Hamstrings: Understanding Muscle Slack, Isolation, and Training Applications

How Do You Put Hamstrings on Slack?

Putting the hamstrings "on slack" refers to positioning the body in a way that shortens the distance between the muscles' origin and insertion points, thereby reducing their passive tension and active contribution to a movement. This strategic positioning is crucial for isolating other muscle groups, managing tension during rehabilitation, or optimizing specific movement patterns.

Understanding Muscle Slack

In exercise science, putting a muscle "on slack" means minimizing the passive tension within that muscle by bringing its origin and insertion points closer together. This concept is particularly relevant for biarticular muscles—those that cross two joints—like the hamstrings. When a muscle is on slack, its ability to generate force is diminished, making it less active or even allowing for greater range of motion at an adjacent joint without its limiting influence.

Hamstring Anatomy and Function

To understand how to put the hamstrings on slack, a brief anatomical review is essential. The hamstring group comprises three primary muscles:

• Biceps Femoris (long head): Originates from the ischial tuberosity (a bony prominence on your sit bone) and inserts onto the head of the fibula and lateral condyle of the tibia (shin bones).
• Semitendinosus: Originates from the ischial tuberosity and inserts onto the medial aspect of the tibia.
• Semimembranosus: Originates from the ischial tuberosity and inserts onto the medial condyle of the tibia.

All three muscles cross both the hip joint and the knee joint. Their primary actions are:

• Hip Extension: Pulling the leg backward (e.g., standing upright from a bent-over position).
• Knee Flexion: Bending the knee (e.g., a leg curl).

Because they act on both joints, the position of both the hip and the knee significantly influences the tension within the hamstrings.

Methods to Put Hamstrings on Slack

Putting the hamstrings on slack involves specific movements at the hip and/or knee joints that reduce the distance between their origin (ischial tuberosity) and insertion points (tibia/fibula).

1. Knee Flexion

• Mechanism: Bending the knee directly shortens the hamstrings from their distal (lower leg) attachment points towards their origin.
• Example: In a lying or seated leg curl, as you flex your knee, your hamstrings shorten significantly. If you were to perform a hip extension movement (like a glute kickback) while simultaneously flexing your knee, the hamstring's contribution to hip extension would be reduced.

2. Hip Extension

• Mechanism: Extending the hip (moving the thigh backward relative to the pelvis) shortens the hamstrings from their proximal (hip) attachment points towards their insertion.
• Example: When you stand upright from a bent-over position, or when performing a glute bridge or hip thrust, your hips move into extension. In a standing position with the hips fully extended, the hamstrings are in a relatively shortened state at the hip joint.

3. Combined Movements

The most effective way to put the hamstrings on significant slack often involves a combination of hip extension and knee flexion.

• Example: Consider the top position of a lying leg curl. Your knees are fully flexed, and your hips are typically in a neutral or slightly extended position (especially if your hips are slightly lifted off the bench). In this position, the hamstrings are maximally shortened and thus "on slack."
• Practical Application for Isolation: This principle is heavily utilized in exercises designed to isolate the gluteal muscles. In a hip thrust, the primary goal is often to maximize glute activation. By starting with the knees significantly bent (knee flexion) and extending the hips, the hamstrings are put on slack at the knee, reducing their ability to contribute to hip extension. This allows the glutes, which primarily act as hip extensors without crossing the knee joint, to become the dominant prime movers.

Practical Applications in Training and Rehabilitation

Understanding how to put hamstrings on slack has several important applications for fitness enthusiasts, trainers, and therapists:

• Targeting Specific Muscle Groups:
  • Glute Isolation: As mentioned, exercises like hip thrusts, glute bridges, and certain glute kickbacks utilize hamstring slack to minimize their involvement and maximize glute activation.
  • Quadriceps Isolation: While less about putting hamstrings on slack, understanding the opposite (putting them on stretch) is relevant. For instance, in a leg extension, the hamstrings are stretched over the knee, preventing them from assisting the quadriceps.
• Injury Rehabilitation: In the early stages of hamstring strain recovery, exercises performed with the hamstrings on slack can allow for gentle, controlled movement without placing excessive tension on healing tissues. This might involve light knee flexion exercises or very limited range hip extension movements.
• Improving Range of Motion: Sometimes, putting the hamstrings on slack at one joint can allow for greater range of motion at the other. For example, if you want to achieve maximum hip flexion (e.g., bringing your knee to your chest), flexing the knee will put the hamstrings on slack, allowing the hip to flex further before hamstring tension becomes a limiting factor.
• Reducing Cramping or Over-activity: In individuals prone to hamstring cramping or excessive tension during certain movements, strategically placing the hamstrings on slack can help alleviate discomfort or prevent over-recruitment.

Important Considerations

• Context is Key: The goal is not always to put hamstrings on slack. Many exercises, like Romanian Deadlifts (RDLs) or Good Mornings, specifically aim to stretch and load the hamstrings to enhance strength and flexibility.
• Relative Slack: "Slack" is a relative term. A muscle can be more or less on slack depending on the degree of joint angles.
• Biarticular Nature: Always remember the hamstrings cross two joints. Manipulating one joint's position will affect the tension across the other.

Conclusion

Mastering the concept of putting hamstrings on slack provides a powerful tool for optimizing your training and understanding muscle mechanics. By strategically positioning the hip and knee joints, you can dictate which muscles contribute most to a movement, enhance isolation, support rehabilitation, and improve overall biomechanical efficiency. This nuanced understanding moves beyond simply "doing an exercise" to truly "training a muscle" with precision and purpose.