Investigation of the Relationship Between Frontal Plane Knee Projection Angle and Dynamic Balance, Agility, and Ankle Dorsiflexion Mobility in Adolescent Female Volleyball Players
This cross-sectional study of 66 adolescent female volleyball players found no significant association between frontal plane knee projection angle (FPPA) during a single-leg squat and dynamic balance, agility, or ankle dorsiflexion mobility, suggesting FPPA should not be used as a standalone functional screening measure.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
In the world of youth sports, the body is a machine under constant construction. For adolescent girls, whose bodies are changing rapidly, the way they move can sometimes put their joints at risk, particularly the knees. One specific movement pattern that doctors and coaches watch closely is the knee collapsing inward while the foot stays planted. This is often called dynamic knee valgus, a three-dimensional shift where the hip, knee, and ankle all twist in ways that can strain the ligaments. To catch this early, researchers often look at the angle the knee makes when a person squats down on one leg. This measurement, known as the frontal plane knee projection angle, is a quick way to see if the knee is drifting inward. Alongside this, coaches frequently use other field tests to check an athlete's overall health: tests for balance that see how far a player can reach while standing on one foot, tests for agility that measure how fast they can change direction, and tests for ankle flexibility that see how far the knee can move forward over the toes. The big question for sports medicine is whether these different pieces of the puzzle fit together. Does a knee that collapses inward during a squat also mean an athlete has poor balance, is slower on their feet, or has tight ankles?
A team of researchers in Turkey set out to answer this question by studying a group of young female volleyball players. They gathered sixty-six athletes, ranging in age from ten to nineteen, who were training regularly at a volleyball academy in İzmir. The researchers wanted to see if the angle of the knee during a single-leg squat was linked to how well the players performed on the balance, agility, and ankle flexibility tests. To do this, they filmed each player performing a squat on one leg, using a camera placed directly in front of them to measure the angle of the knee. They then had the same players perform the other tests: a balance challenge where they reached in different directions, a running drill that required sprinting forward, shuffling sideways, and running backward, and a mobility test where they lunged forward to see how far they could get their knee toward a wall without lifting their heel.
The results of this investigation were clear and somewhat surprising. The researchers found no meaningful connection between the angle of the knee during the squat and the other physical abilities they measured. A player whose knee collapsed inward did not necessarily have worse balance, nor were they slower in the agility drill, nor did they have less flexible ankles. The data showed that these different aspects of movement were largely independent of one another in this group. For instance, the time it took to complete the agility run did not predict how the knee moved during the squat, and the distance a player could reach during the balance test offered no insight into their knee alignment. Even when the researchers looked at the group as a whole, trying to see if age, height, or body mass could explain the knee angles, the picture remained the same. The tests they used to measure balance, speed, and flexibility simply did not act as reliable indicators for the specific way the knee was moving during the squat.
There was one small observation that stood out, though the researchers treated it with caution. They noticed that taller players tended to have slightly larger knee angles during the squat. However, because the study did not measure the biological maturity of the athletes—such as exactly where they were in their growth spurts—this link between height and knee angle cannot be taken as a firm rule. It might be that taller girls are at a different stage of development, or it might be a coincidence. The researchers emphasized that this finding needs more study before it can be considered a fact. The main takeaway from the work is that coaches and medical professionals should not assume that a player who fails a balance test or has a stiff ankle will automatically have poor knee mechanics, or vice versa.
This study suggests that using a single test to screen for all lower-body movement risks is not enough. The tools that measure balance, agility, and ankle mobility capture different parts of how an athlete moves, and they do not overlap enough to replace a direct look at the knee itself. While the single-leg squat is a practical way to see how the knee behaves, it does not tell the whole story of an athlete's physical readiness. To truly understand the risk of injury in young female volleyball players, the researchers conclude that we need to look at these factors separately and perhaps include other measurements, like hip strength or how the body matures, in future studies. For now, the evidence indicates that a player's ability to balance or change direction quickly does not predict how their knee will align when they squat, and each of these qualities must be assessed on its own terms.
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