Journal of Elecrromyography zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA and Kinesiology Vol. I, No. 3, pp 191-198 8 1991 Raven Press. Ltd.. New York Effect of Knee Musculature on Anterior Cruciate Ligament Strain In Vivo M. H. Pope, C. J. Stankewich, B. D. Beynnon, and B. C. Fleming zyxwvutsrqponmlkjihgfedcbaZ McClure Musculoskeletal Research Center, Department of Orthopaedics and Rehabilitation, University of Vermont College of Medicine, Burlington, Vermont, U.S.A. Summary: Squatting is a commonly prescribed exercise following reconstruc- tion of the anterior cruciate ligament (ACL). The objective of this paper was to measure the in vivo strain patterns of the normal ACL and the load at the knee for the simple squat and for squatting with a “sport cord”. A sport cord is a large elastic rubber tube used for added resistance. Strain patterns were de- duced using displacement data from a Hall Effect Strain Transducer (HEST), while joint loads were determined by a mathematical model with inputs from a force plate and eiectrogoniometers. ACL strain for the free squat in one subject had a maximum of -2% at a knee angle of IO” and was slack for knee angles > 17”.In squatting with a sport cord, peak strain was - 1% at 10” and was slack at zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA knee angles > 14”. Since these peak strains are low, squatting appears to be a safe exercise for conservative rehabilitation of ACL reconstruction patients. In addition, the sport cord is a recommended augmentation to the activity. We believe that the decrease in strain with the sport cord results from added joint stiffness due to greater compressive forces at the tibiofemoral joint. This greater compressive force results from the approximately 10% increase in quadriceps activity. From shear force data predicted by the mathematical model, the maximum anterior drawer force for free squatting (50 N) was con- siderably Iess than for sport cord squatting (430 N). Therefore, the value of shear force at the tibiofemoral joint only partially determines the load placed on the ACL. Key Words: Anterior cruciate ligament-Knee-Quadriceps- Hamstrings-Rehabilitation. After reconstruction of the anterior cruciate liga- ment (ACL), proper muscle training is essential to avoid muscle atrophy and deterioration. Muscles, including the quadriceps and hamstrings, must also remain active to preserve proper joint function. If activity is not maintained, the muscles of the joint will become increasingly weak, making the ACL prone to injury. To condition these muscles fully, exercise must be performed throughout the range of knee angles because only specific motor units are Address correspondence and reprint requests to Mr. C. J. Stankewich at Department of Orthopaedics and Rehabilitation, University of Vermont College of Medicine, Burlington, VT 05405, U.S.A. recruited at a given joint angle (11). Muscle strength has been shown to increase in isometric exercise only at the knee angle at which conditioning took place (6,13). Excessive load or strain that might damage the graft must also be considered. The func- tion of the healthy ACL is to resist anteriorly di- rected shear forces at the tibiofemoral joint. Ac- cording to Yasuda: “[Tlhe shear force exerted at the tibiofemoral joint is equal to the drawer force applied to the tibia and the value of the shear force is a good index of the load placed on the ACL.” (19). We explored the validity of this hypothesis. The ACL has also been shown to exhibit a strain pattern during passive flexion-extension (PROM) (3), with maximum ACL strain occurring at full ex- 191