Significant differences (P < 0.05) in cruciate ligament force at specified knee angles between the descent and the ascent phases of each squat exercise are shown in Table 1. Mean PCL force was significantly greater in the ascent phase compared with the descent phase between 60° and 80° knee angles for the wall squat long, 70°-90° knee angles for the wall squat short, and 20°-70° knee angles for the one-leg squat. Descriptive data of mean quadriceps and hamstrings force values during wall squat and one-leg squat exercises are shown in Table 2. Quadriceps force ranged approximately between 30 and 720 N and generally increased with knee flexion, whereas hamstring force ranged approximately between 15 and 190 N. At each knee angle, quadriceps and hamstrings forces were generally greater during the ascent compared with the descent.
It is not well understood what PCL or ACL force magnitudes become injurious to the healthy or reconstructed ACL and PCL. In healthy adults, the ultimate strength of the ACL and PCL is approximately 2000 N (36) and 4000 N (27), respectively, although these values depend on age and anatomical factors. Therefore, the ACL and the PCL loads generated during the one-leg squat and the wall squat exercises appear to be well within a safe limit for the healthy ACL and PCL. The reconstructed ACL and PCL have similar ultimate strengths compared with the healthy ACL or PCL, although these values can change considerably depending on graft type and donor characteristics (e.g., autograft vs allograft; patellar tendon vs hamstrings graft) (4,28). However, the healing graft site may be injured with considerably less force compared with the ultimate strength of the graft, although it is not well understood how much force to the graft site is too much and how soon force can be applied after reconstruction. Therefore, the mean peak PCL forces of approximately 400 N during the one-leg squat and approximately 750 N during the wall squat exercises may be problematic early after PCL reconstruction when the graft site is still healing. Moreover, during PCL reconstruction, at the same relative intensity, it may be appropriate to use the one-leg squat before wall squat exercises due to less PCL loading during the one-leg squat, especially compared with the wall squat long. In addition, it may be prudent to use smaller knee angles (e.g., 0°-50°) before progressing to larger knee angles (e.g., 50°-100°) because PCL forces generally increase as knee angle increases. In contrast, wall squat exercises may be a better choice compared with the one-leg squat early after ACL reconstruction due to ACL forces generated during the one-leg squat. However, because peak ACL force during the one-leg squat were only approximately 60 N, it is not likely that the one-leg squat will produce forces that would be injurious to the healing ACL graft, and mild strain to the graft may enhance the healing process (13). Nevertheless, after ACL reconstruction, it may be safer to start with wall squat exercises and progress to the one-leg squat and use larger knee angles (e.g., 50°-100°) before progressing to smaller knee angles (e.g., 0°-50°) because ACL forces may be generated at smaller knee angles less than 50°.
As hypothesized, ACL forces were greater in the one-leg squat compared with the wall squat long and occurred at knee angles between 0° and 40° with a peak magnitude of approximately 60 N at 30° knee angle. During the one-leg sit-to-stand, which is similar to ascent phase of the one-leg squat, Heijne et al. (15) reported a peak 2.8% ACL strain (calibrated to approximately 100 N) at 30° knee angle. Moreover, Kvist and Gillquist (19) reported a peak anterior shear ACL force of less than 90 N at 30° knee angle during the one-leg bodyweight squat, which is similar to the results in the current study. Butler et al. (5) demonstrated that the ACL provides 86% of the total resistance to anterior drawer (caused by an anterior shear force) and the PCL provides approximately 95% of the total restraining force to posterior drawer (caused by a posterior shear force). Therefore, the anterior shear force is resisted primarily by the ACL, and posterior shear force is resisted primarily by the PCL. Moreover, ACL forces as a function of knee angle in the current study are similar to ACL forces and knee angles in the squat literature (3,15,25,32). However, both the ACL and the PCL forces that are generated while performing squatting exercises are dependent on which exercise technique is used and whether external resistance is used. For example, in Beynnon et al. (3), it appears that the subjects may have squatted using a more upright trunk position with relatively little forward trunk tilt, which suggests that these subjects may use their quadriceps to a greater extent than their hamstrings (26). This is important because hamstrings force has been shown to unload the ACL and to load the PCL during the weight-bearing squat exercise (11,21,26). Ohkoshi et al. (26) reported no ACL strain at all knee angles tested (15°, 30°, 60°, and 90°) while maintaining a squat position with the trunk tilted forward, with 30° or more forward trunk tilt being optimal for eliminating or minimizing ACL strain throughout the knee range of motion and recruiting relatively high hamstrings activity.
The exercises that had the greatest amount of anterior knee movement beyond the knees, the one-leg squat (10 ± 2 cm) and wall squat short (9 ± 2 cm), also generated the greatest ACL forces and least PCL forces. These exercises may be preferable to the wall squat long during PCL rehabilitation. In contrast, as hypothesized, the wall squat long, in which the knees did not move beyond the toes, generated the highest PCL forces and no ACL forces and may be problematic during PCL rehabilitation. Anterior knee movement beyond the toes can influence quadriceps activity and patellar tendon force, which in turn can influence cruciate ligament loading. Zernicke et al. (40) estimated the force in the patellar tendon at approximately 17 times bodyweight in a subject that used a considerable external load during a squat descent with excessive anterior knee movement beyond the toes. Although 17 times bodyweight may be an over estimate of the actual force in the patella tendon, large patellar tendon forces tend to load the ACL at smaller knee angles less than approximately 60° (primarily between 0° and 30°) but load the PCL at larger knee angles greater than approximately 60° (9,17,18). Although patellar tendon force from quadriceps activity can load either the ACL or the PCL depending on knee angle, it is difficult to make definite conclusions regarding how quadriceps activity and anterior knee movement may influence cruciate ligament loading while performing squat exercises, and additional research in this area is needed.
Although the wall squat short and one-leg squat both resulted in similar amounts of anterior knee movement at maximum knee flexion, PCL forces were significantly lower in the one-leg squat compared with the wall squat short between 90° and 70° knee angles during the squat ascent (Table 1 and Fig. 6). One explanation of the greater PCL forces between 90° and 70° knee angles in the wall squat short compared with the one-leg squat is greater quadriceps forces that are generated during the wall squat short because quadriceps forces at knee angles greater than 60° load the PCL (9,17,18). Between 90° and 70° knee angles during the ascent, the estimated quadriceps forces in the current study were approximately 30-50% greater in the wall squat short compared with the one-leg squat. Although hamstrings forces between 90° and 70° knee angles also load the PCL, hamstrings forces were only 20-30 N greater in the one-leg squat compared the wall squat short. In contrast, quadriceps force magnitudes were approximately 150 N greater in the wall squat short compared with the one-leg squat, therefore loading the PCL to a great extent compared with the hamstrings.
Although hamstrings forces were greatest in the one-leg squat between 0° and 30° knee angles, the hamstrings are not effective in either unloading the ACL or loading the PCL due to a small insertion angle into the tibia that results in most of the hamstrings force being directed parallel instead of perpendicular to the tibia. Hamstrings force is most effective in generating posterior shear force and in loading the PCL when the knee is flexed approximately 90° (20). The relatively low hamstrings force (typically less than 50 N) generated during the wall squat exercises throughout the knee range of motion implies that wall squat exercises primarily target the quadriceps and not the hamstrings, whereas the one-leg squat is more effective in recruiting the hamstrings. One reason for greater quadriceps force and less hamstrings force in the wall squat short compared with the one-leg squat is because the trunk is erect in the wall squat short (greater knee extensor torque and less hip extensor torque needed to overcome the effects of gravity) but tilted forward 30°-40° in the one-leg squat (less knee extensor torque and greater hip extensor torque needed to overcome the effects of gravity).
The friction and the normal forces that the wall applied to the subject may also help explain why quadriceps forces were greater in the wall squat short compared with the one-leg squat during the squat ascent. Although friction was minimized during the wall squat by using a smooth wall, the normal force that the wall exerted on the subject's back during the wall squat exercises resulted in an increased friction force on the subject as they slid down and up the wall. Because the friction force opposes motion, it acted opposite the force of gravity during the squat descent but in the same direction as the force of gravity during the squat ascent. Therefore, the friction force made it easier for the subject to control the rate of sliding down the wall by producing a knee extensor torque but made it more difficult for the subject to slide up the wall by producing a knee flexor torque. Because the one-leg squat did not have a friction force compare to the wall squat, this provides one plausible explanation why quadriceps force and PCL force were greater in the ascent phase of the wall squat exercises compared with the one-leg squat.
The friction force also differed between the wall squat long and short. Because during the wall squat long the heels were twice as far from the wall compared with the wall squat short, the normal force must be greater in the wall squat long. Because friction force is directly proportional to the normal force, the downward-acting friction force on the subject during the squat ascent was greater in the wall squat long compared with the wall squat short, which makes the wall squat long more difficult to perform. This may partially explain why PCL forces were greater in the wall squat long compared with the wall squat short.
Cruciate ligament forces tended to be higher in the ascent phase compared with the descent phase, in part because quadriceps and hamstrings forces were also greater during the ascent phase. For the wall squat exercises, significant PCL force differences between squat descent and ascent occurred only at higher knee angles between 60° and 90°. As previously mentioned, quadriceps force at knee angles greater than 60° loads the PCL, and the greater quadriceps force was greater during the ascent than the descent in part due to having to overcome gravity and the downward-acting friction force. A different pattern occurred during the one-leg squat, in which between 20° and 70° knee angles PCL forces were significantly greater during the squat ascent compared with the squat descent. These findings are in agreement with the squat literature, in which cruciate forces have been reported to be greater in the squat ascent compared with the squat descent (11,12).
There are limitations in this study. Firstly, muscle and cruciate ligament forces were estimated from biomechanical modeling techniques and not measured directly because it is currently not possible to measure cruciate ligament forces in vivo while performing wall squat and one-leg squat exercises in healthy subjects. However, both Beynnon et al. (3) and Heijne (15), who implanted strain sensors in patients within the anteromedial bundle of an ACL during arthroscopic surgery for partial minisectomies or capsule/patellofemoral joint debridement, after surgery had these patients perform one- and two-leg squat-type exercises. These authors reported a peak ACL strain of approximately 2.8-4% (approximately 100-150 N) at knee angles between 0° and 30°. These ACL force magnitudes and knee angles from Beynnon et al. (3) and Heijne (15) are similar to the current study. Unfortunately, there are no studies that have quantified PCL forces in vivo while performing a squat exercise, so it is not possible to compare the modeled PCL force results in the current study to in vivo PCL forces.
The current study was limited to sagittal plane motion only, and only subjects who could perform all exercises without discernable frontal or transverse plane movements were used in this study. Future three-dimensional biomechanical analyses of the knee during squatting are needed to investigate the effects of transverse plane rotary motions and frontal plane valgus and varus motions on cruciate ligament loading. Slightly different cruciate ligament loading patterns during squatting may occur between two- and three-dimensional analyses, although normal squatting is primarily sagittal plane movements. A normal range of motion of 5°-7° knee valgus and 6°-14° of knee varus has been reported during the one-leg squat (39), although these relatively small amounts of valgus and varus may not affect cruciate ligament loading. However, excessive knee valgus has been shown to be associated with an increased risk of ACL ruptures (22,39). Transverse and frontal plane hip joint motions have also been shown to be associated with an increased risk of ACL ruptures and are relatively common in individuals with weak hip abductors and external rotations (22).
In conclusion, throughout the 0°-90° knee angles, the wall squat long generally exhibited significantly greater PCL forces compared with the wall squat short and the one-leg squat. There was generally no significant difference in PCL force between the wall squat short and the one-leg squat, except at 80° and 90° knee angles, where PCL forces were greater in the wall squat short. Throughout the 0°-90° knee angles, the wall squat exercises generated PCL force magnitudes ranging approximately from 100 to 790 N, with PCL magnitudes generally decreasing between 0° and 30° knee angles and increasing between 40° and 90° knee angles. Moreover, the one-leg squat generated PCL force magnitudes ranging approximately from 60 to 410 N, with PCL magnitudes generally increasing between 50° and 90° knee angles during the descent and 10°-90° knee angles during the ascent. ACL forces were only found in the one-leg squat, which generated relatively small magnitudes of approximately 20-60 N between 0° and 40° knee angles. The one-leg squat, the wall squat long, and the wall squat short all appear to load the ACL and the PCL within a safe range in healthy individuals.
The authors would like to thank Lisa Bonacci, Toni Burnham, Juliann Busch, Kristen D'Anna, Pete Eliopoulos, and Ryan Mowbray for all their assistance during data collection and analyses.
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Keywords:©2009The American College of Sports Medicine
BIOMECHANICS; KINETICS; CLOSED CHAIN EXERCISES; KNEE