Maximum Grf And Knee Joint Angle In Skilled And Non-skilled Jumpers

Introduction

The patellar ligament force has been shown to be higher than the quadriceps tendon force in cadaver knees at small amounts of knee flexion [1]. This discrepancy disappears as knee flexion increases beyond 45º. This is a concern in athletes who frequently jump because the knee joint experiences high loading during landing [2]. In addition, the knee joint is only slightly flexed at foot contact when landing from a jump, which puts the athlete at risk of a high force discrepancy across the patella. The present study examined skilled and non-skilled jumpers in order to see if predictions from cadaver knee studies [1] and modeling studies [1] apply to these athletes. Specifically, the purpose of this study was to examine if the maximum GRF occurs when knee flexion angle is less than 45º at take-off and landing in skilled and non-skilled jumpers. For take-off and landing, we also calculated the ratio of patellar ligament force-to-quadriceps tendon force using the approach of Huberti et al. [1].

Methods

Volunteers were placed in two groups: four skilled jumpers were male volleyball players with at least 10 years experience; four non-skilled jumpers were healthy, physically active males (table 1). All participants were asked to perform five maximal vertical jumps with a preparatory counter-movement. Two-dimensional kinematic data and verical ground reaction forces (GRF) were collected for each attempt. A 60-Hz digital camera (model TK-C1480; JVC) recorded the motion of the right side of body and the subject’s hip, knee, ankle, toe, and heel were subsequently digitized. A 40 x 60 cm force platform (Kistler) was used to measure the GRF during the jumps (digitized at 480 Hz). An inverse dynamic approach was used to compute the kinetics of the knee joint. Based on the model of Huberti et al. [1], the average knee flexion angles and quadratic equation were used to estimate the force ratios of patellar ligament to quadriceps tendon. T-tests were used to statistically test if the knee flexion angles were less than 45º, the difference of the GRF between skilled and non-skilled jumpers, and the difference of the force ratios between skilled and non-skilled jumpers at take-off and landing.

Table 1. Body height and weight of subjects

	Body Height	Body Weight
Skilled	188 cm (± 2.6 cm)	889.4 N (± 58.2 N)
Non-Skilled	186.8 cm (± 2.5 cm)	747.5 N (± 60.6 N)

Results

The tables summarized the results. Only one condition (table 2) showed a knee flexion angle significantly less than 45º when jumpers produced maximum GRF (p<.05). The result in table 3 indicated a significant difference of force ratio between skilled and non-skilled jumpers at landing.

	Table 2. Average of knee flexion angle		Table 3. Average force ratio		Table 4. Average GRF (BW)
	Non-Skilled	Skilled	Non-Skilled	Skilled	Non-Skilled	Skilled
T (v)	83.42º	71.65º	0.77	0.83	2.58	2.75
L (v)	41.84º *	49.02º	1.07*	1.00*	6.54	5.84

Note: * p<.05

Discussion/Conclusions

The results showed the maximum GRF occurs when knee joint angle is less than 45º at landing of the non-skilled jumpers group only. Zhang et al. [2] showed that the maximum patellar ligament force occurred when knee flexion angle was 39.7º at landing, close to the value exhibited by the non-skilled jumpers in the present study. Skilled jumpers, however, exhibited a knee flexion angle of 49.02º during landing of the skilled jumpers. Calculations also show that the force ratio of patellar ligament-to-quadriceps tendon is higher at landing in non-skilled jumpers (table 4). High force differences across the knee joint may contribute to chronic injuries such as jumper’s knee. Jumper’s knee is induced by the repetitive overloading and intensity of load within the knee extensor mechanism. The present results suggest that kinematic adjustments, in addition to traditional strength-training prescriptions, may be warranted for athletes to protect them from high force discrepancies across the patella and potential chronic injury. Skilled jumpers, on the other hand, were able to produce larger GRF’s at take-off and smaller GRF’s at landing when compared to non-skilled jumpers. From a coaching perspective, further study of skilled jumpers is warranted so that succesful, kinematic strategies may be identified for optimal performance and reduced potential for injury.

References

1. Huberti, H. H. et al. (1984). J of Ortho Res, 2, 49-54.
   
2. Zhang, S. et al. (1996, Oct). Presented at the 20th annual meeting of the American Society of Biomechanics, Atlanta, GA.