May 2010

PLANTER’S PUNCH: Kicking and ACL injury risk #1038889

Most studies of kicking in soccer have focused on the kicking leg. But the biomechanics of the plant leg, which often functions at odd and unexpected angles, may have greater implications for injury.

By Sarah E. Clagg, DPT, Angela R Warnock, MPT, and James S. Thomas, PT, PhD

Over the past 20 years, female athletic participation has grown, with more opportunities for female athletes to participate in competitive athletics than ever before.1 However, with an increasing number of female athletes participating in sports, there is an increased incidence of injury. There has been a dramatic increase in the number of knee injuries, specifically anterior cruciate ligament (ACL) injuries, in female athletes who participate in certain competitive sports, like soccer. A meta-analysis of the incidence of ACL tears as a function of sport and gender in collegiate soccer studies found 11,754,568 incidences and a mean female collegiate rate of 0.32 and a mean male rate of 0.12 with a female-male ratio of 2.67.2 It is also known that more than half of all female ACL injuries occur during non-contact athletic participation.3 In a 13-year study of the National Collegiate Athletic Association injury database, Agel et al reported that 67% of all the ACL injuries sustained in females were due to non-contact mechanisms.3 The influence of gender on lower extremity jumping and cutting mechanics has been extensively studied in an attempt to understand the possible risk factors related to female non-contact ACL injury, with many studies looking at female athletes in general.4-7 While studies have also specifically examined the lower extremity kinematics and kinetics of female soccer players during landing and cutting maneuvers,8-12 few studies have looked at lower extremity mechanics involved in sport-specific maneuvers, such as kicking in soccer. #3984059

Most of the research on kicking in soccer has focused on the mechanics of the kicking leg, but one study found that 99% of all soccer-related ACL injuries recorded in their study occurred to the limb that was in contact with the ground.13 Because of the increasing incidence of ACL injury in female soccer athletes, it is necessary to examine lower extremity mechanics as possible predisposing factors related to injury. Thus an important first step to understanding lower extremity mechanics in soccer is to characterize the mechanics of the plant leg in healthy individuals performing soccer kicks.

Studies on kicking leg mechanics have primarily focused on the optimal position of the kicking leg to achieve high ball velocities and kicking accuracy in regards to performance.14,15 Research on kicking leg mechanics and performance has found that maximum swing leg and ball velocities occur at a large approach angle to the ball, between 30º and 45º.14 There have also been numerous studies performed on kicking leg dominance.15,16 This research has found that higher ball velocities are achieved when soccer players kick with their dominant or preferred kicking leg, as a result of higher foot speed. 17

Plant science

Although the mechanics of the kicking leg have been extensively studied to determine factors related to kick performance, few studies have examined the mechanics of the plant leg during a soccer kick, even though is it known that soccer injuries most often occur to the plant leg during competition.13 In the studies that have examined plant leg mechanics, the focus has been on ground reaction forces, joint displacements, strength of the plant leg, and lower extremity joint torques.

Orloff et al analyzed the kinetics and kinematics of the plant leg position during instep kicking in 11 male and 12 female collegiate soccer athletes. They found that there were no significant gender differences in plant leg position, meaning the distance from the ball to the plant foot was the same for male and female players; however, during kicking, female athletes demonstrated significantly greater trunk lean and lower leg angle of the plant leg, resulting in greater peak mediolateral ground reaction forces than male athletes.18 Research has found that increased ground reaction forces with a more extended knee may contribute to risk of ACL injury.19 #9561172

In another plant leg study of 10 male soccer players, Kellis et al revealed that medial and posterior ground reaction forces of the plant leg during instep kicking increase with an increase in approach angle (the angle between the player’s starting position and the position of the ball).20 They hypothesized that increased medial and posterior ground reaction forces may result in an altered stance of the plant leg during kicking, affecting balance and creating large shear forces that may cause significant loading of the lower extremity joints and increase stress on the medial aspect of the knee. The same study also found high external rotation displacements and high knee angular velocities in the plant leg when kicking from an increased approach angle, at 45º or 90º, resulting in altered balance. Off-balance kicks can cause changes in knee joint mechanics, which may increase loads to the knee, increasing ACL injury risk.20

One of the few studies to examine the strength of the plant leg found that ball velocity was correlated with specific strength measures in the plant leg when kicking from a large approach angle. Masuda et al examined the relationship between ball velocity, approach angle, and strength of the plant leg.21 The approach angles used in the study were set so that 0º represented the direction in which the ball was kicked. Subjects performed a designated number of kicking trials aimed at a target from three approach angles (i.e. self-selected kick or free kick, 90º, or 135º). The authors found that higher ball velocities during the 135º kick were associated with greater knee flexor, hip extensor, and hip abductor strength, suggesting that strength in these muscles is required to stabilize the trunk for generating high ball velocities during an off-balance kick.21

Axis powers

An example of the kicking paradigm used to study the effect of plant leg on lower extremity joint torques in female soccer players.

In the most recent study out of our lab, we examined the effect of plant leg and approach condition on the lower extremity torques of the hip, knee, and ankle in soccer kicking tasks.22 In the study, nine female collegiate athletes performed a series of kicking tasks with their dominant and non-dominant kicking leg from three different approaches (Figure 1). The approach conditions included a straight-on or center kick, an on-axis kick, and an off-axis kick. The on-axis kick, when the plant leg angle and approach angle relative to the ball were the same, represented the natural fluid motion of kicking. The off-axis kick, when the plant leg angle and approach angle were opposite, required many of the subjects to change their kicking strategy in order to achieve ball contact. The off-axis kick was designed to mimic the unpredictable and variable kicking approaches that often occur during competitive soccer competition. However, the off-axis kick mimics cross-body or cross-midline stepping, as opposed to the type of lateral stepping involved in cutting maneuvers, which have been extensively studied in the ACL literature.

In our study we found differences in peak hip, knee, and ankle joint torques between the dominant and non-dominant plant leg, even in elite college athletes who are trained to be proficient in kicking with either leg. We also found that there were minimal differences in peak flexor and extensor torque between the dominant and non-dominant plant legs in college female athletes when using an on-axis approach; however, when the subjects kicked from an off-axis approach, significant differences in peak hip and ankle flexor and extensor joint torque emerged between the left and right plant legs. Peak knee flexor and extensor joint torques showed a similar trend, although the differences were not statistically significant. In addition, we found that peak abduction torque at the hip, knee, and ankle were greater for the non-dominant plant leg than the dominant plant leg, while the opposite was true for peak adduction torque for all three approach conditions. This suggests that even at a collegiate level of play, participants had to change their kicking strategy when kicking with their dominant versus non-dominant leg and when kicking from different approach conditions.22 #9361157

Barfield examined the effects of selected kinematic and kinetic variables during instep kicking and found that when athletes kicked the ball using their non-dominant leg, they demonstrated more variability compared to the dominant limb with regard to the measures that correlated with ball velocity, especially angular position.23 Some of the measures that were associated with ball velocity included: linear velocity of the big toe at ball contact, maximum angular velocity at the knee, maximum angular position at the knee, maximum knee flexion and extension torque, and angular position of the ankle from support foot (i.e., the foot of the plant leg) contact to ball contact. We found that the peak joint torques of the dominant and non-dominant plant legs differed when participants kicked with their dominant vs non-dominant kicking limb. We speculate that the differences we observed in plant legs during an off-axis kick are consistent with findings from Barfield;23 specifically, that the increased variability may indicate a difference in the control of the non-dominant plant leg (i.e., the leg not usually in contact with the ground during kicking).

Furthermore, when the female athletes in our study planted with the non-dominant leg, they produced greater amounts of braking torque (i.e., extension, external rotation, and abduction) than when planting with their dominant leg. On the other hand, the acceleration torques (i.e., flexion, internal rotation, and adduction) were lower for the non-dominant plant leg.  This suggests that when the participants planted with their non-dominant plant leg, they produced a larger braking force to control the movement; therefore, utilizing a more novice kicking strategy to compensate for decreased stability in the non-dominant plant leg. Interestingly, Fagenbaum and Darling found that compensatory hamstring activity increased braking force during jumping tasks in healthy female athletes, which could indicate an attempt to decrease joint laxity and in turn prevent knee injury.24 Although that study involved basketball players, the proposed mechanism may help to explain our findings.

Implications for injury #10595976

It is known that ACL injury occurs when the foot is in contact with the ground; however, to this date no studies have specifically examined the risk of ACL injury or strain on the ACL of the plant leg during kicking tasks.3,13 Thus these questions remain unanswered: Does increased strain occur to the ACL during rapid deceleration or quick stopping prior to kicking in soccer?  Are soccer athletes, especially female athletes, more at risk of injuring the ACL of the plant leg when kicking with their non-dominant leg than their dominant leg and/or when using an off-axis approach?

The results of our study confirmed that collegiate soccer players without prior injury display different joint torque strategies during kicking with their dominant vs non-dominant plant legs and when kicking from an off-axis approach. If these differences occur in healthy female soccer athletes, the next step would be to determine if differences also occur in those who may demonstrate other proven risk factors for ACL injury and those who have already suffered an ACL injury. This would further explore and provide insight into the association of lower extremity biomechanics and ACL injury, enhancing our ability to predict injury and further identify exact mechanisms of ACL injury in specific sports like soccer.

In conclusion, the findings from our study provide new information on the mechanics of the plant leg during maximal effort kicking. Collegiate level female soccer players display very different joint torque strategies in the dominant and non-dominant plant legs during off-axis kicks. It needs to be determined if differences in the mechanics of off-axis kicks emerge between healthy participants and those who have suffered an ACL injury. The differences in peak joint torques of the plant leg during kicking found in our study and the findings from other plant leg studies advance the understanding of lower extremity mechanics in female athletes, who are at an increased risk of ACL injury.  Further investigation must be made into lower extremity biomechanics and plant leg control in those who have suffered an ACL injury, in order to identify exact mechanisms of injury and possibly identify those who are more susceptible to injury.

Sarah E. Clagg, DPT, ATC, is a practicing sports medicine physical therapist and certified athletic trainer at Good Samaritan Hospital in Dayton, OH. Angela R. Warnock, MPT, is a practicing physical therapist at Ohio State University Children’s Hospital Sports Medicine Center in Columbus, OH. James S. Thomas PT, PhD, is an associate professor in the school of physical therapy and director of the Motor Control Laboratories at Ohio University in Athens, OH.


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19. Podraza JT, White SC. Effect of knee flexion angle on ground reaction forces, knee moments and muscle co-contraction during an impact-like deceleration landing: Implications for the non-contact mechanism of ACL injury. Knee 2010;Mar 17.[Epub ahead of print]

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3 Responses to PLANTER’S PUNCH: Kicking and ACL injury risk

  1. A very interesting topic as we have been working with the Bio-Engineering group at Binghamton University on a technology that measures muscle force during functional activities. Our focus has been on the muscles surrounding the knee. In our limited testing we have found that the non dominant kicking leg has favorable quadriceps to hamstring ratios. Our assumption is that this is a function of the repetitive eccentric loading of the plant leg.

  2. Great to see the discussion on the plant leg. Thought you might be interested in the work we are doing – and the blog post in which we discuss the importance of the plant leg in assessing propensity for ACL tears.

  3. dr.M.Neethi says:

    excellent work

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