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Landing adaptations after ACL reconstruction


Medicine & Science in Sports & Exercise: September 2002 - Volume 34 - Issue 9 - pp 1408-1413
CLINICAL SCIENCES: Clinical Investigations

DECKER, M. J., M. R. TORRY, T. J. NOONAN, A. RIVIERE, and W. I. STERETT. Landing adaptations after ACL reconstruction. Med. Sci. Sports Exerc., Vol. 34, No. 9, pp. 1408–1413, 2002.

Purpose: The purpose of this study was to determine whether fully rehabilitated ACL reconstructed (ACLr) recreational athletes utilize adapted lower-extremity joint kinematics and kinetics during a high-demand functional task.

Methods: The kinematic and kinetic performance of 11 healthy and 11 hamstring ACLr recreational athletes were compared during a 60-cm vertical drop landing.

Results: At initial ground contact, the ACLr group demonstrated greater hip extension and ankle plantarflexion compared with the healthy group. The peak vertical ground-reaction force was not different between groups, but the ACLr group delayed the time to its occurrence. The knee extensors provided the major energy absorption function for both groups; however, the ACLr group performed 37% more ankle plantarflexor work and 39% less hip extensor work compared with the healthy group.

Conclusions: The hamstring ACLr recreational athletes utilized an adapted landing strategy that employed the hip extensor muscles less and the ankle plantarflexor muscles more. The harvesting of the medial hamstring muscles for ACL reconstruction may contribute to the utilization of this protective landing strategy.

Biomechanics Research Laboratory, Steadman-Hawkins, Sports Medicine Foundation, Vail, CO

Submitted for publication July 2001.

Accepted for publication February 2002.

Anterior cruciate ligament reconstruction with the combined loops of the semitendinosus and gracilis tendons is common in sports medicine (2). The selection of this graft over the central third of the patellar tendon is typically based on the morbidity associated with graft harvest. Graft site morbidity from the patellar tendon technique has been thought to be the primary mechanism attributed to knee extensor dysfunction characterized by a reduced knee extensor moment during functional activities (4,5,12,28,29). Although quadriceps strength is typically normal in hamstring anterior cruciate ligament reconstructed (ACLr) knees (14,25,31), some authors have reported the harvest of the semitendinosus and gracilis tendons cause decrements in hamstring strength (15,30,31), whereas others have reported no hamstring strength decrements (14,25). The disparity between the results of these studies may indicate the presence of functional adaptations in the involved leg; however, there is limited information regarding whether functional adaptations are present after hamstring ACL reconstruction.

Landing is ideally suited for a performance study, as it requires large eccentric quadriceps and hamstrings forces during the control of joint flexion (deceleration) and mimics the muscular stresses experienced during athletic competition. Examination of the landing strategy by which hamstring ACLr individuals control lower-extremity joint motion and dissipate the large loads experienced during impact may provide insight to the functional capacity of the knee as well as the entire lower extremity (10,19). Thus, it may be possible to determine whether a fully rehabilitated ACLr knee functions similar to normal, or whether other neuromuscular compensations are present. The purpose of this study, therefore, was to evaluate and compare the kinetic and kinematic landing performances of healthy and hamstring ACLr individuals.

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Eleven healthy (age, 26.9 yr; height, 1.8 m; mass, 80.7 kg) and 11 ACLr (age, 27.3 yr; height, 1.7 m; mass, 73.7 kg) recreational athletes, who were involved in jumping and landing sports at least three to five times a week, participated in this study. Selection criteria for the ACLr subjects were as follows: unilateral ACL reconstruction with double-loop semitendinosus-gracilis (DLSTG) technique, time from reconstruction was greater than 1 yr, time from injury to reconstruction was less than 3 months, KT-1000 manual maximum difference less than or equal to 5.0 mm, and no other injuries or surgeries to any lower-extremity joint. Although different surgeons operated upon the ACLr subjects, they all performed the same rehabilitation protocol that emphasized early weight bearing, strength, and range of motion (26). The healthy subjects were matched to the ACLr subjects for age, gender, athletic experience in jumping and landing sports, footfall patterning, and landing stiffness. Landing stiffness was practically defined as the minimum knee flexion position, where positions greater than and less than 90° were used to classify soft- and stiff-landing techniques (3,32). Thus, each subject was classified as either a soft or stiff lander, and this labeling was subsequently used to match the healthy to the ACLr subjects.

This study was approved by the Vail Valley Medical Center Internal Review Board. Upon signing the written informed consent, the subjects were fitted for a standardized court shoe (Turntec, model no. TM08061) and then asked to warm-up on a treadmill for 5 min. After the subjects practiced the landing task and felt comfortable with the performance requirements, eight vertical drop-landings were collected. In a pilot study, five to seven practice trials followed by the eight trials during data collection were determined to sufficiently capture the true landing performance without extraneous performance variability. The landing task consisted of stepping off a 60-cm box onto a landing platform. The subjects were instructed to fold their arms across their chest and step off the box, without jumping up or stepping down, and to land as naturally as possible with both feet on the landing platform. One foot landed upon a force plate (Bertec, Corp., Columbus, OH), and the other landed next to the force plate on the landing platform.

The force plate was used to measure ground-reaction forces (1200 Hz). Three-dimensional lower-extremity kinematics were captured (120 Hz) with a five-camera motion-analysis system (Motion Analysis Corp., Santa Rosa, CA) during the landing phase delineated from initial touchdown to minimum knee flexion. Force plate, kinematic, and anthropometric data were combined in an inverse dynamics solution utilizing a four-segment rigid link model (11) to calculate sagittal plane, lower-extremity internal-joint moments during the impact phase delineated from initial touchdown to the first 100 ms (21,22).

Hip, knee, and ankle muscle powers were calculated as the product of the joint moment and joint angular velocity. The muscle power curves were mathematically integrated to calculate negative and positive joint-work values. Positive and negative work values indicate energy production and absorption through concentric and eccentric muscular contractions, respectively. All joint power parameters were normalized to the product of body weight and height (%BW·ht). The data were interpolated to 100 points during the impact phase for graphical purposes only.

A 2 × 3 (group × joint) mixed-factor ANOVA was computed from the eight trial means for contact position, ROM, peak joint angular velocity, maximum extensor joint moments, minimum joint powers, and negative work during the impact phase of landing. Tukey post hoc analyses were used to determine specific differences when appropriate (alpha = 0.05). An unpaired t-test was used to contrast between group landing phase times and the magnitude, time, and loading rates of the peak vertical ground-reaction forces (VGRF).

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Group means and standard deviations for the impact phase kinematics are located in Table 1 and graphically presented in Figure 1. Compared with the healthy group, the hip and knee tended to be more extended, and the ankle more plantarflexed for the ACLr group; however, only the hip and ankle initial ground contact positions were statistically different between groups (both P < 0.05). The greater ankle contact position afforded more ankle ROM with greater peak angular velocities (both P < 0.05), yet the hip and knee remained in a more extended position throughout the impact phase, and yielded similar ROM and angular velocities compared with the healthy group (P > 0.05).

Group means and standard deviations for the VGRF parameters are located in Table 2. All subjects utilized a forefoot-rearfoot landing strategy that demonstrated two distinct peaks on the VGRF record. No between group differences were found for the first (F1) or second (F2) peak VGRF (both P > 0.05). Compared with the healthy group, however, the ACLr group delayed the time and reduced the loading rates to both force peaks (all P < 0.05) (Fig. 2).

Group means and standard deviations for the impact phase kinetics are located in Table 3 and graphically presented in Figures 3 and 4. Two distinct peaks were analyzed from the knee extensor moment and negative knee power curves, and one peak from each of the hip and ankle extensor moment and negative power curves. Within each group, the peak hip extensor moment was significantly larger than the peak ankle and knee extensor moments (all P < 0.05). The peak negative hip power was larger than the peak negative ankle and knee powers for the healthy group (all P < 0.05), but the ACLr group demonstrated no statistical differences between the lower-extremity peak negative powers (all P > 0.05). The healthy group revealed larger peak values for the hip extensor moment and negative hip power (both P < 0.05), and first knee extensor moment and first negative knee power (both P < 0.05) compared with the ACLr group.

Group means and standard deviations for negative joint work and relative contributions to energy absorption are presented in Table 4. The ACLr group demonstrated greater energy absorption from the knee and ankle compared with the hip (P < 0.05), whereas the healthy group demonstrated no energy absorption differences between the lower-extremity joints (all P > 0.05). Both groups utilized the knee as the primary joint to absorb energy; however, the ACLr group performed 39% less negative hip work (P < 0.05) and 37% more negative ankle work (P < 0.05) compared with the healthy group.

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The purpose of this study was to evaluate and compare the kinetic and kinematic landing performance of healthy and ACLr subjects to determine whether the injured limb functioned similar to normal after hamstring ACL reconstruction or whether other neuromuscular compensations were present. Given a matched landing stiffness, the ACLr subjects demonstrated a more erect landing posture at initial ground contact and reduced the rate of force application to the body. This reduced loading strategy revealed less energy absorption from the hip extensors and greater energy absorption from the ankle plantarflexors. These results were interpreted to represent a net reduction or avoidance in hip extensors use, namely the hamstrings, and an accentuation of the ankle plantarflexors use. Thus, the results of the current study reveal that lower-extremity compensations are present in fully rehabilitated ACL-injured subjects reconstructed with the DLSTG technique.

Although both groups performed soft landings with minimum knee flexion angles greater than 90° (healthy, 92°; ACLr, 97°), the kinematic landing differences were similar to the differences other researchers have noted between the performances of soft- and stiff-landing techniques (3,8,16,24,32). Similar to a stiff-landing technique, the ACLr group performed greater hip extension and ankle plantarflexion at initial ground contact compared with the healthy group. In addition, the hip maintained a more extended position throughout the landing phase yielding smaller, yet statistically similar, ROM and angular velocity values compared with the healthy group. However, similar to a soft-landing technique, the ACLr group performed greater ankle ROM with an increased angular velocity. The more restrained use of hip flexion was hypothesized to reflect a movement strategy to protect the remnant of the autogenous ACL donor site from excessive tension. This strategy would dictate greater ankle plantarflexion at initial ground contact to keep the total body center of mass over the base of support and allow maximum ankle ROM and energy absorption during the landing phase.

The magnitude and timing of the bimodal VGRF force peaks are in agreement with the results of other studies that have investigated vertical drop landings from a 60-cm height (6,13,23,32). However, the peak VGRF for the ACLr group was generally in the lower range of reported values for soft landings. The temporal features of the VGRF peaks showed that a lower peak VGRF was achieved when the time to occurrence was progressively greater. This has been shown to occur with the performance of greater hip (27) or knee flexion (3,6,32). In this study, increased ankle ROM demonstrated by the ACLr group may have accounted for the increased times and decreased loading rates of the bimodal VGRF peaks, because minimum hip and knee flexion angles and ROM were not different between groups. This decreased loading strategy was preplanned as the temporal features of the VGRF force peaks occur before a reflexive (18) or voluntary (7) response can directly react to contact with the ground.

The reductions in the peak hip and knee extensor joint moments and negative powers, noted for the ACLr group, represent decreased maximum efforts of the hip extensor muscles, including the hamstrings, and the quadriceps muscles in energy absorption. However, the maximum extensor moments and negative powers for both groups are within the ranges of reported values (9,32) and therefore support previous studies that have found normal isokinetic knee extension and flexion strength after hamstring ACL reconstruction (14,25,31). Alternatively, the presence of a neuromuscular strategy facilitated by the ankle’s increased ROM may have allowed for a more even distribution of muscular force within the ankle, knee, and hip throughout the landing phase. This strategy would ultimately transmit a smaller maximum tension from the rectus femoris to the anterior hip (20), subsequently reducing the position and velocity of hip flexion. Thus, the functional role of this neuromuscular strategy would serve to maintain the trunk in an erect landing posture and reduce the muscular output of the hip extensor muscles, including the hamstrings, to decelerate and halt the forward flexion of the trunk.

This neuromuscular strategy was also evident in the energy dissipation pattern. The knee extensors provided the major energy absorption function for both groups (healthy, 40%; ACLr, 41%) and may indicate their functional recovery after hamstring ACL reconstruction and rehabilitation. However, the ACLr group had a reduced contribution of energy absorption from the hip extensors (healthy, 32%; ACLr, 20%) and increased contribution from the ankle plantarflexors (healthy, 28%; ACLr, 39%). Although both energy dissipation patterns have been documented for healthy subjects (3,17), the group differences in landing strategy may indicate that the ACLr group increased the muscular function of the ankle plantarflexors to reduce the energy transmitted to the more proximal joints, name-ly the hip (20). Similar to other studies (3,17,32), both groups demonstrated the hip to have the largest extensor moment and negative power magnitudes but did not perform the greatest energy absorption function. The hip extensor joint moments and powers may have a greater role in controlling the position and velocity of hip and trunk flexion, rather than reducing the velocity of the total body center of gravity.

It is reasonable to assume that the harvest of the medial hamstring muscles has some role in the origin of this energy dissipation strategy. In ACL deficient and patellar tendon ACLr knees, for example, the loss of the ACL, or the harvest of the central third patellar tendon, tends to cause neuromuscular adaptations of the entire lower extremity during functional activities (1,4,5,12). For these individuals, the hip extensors generally output larger joint work values to allow for reduced knee extensor work thereby protecting other soft tissues in the knee or the ACL donor site. Thus, the neuromuscular adaptations noted for the hamstring ACLr subjects during landing may be a protective function to limit the muscular output of the hip extensors, including the hamstrings. Rehabilitation protocols for this population, therefore, may need to focus on the development of muscular power for the entire lower extremity during functional activities.

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Differences in landing strategies were observed from the kinematics, ground-reaction forces, and internal joint moments and powers. These landing strategies provide insight to the selective processes by which ACLr recreational athletes control joint motion and attenuate the load experienced when landing from a 60-cm height. This study revealed that landing strategies are preselected and can be designed to mediate stresses to a specific joint while allowing adequate performance of a high demand functional task.

This project was supported by a grant from the NFL Charities, New York, NY. The authors thank Philip K. Schot, Forrest Pecha, Mike Kain, and Chris Rich for their assistance with this project.

No author or related institution has received any financial benefit from research in this study.

This paper was presented at the 2000 Specialty day program of the American Orthopaedic Society for Sports Medicine in Orlando, Florida.

Address for correspondence: Michael J. Decker, Biomechanics Research Laboratory, Steadman-Hawkins, Sports Medicine Foundation, 181 West Meadow Drive, Suite 1000, Vail, CO 81657; E-mail:

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©2002The American College of Sports Medicine