June 2016

Posture-specific strength and landing mechanics

6ACL-shutterstock_393622249-copyRecent research suggests that training designed to maximize strength at deeper knee flexion angles during landing may be warranted, especially in female athletes, to help reduce the risk of anterior cruciate ligament injury without inadvertently compromising jump height.

By Boyi Dai, PhD; Jacob S. Layer; and Taylour J. Hinshaw

Landing is a type of fall in which the person falling is able to strike the surface with controlled movement patterns. During landing activities, the musculoskeletal system responds to the landing surface by generating forces and moments that progressively decelerate the body’s downward velocity.1,2 Inappropriate landing patterns are associated with increased risks of lower extremity injuries, such as anterior cruciate ligament (ACL) injury and patellofemoral pain syndrome.

Video analysis has shown that individuals who suffer ACL injuries typically demonstrate decreased knee flexion angles and increased knee valgus–varus angles during landing near the time point at which the injury occurs.3-7 Prospective studies have suggested that athletes who land with relatively large knee valgus angles and moments, high-impact ground reaction forces, small knee flexion angles,8 and small sagittal plane joint displacements9 during baseline assessments are more likely to suffer ACL injuries. Another prospective study has identified relatively small knee flexion angles and ground reaction forces during landing, as well as relatively weak quadriceps and hamstring muscles, as risk factors for patellofemoral pain syndrome.10

In addition to isometric squat training, transitional front and back squats could also be performed with greater knee flexion angles to achieve training goals.

Based on these identified landing-specific risk factors for ACL injuries, one strategy for at-risk individuals to decrease that risk is to land with more knee flexion, less knee valgus, and lower ground reaction forces. Using different feedback modalities, including verbal, visual, and tactile feedback, investigators have developed many training strategies to modify jump-landing patterns.11-14 Research has shown immediate improvements in both knee flexion angles and ground reaction forces following a short period of feedback training. However, these changes in landing mechanics may not represent a true learning effect because they can result in decreased performance, as indicated by an accompanying decline in jump height and increase in stance time.11,14,15 It is questionable whether competitive athletes will be willing to modify their landing patterns if decreased performance is a possible result, especially during competitions. The ultimate goal of jump-landing training should be for individuals to perform the modified landing pattern automatically and efficiently, without sacrificing performance.14

Strength assessment

Strength is an essential component of sports performance and injury prevention. Previous investigators have hypothesized that lower extremity strength and landing mechanics, in particular, may be related.10,16-19 The lower extremity musculature contracts, primarily eccentrically, to absorb impact ground reaction forces during landing. However, studies designed to identify clinically relevant associations between strength and landing mechanics may be limited by the techniques used to assess strength.

Although strength is typically measured in terms of the maximum torque or force a group of muscles can produce, several factors need to be considered during strength assessments. During a strength assessment, a group of muscles activate to generate internal torques around a single joint or multiple joints. The internal torques may be counterbalanced by external torques generated by external forces. Therefore, strength may be quantified in terms of external forces, external torques, internal torques, or internal forces. External forces and torques are the most commonly used measurements of strength.

For example, when isometric quadriceps strength is assessed using a dynamometer, the force measured by the dynamometer is an external force.10,16 If the dynamometer is applied perpendicular to the shank, the external torque can be quantified by multiplying the external force by the distance between the dynamometer’s point of contact and the knee joint.20 Once the external torques generated by the weights of the shank and foot have been quantified, internal knee extension torques can be calculated using an inverse dynamic approach. Assuming no muscle co-activation, knee extension torque can be divided by the patellar tendon moment arm to estimate the quadriceps force. Internal torques can also be determined using a computerized dynamometer after adjusting for the external torques caused by segment weights and angular accelerations.21

Another factor is the muscle force-length-velocity relationship.22 During concentric, isometric, and eccentric contractions, the length of muscles is shortened, kept the same, or elongated when forces are generated. The muscle force-velocity relationship describes that the maximum force muscle can generate decreases when the velocity of contraction increases during concentric contractions. The maximum force that can be generated is the greatest during eccentric contraction, second greatest during isometric contraction, and lowest during concentric contractions. As such, it is important to control the speed of muscle contraction when measuring strength.

Because the speed of muscle contraction is affected by the speed of joint motion, investigators commonly control joint motion speed during strength assessment; for example, controlling speed of knee flexion and extension when assessing quadriceps and hamstring strength.18 The muscle force-length relationship indicates there is an optimal length for a muscle to generate maximum force. Therefore, the magnitudes of joint angles, which directly affect muscle length, also need to be considered during strength testing.10

Two other important factors are task modalities and body posture. During an open-chain exercise, such as kicking a soccer ball, the distal joints are typically free to move. However, distal joints have little motion during closed-chain movements, such as jumping from the ground. Previously, investigators have found different knee joint contact forces, shear forces, and thigh muscle activations between two closed-chain exercises (squat vs leg press) and between the closed-chain exercises and an open-chain exercise (knee extension).23 In another study, participants performed squat exercises to train for lower extremity strength. Although squat training was associated with gains of more than 20% for both vertical jump height and load in maximal squat, no change was found in isokinetic knee extension torques assessed during an open-chain task.24 A recent study found squat training was more effective than leg press training for improving jump performance.25

These studies highlight the importance of task modality and body posture in strength training and assessments. The greatest strength gain is likely to be associated with the exercises used for training. One should also keep in mind that, in addition to the muscle force-length-velocity relationship, neural adaptation and multijoint coordination also contribute to the specificity of strength training.25

Strength and landing mechanics

Previous studies on the relationship between strength and landing mechanics are not consistent. Although some findings have suggested a correlation between lower extremity strength and knee flexion angles during landing, other findings have indicated a lack of correlation. These inconsistent findings may be related to the use of different strength assessments in the various studies.

The aforementioned prospective study on landing mechanics and risks of patellofemoral pain syndrome found quadriceps and hamstring weakness was associated with risk of injury. In this study quadriceps and hamstring strength were assessed isometrically in sitting or prone positions.10 In a different study, Lephart et al assessed the association between landing mechanics and isokinetic quadriceps and hamstring strength, and found women demonstrated smaller knee flexion angles during landing and less strength than men.18 On the other hand, another study using the same military data set as the patellofemoral pain syndrome study found weak correlations between Landing Error Scoring System scores (a visual assessment of landing mechanics to assess injury risk) and lower extremity muscle isometric strength.16 Shultz et al found that isometric quadriceps and hamstring strength were poor predictors of knee and hip flexion angles during landing.19

In these studies, quadriceps and hamstring strength was mainly assessed isometrically or isokinetically during an open-chain exercise in a seated or prone position. However, landing is a closed-chain task in which the body is in a relatively upright position. The advantage of isometric or isokinetic strength assessment is the ability to isolate different muscle groups, such as knee and hip extensors and flexors. The disadvantage, however, is the inability to accurately simulate the body postures associated with landing.

Our research

To better assess the dynamic strength utilized during landing, we recently completed a study to quantify the relationship between the peak force production during isometric squats at different knee flexion angles and the knee flexion angles during landing.26 Eighteen male and 18 female recreational and collegiate athletes participated. They did a jump-landing-jump task while knee flexion angles were recorded during the landing phase. Four isometric squats at different knee flexion angles were also performed, and the bilateral ground reaction forces were recorded during the squats to assess strength.

Interestingly, we found significant correlations only for women. In our female participants, we found significant and strong correlations between knee flexion angle at initial contact during landing and peak force production during isometric squats at knee flexion angles of 55° and 70°, and between the peak knee flexion angle during landing and the peak force production during isometric squats at knee flexion angles of 55°, 70°, and 90°. These correlations tended to be stronger for the isometric squats performed with greater knee flexion angles.

Based on the findings, we concluded that posture-specific strength may play a role in determining the self-selected knee flexion angles during landing in women, and therefore may contribute to ACL injury risk.

In previous studies in which participants did a landing task from a jump or a drop, the landing could be the end of the task or followed by another jump.10,16-19 Investigators have quantified the differences in kinematics and kinetics between a landing with or without a subsequent jump, and shown that landing with a jump is associated with greater peak anterior tibial shear force, internal knee extension moment,  and internal knee varus moment. Landing without a jump results in decreased knee and hip flexion and increased side-to-side asymmetry in vertical ground reaction force and height of center of mass.27,28

Although ACL injuries have been observed during different landing types, different landing tasks may load the ACL differently and are associated with different ACL injury risk factors.3-7 Findings from studies using different landing protocols should also be compared with caution; doing so may have contributed to some inconsistencies in previous studies.

During the landing phase, the knee and hip joint angles are relatively small at initial contact and increase during the landing phase. Knee and hip joint angles typically vary between 10° and 100° throughout the landing phase.29,30 During a jump-landing-jump task, the landing is followed by a jump for maximum height. Therefore, the goal is to dissipate impact ground reaction forces during the landing phase and then generate ground reaction force during the jumping phase. From an injury prevention perspective, lower impact ground reaction forces are associated with decreased injury risks. From a performance perspective, greater ground reaction forces during the jumping phase are associated with increased jump height. As such, the relationship between posture and the ability to produce force needs to be considered during a jump-landing-jump task, especially during the jumping phase.

Increasing knee and hip angles typically increases the moment arms of the vertical ground reaction force about the knee and hip, and results in decreased magnitude of vertical ground reaction forces relative to internal joint torques. Therefore, increased knee and hip joint angles place the lower extremity in a position of mechanical disadvantage for producing vertical ground reaction force for the jump. It is reasonable to observe that individuals who are stronger at increased hip and knee flexion angles will tend to utilize these deeper flexion angles because they may benefit from the additional range of motion to produce force for the jump. Conversely, individuals who are weaker at deeper hip and knee joint angles may avoid these positions, from which they are unable to produce sufficient force to use the additional range of motion effectively.

Gender considerations

Our study’s observation of significant correlations between strength and landing mechanics only in women is consistent with the findings of two previous studies31,32 that found significant correlations between strength and energy adoption patterns during landing in female participants, but not in their male counterparts. Other factors, such as balance, range of motion, and motor learning of landing techniques,33 may explain the lack of correlation between posture-specific strength and knee flexion angles during landing in men.

Men are generally stronger than women in terms of both raw strength and strength relative to body weight. In the current study, we postulated the lower force production in female participants suggests a more important role in women compared with men of strength as a determinant of landing mechanics. In men, other factors, such as motor learning of landing techniques, may play more important roles in landing mechanics.

Based on our findings, women may benefit from training designed to increase strength at greater degrees of knee flexion from both an injury prevention and performance training perspective. Literature has shown isometric squat training at greater knee flexion can increase force production during squats and joint moments during a jump.34,35 In addition, isometric squat training at certain knee flexion could shift the optimal angle for torque production toward the training angle.36 In addition to isometric squat training, transitional front and back squats could also be performed with greater knee flexion to achieve training goals.37 Future intervention studies are needed to quantify the training effect on landing mechanics in women.

Summary

Impaired landing mechanics are associated with increased risks of lower extremity injury. Strength may play an important role in self-selected landing patterns, particularly in women. The findings of our study suggest posture-specific strength training may be considered a component of intervention to increase knee flexion angles during landing in women.

A jump landing is a closed-chain task that involves muscle eccentric and concentric contractions while the body is in a relatively upright posture; therefore, effective evaluation of the dynamic strength used in landing tasks requires development of assessment techniques superior to those involving the isometric squat. Additional studies on the effects of different posture-specific strength-training interventions on landing mechanics are needed to further quantify the cause-effect relationship between strength and landing mechanics.

Boyi Dai, PhD, is an assistant professor of biomechanics in the Division of Kinesiology and Health at the University of Wyoming in Laramie. Jacob S. Layer and Taylour J. Hinshaw are graduate students in the Division of Kinesiology and Health at the University of Wyoming.

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