QUADRICEPS STRENGTH AND KNEE ABDUCTION MOMENT DURING LANDING IN ADOLESCENT ATHLETES

This study found that isokinetic quadriceps strength is not significantly associated with peak knee abduction moment during landing in adolescent athletes, suggesting that frontal-plane knee loading is driven more by multi-joint movement strategies than by single muscle group capacity.

The Gist

The "Shock Absorber" Myth: Why Strong Thighs Don't Always Save Your Knees

Imagine your knee is like a high-performance car suspension system. When you jump off a box and land, your body needs to absorb a massive amount of impact energy. If the suspension is too stiff, the whole car shakes violently. If it's too loose, the car bottoms out.

For years, coaches and doctors have worried about a specific type of "bad landing" called Knee Valgus. Think of this as your knee collapsing inward like a wobbly tower of Jenga blocks. This inward collapse creates a twisting force (called the Knee Abduction Moment, or KAM) that is a major red flag for tearing the ACL, the crucial ligament that keeps your knee stable.

The big question this study asked was: "If an athlete has weak thigh muscles (quadriceps), will their knees collapse inward more often?"

The common sense theory was: Weak thighs = Stiff landing = Bad knee collapse.

The Experiment: Testing the Theory

The researchers gathered 134 healthy teenage athletes. They did two things:

1. The Strength Test: They measured how hard each athlete could push their leg out using a machine (like a giant, high-tech leg press). This told them how "strong" the shock absorbers were.
2. The Jump Test: They made the athletes jump off a box and land, using high-speed cameras and force plates to see exactly how their knees moved.

They wanted to see if the athletes with the weakest "shock absorbers" were the ones whose knees collapsed inward the most.

The Surprise Result: No Connection Found

The results were surprising. There was almost no link between how strong the thigh muscles were and how much the knees collapsed inward.

In fact, the strength of the thigh muscles explained only 1.3% of why some knees collapsed and others didn't. It's like trying to explain why a car swerves on a curve by only looking at the size of the engine. The engine size matters, but it's not the main reason the car swerves.

The "Big Picture" Analogy

Think of landing a jump like a dance routine.

• The Quadriceps (Thighs) are just the lead dancer's legs.
• The KAM (Knee Collapse) is the whole group falling out of sync.

The study found that even if the lead dancer's legs are super strong, the whole group can still fall out of sync if the hip dancer (hip muscles) isn't holding the frame, or if the trunk dancer (core muscles) is leaning the wrong way.

The researchers realized that landing isn't just about one muscle group doing its job. It's about neuromuscular coordination—how all the muscles in your legs, hips, and core talk to each other at the exact same millisecond.

Why This Matters

This study is a wake-up call for injury prevention.

• The Old Way: "Your knees are collapsing? You must be weak. Do more leg extensions!"
• The New Way: "Your knees are collapsing? Your whole body's movement strategy is off. We need to teach your hips, your core, and your brain how to work together as a team."

The Takeaway

Having strong thighs is great, but it's not a magic shield against knee injuries. If your brain tells your body to land stiffly or with your knees caving in, strong muscles alone won't fix it.

To keep athletes safe, we need to stop looking at muscles in isolation and start training the entire movement system. It's not about building a stronger engine; it's about teaching the driver how to steer through the curve without crashing.

Technical Summary

1. Problem Statement and Background

Anterior Cruciate Ligament (ACL) injuries are a significant burden in adolescent sports, often occurring via non-contact mechanisms during cutting, pivoting, and landing. A primary biomechanical risk factor for these injuries is an elevated Knee Abduction Moment (KAM), which reflects excessive frontal-plane loading (medial knee collapse) and internal tibial rotation.

Current literature suggests that athletes may adopt suboptimal movement strategies where insufficient sagittal-plane shock absorption (controlled by the quadriceps) forces a compensatory shift of load to the frontal plane, increasing KAM. While it is hypothesized that quadriceps weakness leads to "stiffer" landings with limited knee flexion—thereby increasing frontal-plane stress—the specific relationship between maximal quadriceps strength and peak KAM during dynamic landing tasks remains poorly understood. This study aims to clarify whether a direct association exists between these two variables in adolescent athletes.

2. Methodology

Study Design:

• Type: Secondary analysis of previously collected data from the "2024 Baseline ACL Injury Risk Screening and Normative Data" study.
• Participants: 134 healthy adolescent athletes (Mean age: 15.1 ± 1.6 years; 15.1 ± 1.6 m height; 66.5 ± 19.0 kg weight).
• Inclusion Criteria: Recreational to competitive athletes, free of injuries limiting activity, and no suspected pregnancy.

Data Collection Procedures:

1. Quadriceps Strength Assessment:
   • Instrument: Isokinetic dynamometer (Humac-Norm, CSMi).
   • Protocol: Participants performed two sets of reciprocal concentric knee extensions/flexions at 60°/s. The first set was a familiarization trial; the second set consisted of five maximal-effort repetitions.
   • Metric: Peak instantaneous concentric knee extension torque (Nm) on the dominant limb, normalized to body mass (Nm/kg).
2. Biomechanical Landing Assessment:
   • Task: Drop Vertical Jump (DVJ) from a 12-inch (0.30 m) box. Athletes stepped off, landed bilaterally, and immediately performed a maximal vertical jump.
   • Instrumentation:
     • Motion Capture: Qualisys Miqus Video system (110 Hz) using markerless 3D pose estimation (Theia3D).
     • Force Plates: Dual Bertec force plates (880 Hz).
   • Processing: Data filtered at 20 Hz (dual-pass Butterworth). Inverse kinematics calculated in Visual3D.
   • Metric: Peak external KAM (Nm/kg) on the dominant limb during the landing phase (defined from initial contact to toe-off).

Statistical Analysis:

• Model: Simple linear regression.
• Variables: Independent variable = Normalized quadriceps strength; Dependent variable = Normalized peak KAM.
• Power: A priori power analysis indicated a minimum of 54 subjects were required to detect a moderate effect size ($f^2 = 0.15$) with 80% power.

3. Key Results

• Statistical Significance: The linear regression analysis revealed no statistically significant association between normalized quadriceps strength and peak normalized KAM.
  • Regression Coefficient ($\beta$): -0.053 (95% CI: [-0.137, 0.030]).
  • Model Fit: $R^2 = 0.013$ (Quadriceps strength explained only 1.3% of the variance in KAM).
  • F-statistic: $F(1, 119) = 1.62$.
  • p-value: $p = 0.206$.
• Trend: While a negative trend was observed (suggesting stronger quadriceps might correlate with slightly lower KAM), the confidence interval crossed zero, and the effect size was negligible.

4. Key Contributions and Discussion

• Refutation of Hypothesis: The study challenges the assumption that maximal concentric quadriceps strength is a primary driver of frontal-plane knee loading (KAM) during vertical landing tasks.
• Mechanistic Insight: The authors argue that KAM is driven more by multi-joint movement strategies and neuromuscular coordination (involving the hip, trunk, and hip abductors) than by the isolated capacity of the quadriceps muscle group.
• Task Specificity: The lack of association may be due to the mismatch in assessment modalities:
  • Strength was measured concentrically (isokinetic).
  • Landing requires rapid eccentric control.
  • The drop jump is a sagittal-plane dominant task, whereas KAM is a frontal-plane metric. The authors suggest that lateral or cutting-based movements might better elicit the relationship between strength and KAM.
• Clinical Implication: Screening for ACL risk based solely on maximal quadriceps strength may be insufficient. Prevention strategies should focus on whole-body neuromuscular training, trunk control, and hip mechanics rather than isolated strength training.

5. Limitations

• Assessment Modality Mismatch: Comparing concentric isokinetic strength to eccentric landing mechanics limits the ecological validity of the findings.
• Task Specificity: The drop jump may not sufficiently challenge frontal-plane stability compared to sport-specific cutting maneuvers.
• Measurement Variability: Potential inter-tester variability due to multiple administrators, though standardized protocols were used.
• Lack of EMG: The study did not measure muscle activation timing or co-contraction patterns, which could explain the lack of association.

6. Significance

This study provides critical evidence that isolated quadriceps strength is not a meaningful predictor of frontal-plane knee loading during drop jumps in adolescent athletes. It shifts the paradigm for ACL injury prevention, suggesting that risk mitigation requires a holistic approach targeting neuromuscular coordination, trunk stability, and hip control rather than focusing exclusively on strengthening the quadriceps. Future research should investigate the role of rate of torque development (eccentric/isometric) and multi-joint coordination in modulating KAM.