Quadriceps Stiffness Changed Sharply With Hip and Knee Position in Small Study

A new ultrasound study found that passive quadriceps stiffness changed substantially depending on how the hip and knee were positioned, with the highest readings when participants lay supine with the knee bent to 60 degrees.

Quadriceps muscle stiffness is not a fixed property that can be interpreted without context. In a randomized crossover study published September 11 in Scientific Reports, researchers measured the rectus femoris and vastus lateralis of 36 healthy young adults while changing both hip position and knee angle.

The study used shear wave elastography, an ultrasound technique that estimates tissue stiffness from the speed of waves moving through muscle. Participants were assessed while lying down or sitting, with the knee flexed to either 20 or 60 degrees, and muscle relaxation was monitored with electromyography.

Both quadriceps muscles were stiffest in the supine 60-degree condition. Superficial regions were also consistently stiffer than intermediate and deeper regions, and men showed higher average passive stiffness than women in this sample.

Why Joint Position Changed Quadriceps Muscle Stiffness

The rectus femoris crosses both the hip and knee, so changing either joint alters its length. The vastus lateralis crosses only the knee, but its mechanics can still be affected by overall quadriceps configuration and fascial connections.

In the rectus femoris, mean shear-wave velocity was 1.98 m/s in the supine 60-degree condition, compared with 1.60 m/s supine at 20 degrees, 1.63 m/s seated at 60 degrees and 1.49 m/s seated at 20 degrees. The vastus lateralis followed a similar pattern, with its highest value also occurring supine at 60 degrees.

The researchers point to passive structural contributors such as titin, extracellular matrix and fascia. As muscle length increases, these tissues can carry more passive tension, which helps explain why a more lengthened configuration produced higher stiffness readings.

The Muscle Was Not Mechanically Uniform

The ultrasound mapping also showed that stiffness differed within the same muscle. Across positions, superficial regions consistently registered higher stiffness than intermediate and deeper regions.

That finding matters for researchers and clinicians because a measurement taken at one depth may not represent the whole muscle. It also reinforces that muscle tissue is mechanically heterogeneous rather than a single uniform block.

Men had higher average shear-wave velocity than women in this study: about 1.70 versus 1.56 m/s. The authors caution, however, that previous research on sex differences is conflicting and that equipment, protocols, age and tissue characteristics can influence results.

This Was Not a Training or Stretching Trial

It would be easy to turn the 60-degree result into an exercise prescription, but the study does not support that leap. Participants were passive during measurement. They did not complete a training intervention, and the researchers did not test hypertrophy, soreness, injury risk or long-term performance.

That distinction is especially important because passive stiffness and active force production are related but not interchangeable. Fitness Living recently covered an isometric resistance-training study that measured actual changes in strength and rapid force; the new quadriceps paper asks a different, more mechanical question.

Likewise, our report on static stretching after resistance exercise examined vascular recovery, not passive stiffness mapping. Similar words can describe very different outcomes.

Why the Measurement Context Matters

For rehabilitation and sports-science testing, the findings strengthen the case for standardizing joint position. If the same athlete is measured in a different hip or knee configuration on a later visit, a change in stiffness could reflect positioning rather than a true change in the tissue.

The same principle applies to muscle depth. Comparing superficial and deeper regions without a consistent protocol could create apparent differences that are partly methodological.

The authors suggest that future work combine passive stiffness measurements with active strength testing and extend the protocol to athletes and people with musculoskeletal conditions. That would help determine whether the mechanical patterns observed here translate to recovery, performance or rehabilitation decisions.

Important Limitations

Participants were young, healthy and physically active, but they had not performed systematic lower-body strength training during the previous six months. Findings therefore may not apply to trained lifters, older adults or people recovering from knee injuries.

Data collection occurred across multiple sessions over seven days, which may have introduced variation from factors outside the protocol. The same examiner collected all ultrasound measurements, so the study assessed within-examiner reliability rather than whether different clinicians would obtain the same values.

The authors also note that only statistically significant interactions were reported, potentially limiting exploration of subtler differences between the rectus femoris and vastus lateralis.