ACL Research Retreat V

Abstract

Context: Deficits in neuromuscular control of the trunk have recently been correlated to knee injury incidence in collegiate athletes. However, the mechanism by which trunk control may influence ACL injury risk remains unknown. Previous studies have shown that both knee abduction moment and tibial internal rotation moment strain the ACL.Objective: To determine whether trunk control measures are correlated to knee abduction and tibial internal rotation moments during unanticipated run-to-cut maneuvers.Design: Descriptive cohort study.1Setting: Controlled, laboratory setting.Patients or Other Participants: Fourteen subjects (9 male, 5 female; height = 1740.8 ± 98.9 mm; mass = 72.7 ± 14.4 kg; age = 24.3 ± 4.0 yrs) with no current history of lower extremity injury or previous history of lower extremity or abdominal surgery.Interventions: Trunk neuromuscular control was quantified using a previously described sudden force-release (SFR) device, in which sudden, unexpected perturbations are applied to the trunk at the T10 level via a force-release apparatus. Subjects held a fixed position before the release of an isometric load (30% MVIC) and attempted to maintain that position after the load released. Subjects then performed 45° unanticipated cutting, planting with the dominant foot. Lower extremity kinematics were estimated using the Point-Cluster Technique, and inverse dynamics used to estimate net external knee moments.Main Outcome Measures: Trunk control in each direction was defined as the average peak angular deviation of the trunk after release of the cable attached from that direction from 3 SFR trials. Normalized peak knee abduction moment (pKAB) and peak tibial internal rotation moment (pTIR) [%BW * ht] from 3 successful cutting trials were calculated. Correlations between trunk control and knee moments during cutting were analyzed.Results: Significant correlations were found between SFR performance and knee moments during cutting. pKAB during cutting had a significant positive correlation to SFR peak angle in the nondominant direction (R = 0.665, P = .01), while pTIR during cutting had a significant positive correlation to SFR peak angle in the anterior direction (R = 0.532, P = .05). Correlations between pKAB and peak angle after back and dominant side SFR approached, but did not reach, significance. pTIR was not correlated with any other SFR peak angle.Conclusions: The significant correlations between both lateral and anterior/posterior trunk control and pKAB and pTIR suggest that trunk control directly affects the dynamic mechanical environment of the knee. In this study, the different directions used in the sudden force-release task were found to be specific to the two knee moments examined. pKAB was correlated to lateral performance on the nondominant side but not to anterior performance, while pTIR was correlated to anterior performance but not to lateral performance. These results suggest that the different muscles of the trunk are recruited differently to control coronal vs. transverse plane knee moments.Previously presented at the 56th Annual Meeting of the Orthopaedic Research Society; March 6–9, 2009; New Orleans, LA.Context: Decreased hip strength has been suggested to contribute to dynamic malalignment of the lower extremity during landing tasks. However, a relationship between hip strength and lower extremity joint motion has not been confirmed. To date, the frontal plane position of the trunk has not been examined. If the trunk's mass is more axially positioned, relative to its base of support, it would decrease the demands on the posterior lateral hip muscles. Thus, it may be a potential compensatory strategy for insufficient hip strength.Objective: To determine the relationship between hip strength and frontal plane trunk position during a single-leg hop (SLH).Design: Descriptive cohort.Setting: Controlled laboratory.Participants: Seventy three (37 M, 36 F) healthy participants (22.2 ± 3.6 yrs, 169.9 ± 10.2 cm, 71.6 ± 16.1 kg).Interventions: Hip strength and 3D kinematics during SLH trials were assessed on the dominant stance leg. Hip abduction (standing, hip abducted 5°), external rotation (semireclined, hip flexed 40°, knee flexed 90°), and extension (supine, hip flexed 90°) torques were measured during maximal isometric voluntary contractions using an instrumented dynamometer. SLH trials began while standing on the dominant stance leg and taking a hop forward, landing on the same leg (hop distance = 40% of height, minimal vertical height = 5″). The peak torque over 3 trials for each strength measure and average frontal plane trunk position at initial ground contact (GRF ≥ 10 N) over 5 SLH trials were used for analysis. Step-wise linear regression determined the extent to which hip torques predicted frontal plane trunk position at initial ground contact.Main Outcome Measures: Hip abduction, external rotation and extension torques were normalized to body mass (Nm/kg). Frontal plane trunk position (cm) represented the medial/lateral linear displacement of thorax relative to foot center of pressure at initial ground contact.Results: Means ± SDs for normalized hip abduction, external rotation, and extension torques were 0.71 ± 0.17 Nm/kg, 0.84 ± 0.22 Nm/kg, and 4.35 ± 1.02 Nm/kg, respectively. Frontal plane trunk position at initial contact was 3.0 ± 17.6 cm lateral to the foot center of pressure. Decreased hip extension (R2 = .417, P < .001), external rotation (R2change = 0.059, P = .006), and abduction (R2change = 0.019, P = .116) torques were predictive of greater lateral trunk position (P < .001), explaining 49.5% of the varianceConclusions: Decreased hip extension and external rotation strength was predictive of increased lateral trunk position at initial contact during a SLH. This laterally positioned trunk may be a compensatory strategy for lesser hip strength by decreasing the demands on the hip musculature to control lower extremity motion. This may be one explanation why no relationship has been established between hip strength and lower extremity joint motion. Future research should consider the influence of trunk position on hip muscle function when examining their role in controlling dynamic joint motion.Data were collected during a funded appointment supported by NIH-NIAMS Grant R01- AR53172.Context: The sexes differ in the performance of functional tasks, such as landing and cutting. However, the underlying control mechanisms responsible for these differences remain unidentified.Objective: To evaluate spinal control mechanisms and functional neuromuscular variables in males and females.Design: Cross-sectional.Setting: Research laboratory.Participants: Volunteer sample of 19 males (23.0 ± 4.3 yrs, 177.45 ± 5.44 cm, 77.52 ± 13.18 kg) and 18 females (24.7 ± 2.9 yrs, 165.31 ± 5.85 cm, 62.44 ± 8.76 kg).Interventions: While seated on an isokinetic dynamometer with the ankle of the dominant leg secured at 9°, the following recruitment curves were collected at the soleus: H-reflex, intrinsic presynaptic inhibition (IPI), and extrinsic presynaptic inhibition (EPI). The first derivative of each of the recruitment curves was then determined. Additionally, percent of recurrent inhibition (RI), V-wave (Vmax∶Mmax), rate of torque development (RTD), and electromechanical delay (EMD) of the soleus were assessed. IPI testing used the paired pulse conditioning protocol (interstimulus interval = 100 ms). EPI was measured through common peroneal nerve conditioning (100 ms conditioning interval). RI was assessed by setting stimulus 1 to 25% of Mmax and stimulus 2 to Mmax. V-waves were tested via Mmax stimulation to the tibial nerve during an isometric maximum voluntary contraction (iMVC). Additionally, three trials of iMVC with the instruction to contract as hard and fast as possible were collected to assess RTD. EMD, the time lag between EMG and torque activity, was measured during the RTD trials. A 2 (sex) × 7 (neural variable) MANOVA was used to compare means of the dependent variables.Results: The Wilks lambda multivariate test of overall differences among groups was statistically significant (P = .001). Univariate between-subjects tests revealed males had significantly greater RI (males = 0.86 ± 0.21, females = 0.68 ± 0.30; P = .042). Males also had greater RTD (males = 387.93 ± 180.90 n·m·s−1, females RTD = 263.89 ± 85.15 n·m·s−1; P = .033). The sexes did not differ on first derivative of the following: H-reflex (males = 9.80 ± 3.71, females = 10.38 ± 4.58, P = .773), IPI (males = 2.23 ± 2.27, females = 2.14 ± 2.23, P = .778), or EPI (males = 8.39 ± 4.15, females = 9.79 ± 6.15, P = .668). V-waves (males = 0.22 ± 0.21, females = 0.27 ± 0.17, P = .526) and EMD (males = 46.35 ± 29.76 ms, females = 58.50 ± 23.47 ms, P = .278) were not different.Conclusions: The sexes differ on modulation of spinal control of movement and activation of the neuromuscular system. Males were able to produce maximal torque more quickly than females. Additionally, RI, a postsynaptic regulator of torque output, was greater in males. Based on these findings, males and females clearly utilize neural control mechanisms differently.Previously presented at the 60th National Athletic Trainers' Association Annual Meeting and Clinical Symposia; June 17–20, 2009; San Antonio, TX.Context: Based on a vast collection of biomechanical studies it is widely accepted that men and women move differently in landing and cutting tasks. However, there is little understanding of potential neural control differences between the sexes that may contribute to these observed differences in gross motor tasks.Objective: The objective was to compare spinal reflex profiles between men and women.Design: A cohort study design was utilized.Setting: The study took place in a controlled laboratory setting.Patients or Other Participants: Twenty-eight regularly menstruating women (mean age = 22.4 ± 3.4 yrs) and 15 men (mean age = 22.3 ± 3.7 yrs) participated in the study. Subjects were recruited from a university setting and none had ever experienced a significant lower extremity injury. Each subject volunteered and provided informed consent.Interventions: As part of the larger study, the women reported to the laboratory for testing every other day during the course of one menstrual cycle. The control male subjects reported to the lab for testing every fourth day over 28 days. To account for unequal data points between the sexes, only female data points corresponding in time to the male control data points were utilized for analysis. This resulted in both sexes having data from every fourth day. These 7 data points were averaged and used for analysis. Each data point represents the ratio between the maximum H-reflex and maximum M wave (Hmax/Mmax ratio) collected on that day. These responses were elicited in the soleus muscle through the use of 1-ms square wave pulse delivered to the tibial nerve in the popliteal fossa. EMG electrodes were placed on the soleus muscle of the right leg to measure the evoked responses. Sex was the sole independent variable.Main Outcome Measures: The dependent variable was the mean Hmax/Mmax ratio in the soleus muscle. A two-sample t test was used to determine if the Hmax/Mmax ratio was different between the two sexes (α = .05).Results: The mean ratio for the women (0.86 ± 0.017) was significantly higher (P < .001) than the mean ratio for the men (0.58 ± 0.019).Conclusions: These data are part of a larger project in which the relationships between spinal reflexes and sex hormones were reported to be minimal. The current analysis clearly shows the Hmax/Mmax ratio is higher in women. Interestingly, the Hmax/Mmax ratio is also known to be lower in power-trained individuals compared to endurance runners. It remains unclear as to why this ratio is higher in some groups or populations, but several authors have speculated that a lower ratio suggests a greater ability to produce explosive movements.Context: Anterior cruciate ligament (ACL) injuries represent a serious problem among female team handball and football players and require several months of rehabilitation. The players are recommended to regain muscle strength to the level of at least 90% of their uninjured leg before returning to sport. However, previous studies have shown side differences in strength even several years after injury.Objective: To assess side differences in muscle strength among elite female handball and football players with and without a previous unilateral ACL injury.Design: Cross-sectional study.Setting: Controlled, laboratory.Methods: This study is part of a large cohort study aimed to investigate risk factors for noncontact ACL injuries among elite female handball and football players. Since study start in 2007, a total of 425 players from the Norwegian elite handball (n = 233) and football league (n = 192) have been included. Of these players, 42 (9.9%) had previous ACL injuries (left, 21; right, 16; bilateral, 5).Main Outcome Measures: All players have been tested for isokinetic concentric muscle strength of the quadriceps and hamstrings muscles at 60°/s. Strength normalised to body weight was compared between players with unilateral injuries (n = 37) and noninjured players (n = 377) and between the injured and noninjured leg for injured players. Players with bilateral injuries or incomplete test results were excluded (n = 11).Results: There were no muscular side-to-side differences observed among the noninjured players. Players with a previous injury were significantly stronger in their noninjured leg compared to their injured leg, both for quadriceps (2.52 vs 2.33 Nm/kg, P = .001) and hamstrings (1.49 vs 1.40 Nm/kg, P = .001). However, there were no strength differences between the injured leg compared to noninjured players, neither for quadriceps (2.33 vs 2.39 Nm/kg, P = .35) nor hamstrings (1.40 vs 1.39 Nm/kg, P = .85). Injured players were significantly stronger in their noninjured leg compared to noninjured players, both for quadriceps (2.52 vs 2.39 Nm/kg, P = .01) and hamstrings (1.49 vs 1.39 Nm/kg, P = .001). These findings did not differ between handball and football players.Conclusions: There was no difference in muscle strength between noninjured players and the injured leg of players with a previous ACL injury. Previously injured players were significantly stronger in their noninjured leg compared to their injured leg and compared to players with no previous injury. Side-to-side differences among injured players may leave them prone to new injuries.Acknowledgments: The Oslo Sports Trauma Research Center is established at the Norwegian School of Sport Sciences through grants from the Royal Norwegian Ministry of Culture, the Norwegian Olympic Committee & Confederation of Sport, the Norwegian Eastern Health Corporate, and Norsk Tipping AS.Context: Anterior cruciate ligament (ACL) injury is prevalent in individuals participating in sports, specifically females. Numerous variables have been reported as predisposing factors. Muscle fatigue contributes to alteration in landing mechanics, predisposing athletes to knee injury.Objectives: To investigate the effects of a single session of repeated muscle fatigue on ground reaction forces (GRF) during drop landings.Design: A univariate ANOVA with repeated measures was used to determine differences between gender, fatigue, and GRF. Rate of perceived exertion (RPE), peak power, mean power, and percent power drop were collected to assess fatigue and effort during the fatigue protocol.Setting: Controlled laboratory setting.Participants: Ten female (22.5 ± 0.85 yrs) and ten male (24.1 ± 2.6 yrs), healthy recreational athletes involved in jumping sports at least twice a week with no history of lower extremity injury, cardiovascular disease, pulmonary disorder, or any previous injury that would impair them from exercising.Intervention: Participants performed five experimental conditions. The first condition consisted of five nonfatigued double-leg drop landings from a 60-cm platform onto a force platform, followed by four conditions of a fatigue protocol. Fatigue was induced by a 20-sec Wingate Anaerobic Test (WAT). Following each fatigue condition, participants completed two drop landings with 30 sec rest between drops and 5 min of active rest that included 4 min of cycling at 60–70 W resistance between each fatigue condition. All drop landings were averaged for each condition and peak force was normalized to body weight.Main Outcome Measures: Kinetic data was used to identify peak magnitude of force for forefoot force (F1), rearfoot force (F2), anterior/posterior (AP), and medial/lateral (ML) at both F1 and F2.Results: No main effect was observed between gender across all GRF variables. A main effect was observed (P ≤ .05) between the nonfatigue and fatigue conditions in respect to peak F2 force. The greatest significant difference was shown between the first fatigue drop landing condition (F2 = 7.15 ± 2.68 bw) compared to the last fatigue drop landing condition (F2 = 9.38 ± 2.19 bw) in respect to peak F2 (P ≤ .05). No difference was observed between gender and peak F2 (P ≤ .05), and no difference was observed across AP and ML at peak F1 and F2.Conclusions: A single session of repeated bouts of muscle fatigue induced by WAT caused an initial reduction in peak F2 followed by an increase in peak F2 across conditions. Muscle fatigue consequently alters landing kinetics, potentially increasing the risk for an ACL injury by increasing joint stiffness.Context: The function of the hamstrings (HAMS) in protecting the knee joint from injury is not fully understood. During participation in sport, muscles may become fatigued and the hamstrings may lose their potential ability to protect the ACL. One approach to understanding the role of the hamstrings is to selectively impair their function and observe acute compensating effects.Objective: To determine the effects of weakened hamstrings on knee mechanics during single-leg side cut maneuvers.Design: Descriptive cohort study design.Setting: Controlled laboratory setting.Patients or Other Participants: Ten female healthy college-aged participants (21.3 ± 1.2 years, 170 ± 5 cm, 64.8 ± 9.0 kg) with no current or previous history of lower extremity injury that would affect the alignment of the lower limb.Interventions: Two experimental sessions were conducted. In both sessions, HAMS strength was reduced through a 180°/sec concentric HAMS fatigue protocol on an isokinetic dynamometer. The first two sets consisted of 40 repetitions and the final set continued until 3 consecutive repetitions fell below 25% of the participant's peak knee flexor torque. In one session, strength recovery of the HAMS were assessed 75 seconds postexercise on the dynamometer, which was the time required in the next session to begin recording landing mechanics. In the next session, three-dimensional stance-leg knee kinetics and kinematics were collected on single-leg stride land and cut maneuvers (LC) before and after the exercise protocol. Single-leg vertical jumps (VJ) were also collected as an indicator of fatigue state.Main Outcome Measures: Three-dimensional stance leg knee kinetics and kinematics were calculated and reported as touchdown angles, ranges of motion, and peak moments during stance phase. PRE- and POST-exercise HAMS strength, VJ jump height, and LC stance leg knee mechanics were analyzed using paired t test (P < .05).Results: HAMS strength was significantly reduced (PRE = 59.4 ± 5.6 Nm, POST = 54.7 ± 8.5 Nm, P = .03). However, VJ height was not significantly different (PRE = 0.13 ± 0.03 m, POST = 0.12 ± 0.3 m, P = .08). During the LC maneuvers there were no significant differences in touchdown angles in any plane, but there was decreased range of motion in the transverse plane (PRE = 16.5 ± 7.5°, POST = 13.9 ± 7.2°, P = .02). The peak extensor moment also decreased (PRE = 2.75 ± 0.60 Nm/kg, POST = 2.55 ± 0.42 Nm/kg, P = .03).Conclusions: The fatigue protocol resulted in decreased HAMS muscle force that had not fully recovered by the time postexercise landing data were collected. The kinematic and kinetic changes suggest a protective landing strategy was employed by the subjects. Post hoc analysis revealed that the decreased knee extension moment was also accompanied by a decreased hip extensor moment. Given the hamstrings role in assisting hip extension, the decreased knee extension moment was likely in response to the hip. The results indicate that the role of the hamstrings at the knee are complicated by its biarticular role.Context: Evaluation of movement coordination may provide more integrated information than traditional biomechanical methods with respect to ACL injury risk factors. Relative phase (RP) measures of motion based on dynamical systems theory allow movement coordination and stability to be analyzed for an entire extremity. Few ACL injury risk-factor studies have incorporated RP measures, and none has been performed using fatigued, unanticipated motions. Additionally, it is unclear how interventions like verbal feedback influence these measures.Objective: To quantify effects of fatigue and verbal feedback on coordination and variability of the lower extremity during an unanticipated cutting task.Design: Cross-sectional.Setting: Research laboratory.Patients or Other Participants: Fifty-nine club-sport athlete volunteers (31 M, 28 F; 19.8 ± 1.6 yrs, 176.7 ± 9.2 cm, 71.2 ± 10.0 kg) were randomly assigned to either receive verbal feedback (FB) or no feedback (NFB) postfatigue.Interventions: Subjects performed an unanticipated sidestep cutting task using their dominant leg. Subjects jumped over a hurdle onto a force platform and responded to a randomized directional cue by cutting 60° in the indicated direction. Participants then performed a fatigue protocol, followed by reassessment of the sidestep cutting task. During this second assessment the FB group received instruction to “land softly, keep the knee over the toes, and make the movement smooth,” while the NFB group received no instruction.Main Outcome Measures: Sagittal and frontal plane segment angles and velocities were calculated relative to the global reference system for the foot, shank, thigh, and trunk. Phase-plane plots and RP angles were created for each segment, continuous RP portraits for each segment pairing (foot-shank, shank-thigh, and thigh-trunk) were generated, and the mean absolute relative phase (MARP) and deviation phase (DP) derived. Comparisons were made between the FB and NFB groups prefatigue and postfatigue using mixed-model ANOVAs with Bonferroni post hoc tests (α < .05).Results: Significant fatigue × group interactions for coordination (MARP) were observed in the sagittal plane for foot-shank (F1,55 = 4.641, P = .036), shank-thigh (F1,55 = 4.719, P = .034), and thigh-trunk (F1,55 = 4.967, P = .030), and in the frontal plane for shank-thigh (F1,55 = 4.464, P = .039) and thigh-trunk (F1,55 = 7.708, P = .008). The NFB group displayed decreased sagittal plane MARP for the foot-shank (−18.3%), shank-thigh (−12.7%), and thigh-trunk (−10.8%), while FB resulted only in decreased thigh-trunk frontal plane MARP (−13.1%). Significant main effects for fatigue were also observed for variability (DP). Fatigue caused decreased DP for foot-shank sagittal plane (−11.9%, F1,55 = 13.634, P = .001), foot-shank frontal plane (−19.1%, F1,55 = 41.262, P < .001), shank-thigh sagittal plane (−7.3%, F1,55 = 4.078, P = .048), and shank-thigh frontal plane (−13.1%, F1,55 = 21.093, P < .001).Conclusions: Fatigue causes a more in-phase coordination pattern and a loss of variability during unanticipated sidestep cutting. However, feedback counteracts the coordination changes, suggesting cognitive control over movement organization. These results suggest feedback may be used to acutely retain motion patterns in the lower extremity, which may help prevent ACL injury postfatigue.Context: Hip strength, muscle activation, and fatigue affect lower extremity alignment. Foot type may influence hip biomechanics and landing force attenuation and contribute to noncontact ACL injury risk.Objective: To determine the effect of foot type and fatigue on hip neuromuscular control and lower extremity kinetics during a functional landing task.Design: Cross-sectional design.Setting: Research laboratory.Patients or Other Participants: Twenty-four healthy National Collegiate Athletic Association Division I male and female athletes with either a rectus or planus foot type (14 rectus: 19.5 ± 1.7 years, height = 166.1 ± 6.7 cm, mass = 64.1 ± 4.9 kg, navicular drop = 7.1 ± 0.92 mm; and 10 planus: 20.1 ± 1.3 years, height = 169.2 ± 7.3 cm, mass = 68.4 ± 8.4 kg, navicular drop = 11.9 ± 2.0 mm) volunteered to participate.Interventions: Independent variables were foot type (planus and rectus) and fatigue (pre and post). Vernier calipers were used for the navicular drop test to measure arch height. A MicroFET Hand-Held Dynamometer was used to measure hip strength, a Noraxon Telemyo Electromyography (EMG) system was used to measure muscle activation, and a Kistler 9287-BA Force Plate was used to measure lower extremity kinetics during a standing broad jump-to-vertical jump maneuver in both prefatigue and postfatigue conditions. Statistical analyses consisted of multiple analyses of variance (ANOVA) and t tests. Alpha level was set at P ≤ .05.Main Outcome Measures: Dependent variables were hip extensor, abductor, and external rotator strength (pounds); EMG activation for the gluteus maximus, gluteus medius, and biceps femoris (reactive area by %MVC); and peak vertical, anterior shear, medial shear, and lateral shear ground reaction forces; and rate of loading at ground contact.Results: ANOVA tests revealed the following significance: Postfatigue, the planus group showed a 49% decrease in biceps femoris EMG area (F = 4.53, P = .045, pre = 22.67 ± 18.94, post = 11.45 ± 9.78), a 35% decrease in coagonist gluteus maximus and biceps femoris EMG area (F = 5.47, P = .029, pre = 41.26 ± 27.83, post = 26.77 ± 20.42), and a 31% increase in medial shear force (F = 50.72, P = .001, pre = .174 ± .030, post = .228 ± .030). Rate of lower extremity loading decreased 24% postfatigue (F = 16.97, P = .001, pre = 56.83 ± 21.81, post = 43.43 ± 23.35) for both groups. No other significant differences were noted between foot types or prefatigue and postfatigue.Conclusions: Under fatigue, athletes with a planus foot type have a reduced capacity to attenuate medial shear force. This may influence hip muscle activation strategies and lower extremity force attenuation, potentially increasing the risk of knee valgus and noncontact ACL injury.Previously presented at the 60th Annual National Athletic Trainers' Annual Meeting and Clinical Symposia: Stearne DJ, Sato N, Sitler MR, Tierney RT. Relationship of foot type and fatigue to hip neuromuscular control and lower extremity kinetics; June 17–20, 2009; San Antonio, TX.Context: The ACL is loaded via anterior tibial translation (ATT), and excessive ATT has been identified prospectively as an ACL injury risk factor. ATT lengthens the hamstrings, and the hamstrings respond by generating tensile force which resists further lengthening. Stiffness refers to the ratio of change in force to change in length (ΔForce/ΔLength), and stiffer hamstrings may limit ACL loading by providing greater resistance to ATT. Hamstring strengthening is essential to ACL injury rehabilitation and prevention, and likely influences the hamstrings' ability to resist ATT. However, the relationships between these hamstring properties and ATT have yet to be identified.Objective: To evaluate relationships between hamstring stiffness, hamstring strength, and ATT. We hypothesized that hamstring stiffness and strength would be positively correlated, greater stiffness and strength would correspond with less ATT, and the correlation between stiffness and ATT would be stronger than that for strength and ATT.Design: Cross-sectional.Setting: Research laboratory.Patients or Other Participants: Thirty healthy, physically active volunteers (15 males, 15 females; mass = 74.9 ± 18.1 kg; height = 1.7 ± 0.1 m, age = 22.7 ± 2.4 years).Interventions: Hamstring stiffness was assessed by evaluating the damping effect imposed by the hamstrings on oscillatory knee flexion/extension (ICC [2,1] = 0.70; SEM = 1.63 N/cm). ATT was assessed by applying a 20% body weight load to the posterior proximal shank via a custom-built perturbation device and was defined as the difference in anterior displacements of electromagnetic motion-capture sensors on the thigh and shank (ICC [2,1] = 0.98; SEM = 1.49 mm). Hamstring strength was defined as the peak force during maximal isometric contraction.Main Outcome Measures: Pearson correlation coefficients were used to evaluate relationships between hamstring strength, hamstring stiffness, and ATT. Stiffness and strength distributions were arranged into tertiles (n = 10), and ATT was compared between highest and lowest tertiles using independent-samples t tests. All dependent variables were correlated with body mass and, thus, were standardized to body mass prior to analysis. Statistical significance was established a priori as α ≤ .05.Results: ATT was correlated with hamstring stiffness (r = −0.51, P < .01) but was not related to hamstring strength (r = −0.08, P = .67). Hamstring stiffness and strength were not correlated (r = 0.05, P = .78). ATT was greater in the lowest hamstring stiffness tertile compared to the highest tertile (0.20 vs. 0.07 mm/kg; P = .011) but did not differ between lowest and highest hamstring-strength tertiles (0.15 vs. 0.10 mm/kg; P = .362).Conclusions: Individuals with greater hamstring stiffness demonstrate less ATT. As musculotendinous stiffness can be modified via numerous mechanisms, these findings sugg