Hamstrings in the Squat - A Biomechanical and Anatomical Analysis

Introduction

The role of the hamstrings in the barbell back squat has long been misrepresented in mainstream fitness culture. Prevailing myths, largely driven by bodybuilding-centric interpretations, suggest that squats fail to effectively train the hamstrings due to the simultaneous hip and knee movements that purportedly cancel out their involvement. However, this simplistic anatomical narrative fails to capture the complexity of how these muscles function in real-world biomechanics. This article aims to clarify the role of the hamstrings in squatting, with detailed analysis of muscle physiology, moment arms, torque dynamics, and clinical application.

Anatomy and Functional Role

The hamstrings comprise three muscles:

  • Biceps femoris (long and short heads)

  • Semitendinosus

  • Semimembranosus

These muscles originate from the ischial tuberosity (except the short head of the biceps femoris, which originates from the femur) and insert on the tibia and fibula. Their main actions are:

  • Hip extension

  • Knee flexion

Importantly, three of these four muscle components are biarticular, crossing both the hip and knee joints. This gives the hamstrings a unique functional profile, particularly in compound movements like squats where both joints are active.

Misconception: Muscle Length Equals Muscle Function

A commonly cited notion is that because the hip extends and the knee flexes during the squat, the hamstrings' length remains unchanged, meaning they do not contribute significantly to the movement. This is biomechanically false.

Isometric contraction is a well-established mechanism through which muscles contribute to joint stability and force transmission without changing length (Behm & Sale, 1993). The hamstrings stabilise the pelvis, resist anterior shear at the knee, and maintain posterior chain integrity throughout the lift (Schoenfeld, 2010).

Biomechanical Contribution

1. Kinetic Chain Stability

During the descent:

  • The hips flex

  • The knees flex

During the ascent:

  • The hips extend

  • The knees extend

Despite limited length change, the hamstrings are under constant tension, especially in low-bar back squats where the trunk angle increases and the posterior chain is heavily recruited (Swinton et al., 2012).

2. Moment Arms and Torque

Torque (τ) is a product of the force (F) and the length of the moment arm (r):

  • Hip Moment Arm: Increases with depth, requiring more hip extensor torque

  • Knee Moment Arm: Modulated based on bar position and stance

The hamstrings contribute to the hip extensor moment even when acting isometrically. This is especially apparent in movements like the "good morning" squat recovery, where posterior chain engagement becomes visibly dominant (Glassbrook et al., 2017).

Depth Control and Rebound Mechanics

  • At mid-depth, the hamstrings are elongated at the hip and moderately flexed at the knee.

  • At the bottom, they stabilize the pelvis and knee joint, contributing to the stretch-reflex rebound without concentrically shortening.

This phenomenon is why squats—especially low-bar—train the hamstrings in ways that leg curls do not. The stress is distributed through tension and force transfer, not just shortening contractions.

Functional Outcomes in Practice

Experienced coaches observe consistent hamstring hypertrophy and strength development in athletes who progressively overload barbell squats (Bryanton et al., 2012). Key real-world findings:

  • Increased posterior chain strength

  • Soreness in glutes and hamstrings

  • Greater deadlift performance

Contrast this with machine-based leg training, which lacks the kinetic complexity and systemic recruitment of compound barbell lifts.

Summary

Key PointRole of Hamstrings in SquatPrimary FunctionIsometric contraction for joint stabilityMisconceptionNo activation due to length cancellationReal ContributionKnee stabilization, hip support, posterior chain tensionMost Active PhaseDescent, bottom range, early ascentTraining TakeawayIsometric load = hypertrophy and performance gains

Conclusion

The hamstrings are integral to a well-executed barbell back squat. Although their primary action may not involve visible concentric shortening, their isometric function is essential to knee integrity, pelvic alignment, and hip extension. In biomechanical terms, the hamstrings act as a posterior stabilizer, critical in balancing anterior forces and distributing joint torque efficiently. Coaches and athletes should dismiss reductionist interpretations and respect the complexity of this muscle group's role in squatting.

References
  1. Behm DG, Sale DG. Intended rather than actual movement velocity determines velocity-specific training response. J Appl Physiol. 1993;74(1):359-368.
  2. Schoenfeld BJ. The mechanisms of muscle hypertrophy and their application to resistance training. J Strength Cond Res. 2010;24(10):2857-2872.
  3. Swinton PA, Stewart A, Lloyd R, Agouris I, Keogh JW. A biomechanical comparison of the traditional squat, powerlifting squat, and box squat. J Strength Cond Res. 2012;26(7):1805-1816.
  4. Glassbrook DJ, Helms ER, Brown SR, Storey AG. A review of the biomechanical differences between the high-bar and low-bar back-squat. J Strength Cond Res. 2017;31(9):2618-2634.
  5. Bryanton MA, Kennedy MD, Carey JP, Chiu LZ. Effect of squat depth and barbell load on relative muscular effort in squatting. J Strength Cond Res. 2012;26(10):2820-2828.