockout strength for the bench press. It is a hybrid movement, combining elements of a close-grip bench press and a skull crusher.

• Primary Muscles Targeted: The triceps brachii are the primary movers, but due to its hybrid nature, there is significant synergistic involvement from the anterior deltoids and the upper pectoralis major.
• Movement Pattern: Unlike the skull crusher, the JM press involves both elbow extension and a degree of shoulder flexion/adduction. It's not a pure isolation or a pure compound press but a blend.
• Bar Path: From an extended arm position above the chest, the bar descends in a diagonal path towards the upper chest or neck area. As the bar descends, the elbows tuck in tightly. The upward phase involves a press that pushes the bar slightly away from the body, back towards the starting position.
• Joint Involvement: A compound movement involving both the elbow and shoulder joints, requiring more coordinated effort.
• Origin: Popularized by elite powerlifter JM Blakley, known for his tremendous bench press strength.
• Benefits:
  • Exceptional for building maximal triceps strength and lockout power, especially relevant for pressing sports.
  • Allows for heavier loads compared to a traditional skull crusher due to the compound nature and better leverage.
  • Develops greater overall arm and shoulder stability.

Key Differences in Biomechanics and Muscle Activation

The distinction between the JM press and skull crusher lies in their biomechanical execution and the resulting muscle activation patterns.

• Joint Action:
  • Skull Crusher: Primarily involves elbow flexion and extension. The shoulder joint remains relatively fixed.
  • JM Press: Involves both elbow flexion/extension and a controlled degree of shoulder flexion/extension and adduction. This makes it a multi-joint movement.
• Bar Path and Leverage:
  • Skull Crusher: The bar moves in an arc over the head, placing a long moment arm on the elbow joint, making it challenging to lift very heavy loads.
  • JM Press: The bar path is more direct and compact, descending towards the upper chest/neck. This shorter, more efficient path, combined with shoulder involvement, allows for greater mechanical advantage and thus heavier loads.
• Muscle Synergism:
  • Skull Crusher: Highly isolates the triceps, with minimal assistance from other muscle groups.
  • JM Press: Engages the triceps significantly, but also recruits the anterior deltoids and upper pectorals as synergists, distributing the load across more muscle groups.
• Stabilization Requirements:
  • Skull Crusher: Requires less overall body and shoulder stabilization, as it's an isolated movement.
  • JM Press: Demands greater shoulder and scapular stability due to the compound nature and the need to control the bar's diagonal path.

Similarities Between the Movements

Despite their differences, the JM press and skull crusher share some common ground as highly effective triceps builders:

• Primary Triceps Emphasis: Both exercises are excellent for directly targeting the triceps brachii for strength and hypertrophy.
• Supine Position: Both are typically performed lying supine on a flat bench.
• Equipment: Both can be performed with a barbell, EZ curl bar, or dumbbells.
• Goal: Both contribute significantly to overall arm strength and are particularly effective for improving the "lockout" phase of pressing movements like the bench press.

When to Incorporate Each Exercise

Choosing between the JM press and skull crusher depends on your specific training goals, experience level, and tolerance for joint stress.

• Skull Crusher:
  • For Triceps Isolation: Ideal for directly targeting and developing the triceps without significant involvement from other muscle groups.
  • Hypertrophy: Excellent for promoting muscle growth in the triceps.
  • Accessory Work: Often used as an accessory movement after primary compound presses to further fatigue the triceps.
  • Beginners: A good starting point for learning proper elbow extension mechanics.
• JM Press:
  • Maximal Triceps Strength: Preferred by advanced lifters and powerlifters aiming to build maximal triceps strength and lockout power for the bench press.
  • Heavier Loading: When the goal is to lift heavier loads for triceps development, benefiting from the compound nature.
  • Bench Press Carryover: Highly effective as a specific assistance exercise to improve bench press performance.
  • Experienced Lifters: Requires more coordination and shoulder stability, making it better suited for those with a solid foundation.

Safety and Proper Technique Considerations

Regardless of which exercise you choose, prioritizing proper form and safety is paramount to prevent injury and maximize effectiveness.

• Thorough Warm-Up: Always ensure a comprehensive warm-up for the elbow and shoulder joints before performing either exercise.
• Controlled Movement: Avoid using momentum or bouncing the weight. Focus on a slow, controlled eccentric (lowering) phase and a powerful, controlled concentric (lifting) phase.
• Elbow Health: Both movements place direct stress on the elbow joint. If you experience pain, consider alternative exercises, reduce the load, or adjust your technique.
• Spotter: Especially when performing the JM press or skull crushers with heavy loads, a spotter is highly recommended for safety.
• Grip:
  • Skull Crusher: Typically a narrower, pronated (overhand) grip, often slightly inside shoulder-width.
  • JM Press: A close grip, usually slightly wider than the skull crusher, but still narrower than a conventional bench press grip. Ensure a secure grip with thumbs wrapped around the bar.
• Shoulder Position (JM Press): Maintain scapular retraction and depression (pulling shoulder blades back and down) throughout the JM press to provide a stable base and protect the shoulders.

Conclusion: Distinct Tools for Triceps Development

In summary, the JM press and skull crusher are not the same exercise. While both are powerful tools for developing the triceps, they offer distinct training stimuli due to their differing biomechanics, joint involvement, and muscle activation patterns. The skull crusher is an excellent isolation movement for direct triceps hypertrophy, while the JM press is a hybrid compound exercise that allows for heavier loads and significantly contributes to maximal triceps strength and pressing lockout power. Understanding these differences allows you to strategically incorporate each exercise into your training program to best achieve your specific strength and physique goals.