Effect of Cervical Forward Flexion Compensation on Isokinetic Knee Strength Test Outcomes in University Soccer Players via Wearable Flexible Strain Sensor: A Self-Controlled Trial

In the world of sports medicine and physical rehabilitation, doctors and therapists rely on a precise method to measure how strong a person's muscles are. This technique, known as isokinetic testing, involves a machine that moves a joint, such as the knee, at a constant speed while measuring the force the muscles produce against it. It is considered the gold standard for assessing athletic performance and tracking recovery after an injury. For these measurements to be trustworthy, the person being tested must remain perfectly still in every part of their body except for the specific joint being examined. If the body shifts or compensates in unexpected ways, the numbers recorded by the machine can become misleading, making a strong leg look weak or a weak leg look strong.

For years, researchers have focused on keeping the hips, trunk, and lower legs locked in place during these tests. However, a critical piece of the body has largely been ignored: the neck. The head sits atop the spine, and when a person leans their head forward, it triggers a chain reaction through the entire body. This study, conducted with university soccer players, set out to investigate whether this subtle forward tilt of the head could secretly alter the results of knee strength tests. The researchers wanted to know if a common posture habit, often seen in people who spend long hours looking down at screens, could skew the data used to make decisions about athletic training and injury recovery.

To find the answer, a team of researchers from Southern Medical University and South China University of Technology designed a self-controlled experiment involving thirty-nine university soccer players. The team needed a way to measure the movement of the neck with extreme precision while the athletes were strapped into a heavy testing machine. Traditional cameras or bulky sensors were not suitable for this dynamic environment, so the researchers turned to a new type of technology: a wearable flexible strain sensor. This device is a thin, stretchable strip made of special materials that can detect tiny changes in shape. The team attached this sensor vertically along the back of the neck, just above the shoulders. As the neck bent forward, the sensor stretched, changing its electrical resistance in a way that could be recorded instantly by a computer.

The study proceeded in two main phases. First, the researchers verified that their new sensor could accurately track neck movement. They asked the athletes to move their heads forward and back at a specific speed while the sensor recorded the data. They compared these readings against a standard, high-precision machine designed to measure neck motion. The results showed an almost perfect match between the flexible sensor and the heavy machine, proving that the lightweight, wearable device could capture the subtle nuances of neck posture in real time.

In the second phase, the researchers put the sensor to work during actual knee strength tests. The athletes sat in the isokinetic machine, which was set up to measure the power of their leg muscles as they pushed and pulled against the device. The tests were performed at two different speeds: a slow, controlled speed of 30 degrees per second and a slightly faster speed of 60 degrees per second. During the tests, the athletes were instructed to perform the movements in two different ways. In one set of trials, they kept their heads in a neutral, straight-ahead position. In the other set, they were allowed to let their heads drop forward into a compensatory posture, mimicking the way people often lean when they are exerting maximum effort.

The findings were clear and significant. When the athletes allowed their heads to tilt forward, the strength measurements for their legs changed dramatically. At the slower speed of 30 degrees per second, the forward head posture caused significant alterations in the recorded strength for both the muscles that straighten the knee and the muscles that bend it. When the speed increased to 60 degrees per second, the forward head posture still significantly altered the strength of the muscles that straighten the knee, while the bending muscles remained unaffected at this faster pace. The data showed that the body was using the forward head position to change the mechanics of the test, likely by shifting weight or engaging other muscles, which systematically biased the leg strength measurements.

The researchers concluded that the position of the neck is not a minor detail but a major factor that can invalidate knee strength tests. They found that the forward flexion of the neck acts as a hidden variable that systematically biases the results, making it difficult to compare one athlete's performance to another's or to track an individual's progress accurately over time. To fix this, the study proposes a new, strict standard for these tests: the person being tested must keep their head perfectly neutral, looking straight ahead, with their back firmly supported. This simple adjustment ensures that the machine measures only the strength of the knee muscles, without interference from the rest of the body.

Beyond the specific findings for soccer players, this research highlights a broader lesson for medical testing: the body functions as a connected system, and you cannot isolate one part without considering the whole. The study also demonstrated the practical value of the flexible sensor technology. By proving that a small, wearable strip could accurately monitor complex movements like neck flexion, the researchers opened the door for more portable and accessible ways to monitor human movement in clinics and on the field. For now, the message to coaches, therapists, and athletes is straightforward: if you want to know how strong your legs really are, you must keep your head up.