Oslo Sports Trauma Research Center

Description

In a number of sports, athletes are required to perform jumping actions or rapid accelerations, thereby placing great demands on their knee extensor apparatus. Many of these athletes complain of pain at the distal or proximal insertion of the patellar tendon, or at the insertion of the quadriceps tendon. This syndrome of pain during and after activity is known as "jumper's knee", but is also referred to as patellar tendinopathy. Athletes with jumper's knee are usually unable to recall one specific traumatic event. It is therefore assumed that in most cases the injury results from repetitive overloading of the tendon fibers.
Histological findings of degeneration and fibrotic scarring in the tendon itself, as well as in the bone-tendon junction suggest that the injury consists of an unhealed partial tendon tear. This view is supported by the demonstration of abnormalities in the tendon tissue using ultrasonography, computerized tomography and magnetic resonance imaging.
Our understanding of the etiology of partial ruptures of the patellar tendon is incomplete. Epidemiological studies show that the prevalence of jumper's knee among volleyball players is related to the frequency of training sessions and also that the prevalence is higher among players training on hard surfaces. Similar epidemiological data from other sports is not available, and it is not known why some athletes have problems whereas others do well despite an equally high training volume.

It is conceivable that an athlete, who is able to generate a high impulse during take-off when jumping or running, is at risk of injury due to high repetitive loads on the extensor apparatus.

Thus, the aim of the present study was to characterize the performance ability of the leg extensors in a group of athletes with jumper's knee using a standardized program of jump and power tests, and to compare the results with those of a matched control group.

Method: Patients and control groups (12 players in each) were selected from a population of 141 well-trained male Norwegian volleyball players, of which 55 (39%) satisfied the diagnostic criteria for jumpers knee. The testing program consisted of a standing jump, a countermovement jump, a 15-second rebound jump test, a standing jump with a 20-kg load, and a standing jump with a load corresponding to one-half of the subjects body weight. Jump height and power were measured with a Bosco mat, which consists of a contact mat connected to an electronic timer/computer.

Results: In the jump tests, the patient group performed better than the control group in the counter-movement jump (15% increase), power during rebound jump (41%) work done in standing jump (12%) and countermovement jump (22%), and the difference between countermovement jump and standing jump (effect of adding eccentric component). Athletes with jumpers knee demonstrated better performance in jump tests than uninjured athletes, particularly in ballistic jumps involving eccentric force generation.
It is interesting to note that the test results did not differ between the groups for all modes of jumping. The standing jump is designed as a "pure" concentric movement, and the results for unloaded jumping did not differ between the groups. However, for the countermovement jump, which consists of a ballistic movement of a rapid eccentric muscle action immediately followed by a maximal concentric contraction, there was a significant difference between the groups. We also observed a significant difference in the 15-second rebound jump test, which consists of a series of ballistic jumping movements.

Consequently, it may be assumed that the main difference between the groups was the way in which they were able to utilize the eccentric pre-stretch component of the ballistic motion to increase their jumping height. Thus, eccentric force production is suggested to be a primary cause of microruptures, since eccentric force production may exceed conventional concentric and isometric forces threefold and therefore also exceed the inherent strength of the tendon.