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.
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.
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: firstname.lastname@example.org.
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Keywords:©2002The American College of Sports Medicine
KNEE; KINEMATICS; KINETICS; SURGERY; PERFORMANCE; BIOMECHANICS