How Muscle Forces Affect Loads on the PCL
March 1, 2000 3 minutes read
How Muscle Forces Affect Loads on the PCL
Abstract & Commentary
Synopsis: Combined hamstring and quadriceps loads decrease the in situ force on the PCL.
Source: Hoher J, et al. In situ forces in the human posterior cruciate ligament in response to muscle loads: A cadaveric study. J Orthop Res 1999;17:763-768.
The main purpose of this study was to determine the effects of hamstring and quadriceps muscle loads on knee kinematics. Nine cadaver knees were studied with a robot that has the capability to measure the forces within the posterior cruciate ligament (PCL). Measurement was made of loads placed on the PCL by quadricep and hamstring activation at specific knee flexion angles (30°, 60°, 90°, and 120°). The PCL forces were low at all flexion angles under nonloading conditions. The PCL forces increased at all flexion angles when hamstring loads were applied. The PCL forces decreased at all flexion angles when a quadriceps load was added to the hamstring load when compared to the isolated hamstring-loaded condition. Finally, the forces in the PCL increased as knee flexion increased from full extension to 90° under all conditions.
Comment by James R. Slauterbeck, MD
The basic science, biomechanics, and clinical data for the PCL are still in their infancy when compared to those for the ACL. This article, in addition to those published by Markolf et al, helps to define how loading conditions affect the in situ forces within the PCL.1-3
Results of surgical reconstructions for isolated and combined ligament injury involving the PCL are not as good as for those involving the ACL. Although patients may be satisfied with the results following PCL reconstruction, measured laxity—when compared to the other side—generally is present. Clinically, patients who are PCL deficient and maintain strong quadriceps often do well and even compete in the NFL without surgery.
This study helps to define the protective effect of the quadriceps to decrease forces on the PCL when quadriceps activity is added to hamstring activity. This project supports the early use of combined quadriceps and hamstring rehabilitation following PCL injury and possibly reconstruction (although reconstruction was not addressed in this article). Certain closed-chain or seated squatting exercises would be preferred exercises secondary to co-contraction of the quadriceps and hamstrings rather than open-chain exercises. More important, aggressive hamstring strengthening is probably exposing the PCL to higher-than-desired forces at greater flexion angles. One should consider avoiding the use of early bike riding following a PCL injury or reconstruction because of the hamstring activation during the cycling stroke. Also, the high knee flexion angles at the top of the stroke may increase the force in the PCL more than desired.
I would recommend using the information from this study to help define post-injury/surgery rehabilitation by avoiding open-chain hamstring exercises and emphasizing closed-chain, co-contraction exercises. This article concludes that increasing knee flexion increases PCL forces and that combining quadriceps and hamstring loads decreases the forces in the PCL.
References
1. Markolf KL, et al. A biomechanical study of replacement of the posterior cruciate ligament with a graft. Part II: Forces in the graft compared with forces in the intact ligament. J Bone Joint Surg Am 1997;79(3): 381-386.
2. Markolf KL, et al. A biomechanical study of replacement of the posterior cruciate ligament with a graft. Part I: Isometry, pre-tension of the graft, and anterior-posterior laxity. J Bone Joint Surg Am 1997;79(3): 375-380.
3. Markolf KL, et al. Effects of combined knee loadings on posterior cruciate ligament force generation. J Orthop Res 1996;14(4):633-638.
The addition of a quadricep force to an already applied hamstring force will have what effect on the in situ force in the PCL?
a. Decrease the force in the PCL
b. Increase the force in the PCL
c. Have no effect on the PCL force
d. The force in the PCL decreases as knee flexion increases.
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