What really determines knee extensor strength?

Knee extensor strength is central to both athletic performance and everyday function. Tasks such as rising from a chair, stair negotiation, landing, deceleration, and gait stability all depend heavily on quadriceps force production. As a result, clinicians often assess muscle architecture and tissue quality—fascicle length, pennation angle, stiffness, or “muscle quality”—to explain strength differences and guide intervention.

A recent study by Hirono et al. (2025) provides important clarity. By examining multiple muscle properties simultaneously, the authors addressed a key clinical question: which muscle characteristics actually independently explain knee extensor force production?

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What the researchers measured

The study assessed 48 healthy young adults, measuring quadriceps strength and architecture using ultrasound and dynamometry. Strength outcomes included:

• Peak isometric knee extension torque
• Rate of torque development (RTD) at early (0–50 ms) and later (0–200 ms) phases

These were tested at three knee angles (40°, 70°, and 100° of flexion) to account for muscle length effects.

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Key finding: muscle size dominates

When each variable was examined alone, many muscle properties appeared related to strength. Larger muscles, longer fascicles, lower echo intensity, and greater stiffness all correlated with higher torque and faster force production.

However, once these factors were analysed together, a clear pattern emerged:

👉 Quadriceps cross-sectional area was the only consistent independent predictor of maximal isometric strength at all knee angles.

Neither pennation angle, echo intensity, nor muscle stiffness independently explained peak torque when muscle size was accounted for.

In simple terms:
bigger muscles produced more force—regardless of architecture or tissue quality.

This reinforces a foundational principle of muscle physiology: force capacity is primarily determined by the amount of contractile tissue available.

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What about explosive strength (RTD)?

Rate of torque development is often thought to reflect “muscle quality” or stiffness rather than size alone. The results partially supported this—but only under specific conditions.

• Muscle cross-sectional area remained the strongest predictor of RTD at most knee angles and time windows.
• Fascicle length made an additional contribution only to late RTD (0–200 ms) when the knee was flexed to 100°—a lengthened muscle position.

This suggests that when muscles are stretched and passive structures are engaged, longer fascicles may enhance force transmission and rapid force production. At shorter muscle lengths, however, architecture and stiffness added little explanatory value beyond muscle size.

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Why other factors mattered less than expected

Several findings are clinically instructive:

• Echo intensity, often interpreted as “muscle quality,” correlated with strength but did not independently predict it. In young, healthy adults, variation in intramuscular fat may simply be too small to meaningfully influence force once size is considered.
• Muscle stiffness (shear elastic modulus) showed associations in simple analyses but did not independently explain strength. This may be because stiffness itself is partly related to muscle size.
• Pennation angle showed no meaningful contribution, despite often being cited as a determinant of force.

These results caution against over-interpreting isolated architectural or tissue measures without accounting for overall muscle mass.

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Joint angle matters—but not how you might think

Interestingly, muscle size explained less variance in strength at 40° of knee flexion than at deeper angles. This suggests that at shorter muscle lengths, non-morphological factors—such as neural drive, motor unit firing rates, or moment arm mechanics—may play a larger role.

For therapists, this reinforces that strength expression is task- and position-dependent, even when underlying muscle size is unchanged.

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Clinical implications for therapists

1. If strength is the goal, prioritise hypertrophy
This study confirms that increasing muscle size remains the most reliable way to improve isometric force capacity in healthy individuals.

2. Muscle “quality” metrics are supportive, not primary
Fascicle length, stiffness, and echo intensity may influence performance, but they do not override the importance of muscle mass—especially in young adults.

3. Train in lengthened positions when targeting rapid force
When late-phase force development matters (e.g., deceleration, landing, deep knee flexion tasks), exercises that load the quadriceps at longer muscle lengths may offer added benefit.

4. Be cautious with mechanistic explanations
Associations seen in isolation do not necessarily imply causation. This study highlights why clinicians should be wary of attributing strength changes to architectural features without considering muscle size.

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Final takeaway

Despite growing interest in muscle architecture and tissue properties, this study reinforces a clear message for practice:

Muscle cross-sectional area is the primary determinant of knee extensor strength and rapid force production.

Other architectural and tissue characteristics may modify performance in specific contexts, but they do not replace muscle size as the dominant contributor. For therapists designing strength-focused rehabilitation or performance programs, this evidence supports a continued emphasis on progressive loading strategies that drive hypertrophy, while using architectural considerations as refinements—not replacements—for sound strength training principles.