Steve T. Jamison

Knee moments during run-to-cut maneuvers are associated with lateral trunk positioning

Non-contact anterior cruciate ligament (ACL) injuries account for approximately 70% of ACL ruptures and often occur during a sudden change in direction or pivot. Decreased neuromuscular control of the trunk in a controlled perturbation task has previously been associated with ACL injury incidence, while knee abduction moments and tibial internal rotation moments have been associated with ACL strain and ACL injury incidence. In this study, the association between movement of the trunk during a run-to-cut maneuver and loading of the knee during the same activity was investigated. External knee moments and trunk angles were quantified during a run-to-cut maneuver for 29 individuals. The trunk angles examined were outside tilt (frontal plane angle of the torso from vertical), angle between the ground reaction force (GRF) and the torso in the plane containing the GRF and shoulders (torso-GRF_shoulders); and angle between GRF and torso in the plane containing the GRF and pelvis (torso-GRF_pelvis). Significant positive associations were found between torso angles and peak knee abduction moments (outside tilt, p=0.002; and torso-GRF_shoulders, p=0.036) while a significant negative association was found between peak tibial internal rotation moment and outside tilt (p=0.021). Because the peaks of these moments occur at different times and minimal axial rotation moment is observed at peak knee abduction moment (-0.29±0.46%BW*ht), the positive association between peak knee abduction moment and torso lean suggests that increasing torso lean may increase ACL load and risk of injury.

The effects of core muscle activation on dynamic trunk position and knee abduction moments: Implications for ACL injury

Anterior cruciate ligament (ACL) injury is one of the most common serious lower-extremity injuries experienced by athletes participating in field and court sports and often occurs during a sudden change in direction or pivot. Both lateral trunk positioning during cutting and peak external knee abduction moments have been associated with ACL injury risk, though it is not known how core muscle activation influences these variables. In this study, the association between core muscle pre-activation and trunk position as well as the association between core muscle pre-activation and peak knee abduction moment during an unanticipated run-to-cut maneuver were investigated in 46 uninjured individuals. Average co-contraction indices and percent differences between muscle pairs were calculated prior to initial contact for internal obliques, external obliques, and L5 extensors using surface electromyography. Outside tilt of the trunk was defined as positive when the trunk was angled away from the cutting direction. No significant associations were found between pre-activations of core muscles and outside tilt of the trunk. Greater average co-contraction index of the L5 extensors was associated with greater peak knee abduction moment (p=0.0107). Increased co-contraction of the L5 extensors before foot contact could influence peak knee abduction moment by stiffening the spine, limiting sagittal plane trunk flexion (a motion pattern previously linked to ACL injury risk) and upper body kinetic energy absorption by the core during weight acceptance.

Footwear matters: influence of footwear and foot strike on load rates during running

INTRODUCTION Running with a forefoot strike (FFS) pattern has been suggested to reduce the risk of overuse running injuries, due to a reduced vertical load rate compared with rearfoot strike (RFS) running. However, resultant load rate has been reported to be similar between foot strikes when running in traditional shoes, leading to questions regarding the value of running with a FFS. The influence of minimal footwear on the resultant load rate has not been considered. This study aimed to compare component and resultant instantaneous loading rate (ILR) between runners with different foot strike patterns in their habitual footwear conditions. METHODS Twenty-nine injury-free participants (22 men, seven women) ran at 3.13 m·s along a 30-m runway, with their habitual foot strike and footwear condition. Ground reaction force data were collected. Peak ILR values were compared between three conditions; those who habitually run with an RFS in standard shoes, with an FFS in standard shoes, and with an FFS in minimal shoes. RESULTS Peak resultant, vertical, lateral, and medial ILR were lower (P < 0.001) when running in minimal shoes with an FFS than in standard shoes with either foot strike. When running with an FFS, peak posterior ILR were lower (P < 0.001) in minimal than standard shoes. CONCLUSIONS When running in a standard shoe, peak resultant and component ILR were similar between footstrike patterns. However, load rates were lower when running in minimal shoes with a FFS, compared with running in standard shoes with either foot strike. Therefore, it appears that footwear alters the load rates during running, even with similar foot strike patterns.

Side-to-side differences in anterior cruciate ligament volume in healthy control subjects

Examination of anterior cruciate ligament (ACL) anatomy is of great interest both in studying injury mechanisms and surgical reconstruction. However, after a typical acute ACL rupture it is not possible to measure the dimensions of the ACL itself due to concomitant or subsequent degeneration of the remaining ligamentous tissue. The contralateral ACL may be an appropriate surrogate for measuring anatomical dimensions, but it remains unknown whether side-to-side differences preclude using the contralateral as a valid surrogate for the ruptured ACL. This study examined whether the ACL volume is significantly different between the left and right knees of uninjured subjects. ACL volumes were calculated for the left and right sides of 28 individuals using a previously validated MRI-based method. The mean ACL volume was not significantly different (p=0.2331) between the two sides in this population. Side-to-side ACL volume was also well correlated (correlation=0.91, p<0.0001). The results of this study show that the volume of the contralateral ACL is a valid surrogate measure for a missing ACL on the injured side. This non-invasive, in vivo technique for measuring ACL volume may prove useful in future large-scale comprehensive studies of potential risk factors for ACL rupture, in quantifying potential loading effects on ACL size as a prophylactic measure against ACL rupture, and in the use of ACL volume as a screening tool for assessing risk of injury.

Knee Joint Kinetics in Relation to Commonly Prescribed Squat Loads and Depths

Abstract Cotter, JA, Chaudhari, AM, Jamison, ST, and Devor, ST. Knee joint kinetics in relation to commonly prescribed squat loads and depths. J Strength Cond Res 27(7): 1765–1774, 2013—Controversy exists regarding the safety and performance benefits of performing the squat exercise to depths beyond 90° of knee flexion. Our aim was to compare the net peak external knee flexion moments (pEKFM) experienced over typical ranges of squat loads and depths. Sixteen recreationally trained men (n = 16; age, 22.7 ± 1.1 years; body mass, 85.4 ± 2.1 kg; height, 177.6 ± 0.96 cm; mean ± SEM) with no previous lower-limb surgeries or other orthopedic issues and at least 1 year of consistent resistance training experience while using the squat exercise performed single-repetition squat trials in a random order at squat depths of above parallel, parallel, and below parallel. Less than 1 week before testing, 1RM values were found for each squat depth. Subsequent testing required the subjects to perform squats at the 3 depths with 3 different loads: unloaded, 50% 1RM, and 85% 1RM (9 total trials). Force platform and kinematic data were collected to calculate pEKFM. To assess the differences among loads and depths, a 2-factor (load and depth) repeated measures analysis of variance with significance set at the p < 0.05 level was used. Squat 1RM significantly decreased 13.6% from the above-parallel to the parallel squat and another 3.6% from the parallel to the below-parallel squat (p < 0.05). Net peak external knee flexion moments significantly increased as both squat depth and load were increased (p ⩽ 0.02). Slopes of pEKFM were greater from unloaded to 50% 1RM than when progressing from 50% to 85% 1RM (p < 0.001). The results suggest that typical decreases in squat loads used with increasing depths are not enough to offset increases in pEKFM.

Time-to-contact demonstrates modulation of postural control during a dynamic lower extremity task §

Postural control deficits are associated with increased risk of loss of balance and potential injury. To assess balance deficits and estimate injury risk, there is a need to evaluate postural control during dynamic activities. Analysis during dynamic activities could assess if an individuals ability to control their posture is a fixed condition or if it is dependent on the demands of a task. The purpose of this study was to evaluate changes in postural control during a dynamic lower extremity task using time-to-contact (TtC) analysis. 3D motion capture with a force plate was used to evaluate 46 healthy recreational athletes performing an anterior reach with the right foot while standing on their left leg. TtC was calculated for nine valid trials. For each trial, the time from the toe leaving the force plate to the toe touching the floor at the maximum reach distance was divided into five epochs of equal duration. TtC was averaged over each epoch. Differences in TtC were evaluated with an unbalanced mixed effects ANOVA and post hoc Tukeys HSD comparisons. Epoch was a significant main effect (p<0.001), with both Epoch 4 and Epoch 5 having significantly greater TtC from all other epochs (p=0.05). Increasing TtC in later epochs suggests a higher demand for postural control when the task becomes more challenging. As an individuals reaching foot extends further from the body, postural control is adjusted to match the changing demands of the dynamic task.

Proximal Risk Factors for ACL Injury: Role of Core Stability

This chapter reviews the role of the core and core stability in preventing ACL injuries. Core stability represents the ability of the core, or the muscles of the abdomen and lower back, to maintain or resume a relative position [or trajectory] of the trunk after a perturbation. Poor trunk control and core stability may place an athlete in a position that results in adverse loading of the knee, leading to injury. Training programs incorporating core-specific exercises have been successful at reducing ACL injury risk, but the extent to which the core-specific exercises influenced the reduction in injury risk is unknown. Reducing trunk angles, medializing the center of mass, and shifting the center of mass anteriorly more over the foot are all associated with reduced knee moments and may lead to reduced ACL injury risk.

Is symmetry of loading improved for injured runners during novice barefoot running

BACKGROUND As barefoot (BF) running provides important sensory information that influence landing patterns, it may also affect loading symmetry. RESEARCH QUESTION The purpose of this investigation was to examine whether symmetry of loading in a group of injured runners would be improved in a novice, barefoot condition. METHODS Cross-sectional design evaluating 67 injured RFS runners. Each subject ran on an instrumented treadmill, first with their habitual shod pattern and then in a BF condition with a FFS pattern, both at the same self-selected speed. Data were averaged over 10 footstrikes. Variables of interest included vertical average load rate, vertical instantaneous load rate, and resultant instantaneous load rate. Symmetry indices (SI) for full population and within quartiles were compared for each loadrate variable (P ≤ 0.05) to evaluate changes between conditions. RESULTS On average, symmetry of loading was similar in a novice BF condition of injured runners compared with their habitual RFS shod condition. However, a subanalysis of quartiles revealed that the injured runners with the highest asymmetry (greatest SI values) displayed significantly lower asymmetry when running BF for all three loadrate measures. SIGNIFICANCE The addition of sensory input during barefoot running only improves symmetry of loading when habitual loading is highly asymmetric.

Dynamic Trunk Control Influence on Run-to-Cut Maneuver: A Risk Factor for ACL Rupture

Anterior Cruciate Ligament (ACL) rupture is one of the most common serious knee injuries in field and court sports, with an estimated 70% of these injuries being non-contact in nature, often from sudden changes in direction or pivoting [3]. ACL injury results in both short- and long-term consequences for the athlete, which may include surgery, decreased activity levels, elevated pain levels during activities and increased risk of osteoarthritis. Previous studies have shown that knee abduction and tibial internal rotation moments independently strain the ACL, and that these moments have an interaction effect at physiologic load levels, creating strains approaching the reported range of ACL rupture [2, 6–8].© 2011 ASME

Differences in Rate of Increased Patellofemoral Joint Reaction Force in the Back Squat Exercise: 2593

The squat exercise is widely utilized in both rehabilitation and strength training programs. Health professionals are often concerned with limiting patellofemoral joint reaction force (PFJRF) while still providing enough stimulatory effect to attain adaptation. PURPOSE: To test the hypotheses that the rate of PFJRF change will increase a.) as squat depth increases; and b.) more rapidly from 50% one repetition max (1RM) to 85% 1RM than when progressing from an unloaded squat to 50% 1RM. METHODS: Sixteen recreationally trained males (22.7 ± 4.5 yrs; 85.4 ± 8.7 kg) with no previous leg surgeries performed single repetition squat trials to squat depths of above-parallel (~90° of knee flexion), parallel (~110°), and below-parallel (~135°), and with loads consisting of no load, 50% 1RM, and 85% 1RM for each depth. Prior to testing, 1RM loads were found for each depth. Force platform and kinematic data were used in a biomechanical model to calculate PFJRF as a function of knee flexion angle and net peak external knee flexion moment. Load progressions were defined as the intervals from no load to 50% 1RM and from 50% 1RM to 85% 1RM. To assess differences among depths and load progression, a two-factor repeated-measure ANOVA (p < 0.05) was used. RESULTS: Squat 1RM significantly decreased as depth increased from 90° (150.4 ± 19.7 kg) to 110° (130.0 ± 19.3 kg) to 135° (125.3 ± 17.4 kg) of knee flexion. Significant effects were found for depth and load progression with no interaction effect. The PFJRF at the 135° depth had a significantly higher rate of increase (31.7 ± 11.7 N/%1RM) than compared with the 90° (38.2 ± 15.2 N/%1RM) and 110° (56.6 ± 19.3 N/%1RM) depths. The rate of PFJRF increase from 50% 1RM to 85% 1RM (28.6 ± 16.5 N/%1RM) was significantly lower than the rate of increase from an unloaded squat to 50% 1RM (55.7 ± 22.2 N/%1RM). CONCLUSION: The rate of increase in PFJRF was significantly higher for below-parallel squats and when progressing from an unloaded squat to a 50% 1RM squat. When designing training progressions that utilize the back squat exercise, health professionals should use caution; the highest rates of increase in PFJRF occur when first adding load and when progressing to below-parallel squats.