Are Gait Patterns during In-Lab Running Representative of Gait Patterns during Real-World Training? An Experimental Study.

Biomechanical assessments of running typically take place inside motion capture laboratories. However, it is unclear whether data from these in-lab gait assessments are representative of gait during real-world running. This study sought to test how well real-world gait patterns are represented by in-lab gait data in two cohorts of runners equipped with consumer-grade wearable sensors measuring speed, step length, vertical oscillation, stance time, and leg stiffness. Cohort 1 (N = 49) completed an in-lab treadmill run plus five real-world runs of self-selected distances on self-selected courses. Cohort 2 (N = 19) completed a 2.4 km outdoor run on a known course plus five real-world runs of self-selected distances on self-selected courses. The degree to which in-lab gait reflected real-world gait was quantified using univariate overlap and multivariate depth overlap statistics, both for all real-world running and for real-world running on flat, straight segments only. When comparing in-lab and real-world data from the same subject, univariate overlap ranged from 65.7% (leg stiffness) to 95.2% (speed). When considering all gait metrics together, only 32.5% of real-world data were well-represented by in-lab data from the same subject. Pooling in-lab gait data across multiple subjects led to greater distributional overlap between in-lab and real-world data (depth overlap 89.3-90.3%) due to the broader variability in gait seen across (as opposed to within) subjects. Stratifying real-world running to only include flat, straight segments did not meaningfully increase the overlap between in-lab and real-world running (changes of <1%). Individual gait patterns during real-world running, as characterized by consumer-grade wearable sensors, are not well-represented by the same runner's in-lab data. Researchers and clinicians should consider "borrowing" information from a pool of many runners to predict individual gait behavior when using biomechanical data to make clinical or sports performance decisions.

Tibial stress during running following a repeated calf-raise protocol.

Tibial stress fractures are a problematic injury among runners. Increased loading of the tibia has been observed following prolonged weight-bearing activity and is suggested to be the result of reduced activity of the plantar flexor muscles. The musculature that spans the tibia contributes to bending of the bone and influences the magnitude of stress on the tibia during running. Participant-specific models of the tibia can be used as a non-invasive estimate of tibial stress. This study aimed to quantify tibial stress during running using participant-specific bone geometry and to compare tibial stress before and after a protocol of repeated muscular contractions of the plantar flexor muscle group. Fourteen participants who run recreationally were included in the final analysis of the study. Synchronized force and kinematic data were collected during overground running before and after an exhaustive, weighted calf-raise protocol. Bending moments and stress at the distal third of the tibia were estimated using beam theory combined with inverse dynamics and musculoskeletal modeling. Bone geometry was obtained from magnetic resonance images. There was no difference in stress at the anterior, posterior, medial, or lateral peripheries of the tibia after the calf-raise protocol compared with before. These findings suggest that an exhaustive, repeated calf-raise protocol did not alter tibial stress during running.

Changes in tibiofemoral contact forces during running in response to in-field gait retraining.

We evaluated the efficacy of an in-field gait retraining programme using mobile biofeedback to reduce cumulative and peak tibiofemoral loads during running. Thirty runners were randomised to either a retraining group or control group. Retrainers were asked to increase their step rate by 7.5% over preferred in response to real-time feedback provided by a wrist mounted running computer for 8 routine in-field runs. An inverse dynamics driven musculoskeletal model estimated total and medial tibiofemoral joint compartment contact forces. Peak and impulse per step total tibiofemoral contact forces were immediately reduced by 7.6% and 10.6%, respectively (P < 0.001). Similarly, medial tibiofemoral compartment peak and impulse per step tibiofemoral contact forces were reduced by 8.2% and 10.6%, respectively (P < 0.001). Interestingly, no changes were found in knee adduction moment measures. Post gait retraining, reductions in medial tibiofemoral compartment peak and impulse per step tibiofemoral contact force were still present (P < 0.01). At the 1-month post-retraining follow-up, these reductions remained (P < 0.05). With these per stance reductions in tibiofemoral contact forces in mind, cumulative tibiofemoral contact forces did not change due to the estimated increase in number of steps to run 1 km.

Accuracy of the SenseWear Armband during Short Bouts of Exercise.

A goal of mobile monitoring is to approximate metabolic energy expenditure (EE) during activities of daily living and exercise. Many physical activity monitors are inaccurate with respect to estimated EE and differentiating between activities that occur over short intervals. The objective of our study was to assess the validity of the SenseWear Armband (SWA) compared to indirect calorimetry (IC) during short intervals of walking and running. Twenty young, fit participants walked (preferred speed) and ran (75%, 85%, and 95% of predicted VO2max run speeds) on a treadmill. EE estimates from IC, SWA, and prediction equations that used the SWA, speed, and heart rate were examined during each 4 min interval and across the whole protocol (Total). The level of significance was p < 0.05. The SWA overestimated EE relative to IC by 1.62 kcal·min-1 while walking and 1.05 kcal·min-1 while running at 75%. However, it underestimated EE at the 85% (0.05 kcal·min-1) and 95% (0.92 kcal·min-1) speeds, but not significantly, and overestimated total EE by 28.29 kcal. Except for walking, our results suggest that the SWA displayed a good level of agreement (ICC = 0.76 to 0.84) with IC measures. Activity-specific algorithms using SWA, speed, and heart rate improved EE estimates, based on the standard error of the estimates, but perhaps not enough to justify extra sensors. The SWA may enable EE estimation of locomotion outside the laboratory, including those with short bouts of high intensity activity, but continued development of the SWA, or devices like it, is needed to enable accurate monitoring.

Lower extremity joint contact force symmetry during walking and running, 2-7 years post-ACL reconstruction.

Premature osteoarthritis after anterior cruciate ligament reconstruction (ACLR) is common among athletes. Reduced knee contact forces after ACLR likely contribute to the multifactorial etiology of the disease. Whether this reduction is accompanied by compensatory increases in joint contact forces (JCF) at adjacent or contralateral joints is unclear. It is also unclear if compensatory effects depend on the task demands. Thus, we compared hip, knee, and ankle JCF symmetry between individuals with reconstruction and a matched control group during walking and running. Thirty participants (19 females), 2-7 years post-unilateral ACLR (mean = 47.8 months), and 30 controls matched on sex, mass, and activity level were recruited. Limb symmetry indices of peak contact forces and force impulses were calculated for each joint during walking and running, and analyzed using two-factor (group, activity) analysis of variances. Lower ACLR group peak knee JCF (p = 0.009) and knee JCF impulse (p = 0.034) during walking and running were observed. An interaction of group and activity was observed for peak hip JCF, with ACLR participants demonstrating greater involved limb peak hip JCF during running (p = 0.012). Ankle JCF and ground reaction force symmetry indices were not different between groups or across tasks. Decreased knee and increased ipsilateral peak hip JCF during running suggests that proximal adaptations exist at 2-7 years after ACLR, particularly during activities with increased task demand. Clinical significance: Knee and hip JCF asymmetry at 2-7 years after ACLR may underscore a need for clinical strategies and follow-up assessments to identify and target such outcomes.

Step width alters iliotibial band strain during running.

This study assessed the effect of step width during running on factors related to iliotibial band (ITB) syndrome. Three-dimensional (3D) kinematics and kinetics were recorded from 15 healthy recreational runners during overground running under various step width conditions (preferred and at least +/- 5% of their leg length). Strain and strain rate were estimated from a musculoskeletal model of the lower extremity. Greater ITB strain and strain rate were found in the narrower step width condition (p < 0.001, p = 0.040). ITB strain was significantly (p < 0.001) greater in the narrow condition than the preferred and wide conditions and it was greater in the preferred condition than the wide condition. ITB strain rate was significantly greater in the narrow condition than the wide condition (p = 0.020). Polynomial contrasts revealed a linear increase in both ITB strain and strain rate with decreasing step width. We conclude that relatively small decreases in step width can substantially increase ITB strain as well as strain rates. Increasing step width during running, especially in persons whose running style is characterized by a narrow step width, may be beneficial in the treatment and prevention of running-related ITB syndrome.

Ankle plantar flexor muscle performance and patient reported outcomes in people following total ankle arthroplasty.

BACKGROUND: The purpose of this study was to prospectively evaluate ankle power generation during gait in people with total ankle arthroplasty, and examine the relationships between postoperative plantar flexor strength, ankle power, and patient outcomes. METHODS: Nineteen people with end-stage ankle arthritis who received a total ankle arthroplasty and 19 healthy matched controls participated in this case-control study. Patient reported outcomes included a region specific measure of foot function and a generic measure of physical function. Gait speed was recorded with the 6-min walk test. Isokinetic plantar flexor strength was measured with an instrumented dynamometer. Motion capture and force plate data were used to calculate peak ankle power generation during walking. Paired or independent t-tests were used to compare ankle power across time and between groups, respectively. Bivariate correlations were performed to examine the interplay of postoperative strength, ankle power, gait speed, and patient reported outcomes. FINDINGS: Ankle power was not different between the preoperative and 6-month postoperative time points (d = 0.20). Six-month postoperative ankle power was less than controls (d = 1.32). Strength, ankle power, and gait speed were directly correlated in the patient group 6-months postoperatively (r or ρ ≥ 0.47). Six-month postoperative strength and ankle power were directly correlated to select 2-year patient reported outcomes (both ρ = 0.54). INTERPRETATION: Lower than normal 6-month postoperative ankle power, which was correlated to strength, gait speed, and longer-term patient reported outcomes, suggests efforts toward improving ankle plantar flexor muscle performance may improve patient outcomes.

Peak and Per-Step Tibial Bone Stress During Walking and Running in Female and Male Recreational Runners.

BACKGROUND: Athletes, especially female athletes, experience high rates of tibial bone stress injuries (BSIs). Knowledge of tibial loads during walking and running is needed to understand injury mechanisms and design safe running progression programs. PURPOSE: To examine tibial loads as a function of gait speed in male and female runners. STUDY DESIGN: Controlled laboratory study. METHODS: Kinematic and kinetic data were collected on 40 recreational runners (20 female, 20 male) during 4 instrumented gait speed conditions on a treadmill (walk, preferred run, slow run, fast run). Musculoskeletal modeling, using participant-specific magnetic resonance imaging and motion data, was used to estimate tibial stress. Peak tibial stress and stress-time impulse were analyzed using 2-factor multivariate analyses of variance (speed*sex) and post hoc comparisons (α = .05). Bone geometry and tibial forces and moments were examined. RESULTS: Peak compression was influenced by speed (P < .001); increasing speed generally increased tibial compression in both sexes. Women displayed greater increases in peak tension (P = .001) and shear (P < .001) than men when transitioning from walking to running. Further, women displayed greater peak tibial stress overall (P < .001). Compressive and tensile stress-time impulse varied by speed (P < .001) and sex (P = .006); impulse was lower during running than walking and greater in women. A shear stress-time impulse interaction (P < .001) indicated that women displayed greater impulse relative to men when changing from a walk to a run. Compared with men, women displayed smaller tibiae (P < .001) and disproportionately lower tibial forces (P≤ .001-.035). CONCLUSION: Peak tibial stress increased with gait speed, with a 2-fold increase in running relative to walking. Women displayed greater tibial stress than men and greater increases in stress when shifting from walking to running. Sex differences appear to be the result of smaller bone geometry in women and tibial forces that were not proportionately lower, given the womens' smaller stature and lower mass relative to men. CLINICAL RELEVANCE: These results may inform interventions to regulate running-related training loads and highlight a need to increase bone strength in women. Lower relative bone strength in women may contribute to a sex bias in tibial BSIs, and female runners may benefit from a slower progression when initiating a running program.

The effects of age and type of carrying task on lower extremity kinematics.

The purpose of this study was to determine the effects of age, load amount and load symmetry on lower extremity kinematics during carrying tasks. Forty-two participants in four age groups (8-10 years, 12-14 years, 15-17 years and adults) carried loads of 0%, 10% and 20% body weight (BW) in large or small buckets unilaterally and bilaterally. Reflective markers were tracked to determine total joint range of motion and maximum joint angles during the stance phase of walking. Maximum hip extension, hip adduction and hip internal rotation angles were significantly greater for each of the child/adolescent age groups as compared with adults. In addition, maximum hip internal rotation angles significantly increased when carrying a 20% BW load. The observation that the 8-10-year-old age group carried the lightest absolute loads and still displayed the highest maximum hip internal rotation angles suggests a particular necessity in setting carrying guidelines for the youngest children. STATEMENT OF RELEVANCE: Bucket-carrying tasks were analysed as a function of age group, load amount and load symmetry. Hip joint rotations significantly increased when carrying 20% BW loads and in children as compared to adults, which suggests a particular necessity in setting carrying guidelines for the youngest age group (8-10 year olds).

Effects of custom and semi-custom foot orthotics on second metatarsal bone strain during dynamic gait simulation.

BACKGROUND: Stress fractures of the lower extremity are common in military and running populations. Research on the effectiveness of orthotics in modifying bone strain is limited. Our hypothesis was that custom and semi-custom foot orthotics would equally decrease bone strain of the second metatarsal. MATERIALS AND METHODS: Eight cadaver specimens were cast for two types of orthotics, a custom and semi-custom device, using neutral plaster casts. Cadaver specimens, mounted to a dynamic gait simulator, walked over a force platform while force and bone strain data were collected. Peak bone strains, strain rates and tendon forces during the stance phase for each condition were analyzed using repeated measures analysis of variance and effect sizes. RESULTS: Condition effects were present for tension strain, shear strain, compression rate and shear rate. Specifically, custom orthotics significantly decreased the aforementioned bone strains and strain rates (< or = 0.01 for all) and the semi-custom orthotic decreased tension strains and shear strain rates (p = 0.05 and 0.03, respectively). The effect of custom and semi-custom devices only differed significantly for compression and shear strain (p= 0.04 and 0.02, respectively) with custom orthotics having a greater effect. CONCLUSION: Both custom and semi-custom orthotics modified the second metatarsal bone strain and strain rate. The use of custom orthotics during simulated walking decreased second metatarsal bone strains and strain rates more effectively than semi-custom orthotics. CLINICAL RELEVANCE: Orthotics may minimize the strain magnitudes and rates of the second metatarsal in walking and therefore are a feasible treatment option for the treatment and prevention of stress injury to the second metatarsal.

Effects of Load Carriage and Step Length Manipulation on Achilles Tendon and Knee Loads.

INTRODUCTION: Longer steps with load carriage is common in shorter Soldiers when matching pace with taller Soldiers whereas shorter steps are hypothesized to reduce risk of injury with load carriage. The effects of load carriage with and without step length manipulation on loading patterns of three commonly injured structures were determined: Achilles tendon, patellofemoral joint (PFJ) and medial tibiofemoral joint (mTFJ). MATERIALS AND METHODS: ROTC Cadets (n = 16; 20.1 years ± 2.5) walked with and without load carriage (20-kg). Cadets then altered preferred step lengths ±7.5% with load carriage. Achilles tendon, PFJ and mTFJ loads were estimated via musculoskeletal modeling. RESULTS: Large increases in peak Achilles tendon load (p < 0.001, d = 1.93), Achilles tendon impulse per 1-km (p < 0.001, d = 0.91), peak mTFJ load (p < 0.001, d = 1.33), and mTFJ impulse per 1-km (p < 0.001, d = 1.49) were noted with load carriage while moderate increases were observed for the PFJ (peak: p < 0.001, d = 0.69; impulse per 1-km: p < 0.001, d = 0.69). Shortened steps with load carriage only reduced peak Achilles tendon load (p < 0.001, d = -0.44) but did not reduce Achilles impulse per km due to the resulting extra steps and also did not reduce peak or cumulative PFJ and mTFJ loads (p > 0.05). Longer steps with load carriage increased PFJ loads the most (p < 0.001, d = 0.68-0.75) with moderate increases in mTFJ forces (p < 0.001, d = 0.48-0.63) with no changes in Achilles tendon loads (p = 0.11-0.20). CONCLUSION: A preferred step length is the safest strategy when walking with load carriage. Taking a shorter step is not an effective strategy to reduce loading on the Achilles tendon, PFJ, and mTFJ.

Walking velocity and step length adjustments affect knee joint contact forces in healthy weight and obese adults.

Knee osteoarthritis is a major public health problem and adults with obesity are particularly at risk. One approach to alleviating this problem is to reduce the mechanical load at the joint during daily activity. Adjusting temporospatial parameters of walking could mitigate cumulative knee joint mechanical loads. The purpose of this study was to determine how adjustments to velocity and step length affects knee joint loading in healthy weight adults and adults with obesity. We collected three-dimensional gait analysis data on 10 adults with a normal body mass index and 10 adults with obesity during over ground walking in nine different conditions. In addition to preferred velocity and step length, we also conducted combinations of 15% increased and decreased velocity and step length. Peak tibiofemoral joint impulse and knee adduction angular impulse were reduced in the decreased step length conditions in both healthy weight adults (main effect) and those with obesity (interaction effect). Peak knee joint adduction moment was also reduced with decreased step length, and with decreased velocity in both groups. We conclude from these results that adopting shorter step lengths during daily activity and when walking for exercise can reduce mechanical stimuli associated with articular cartilage degenerative processes in adults with and without obesity. Thus, walking with reduced step length may benefit adults at risk for disability due to knee osteoarthritis. Clinical Significance: Adopting a shorter step length during daily walking activity may reduce knee joint loading and thus benefit those at risk for knee cartilage degeneration. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:2679-2686, 2018.

Lower extremity mechanics of iliotibial band syndrome during an exhaustive run.

Injury patterns in distance running may be related to kinematic adjustments induced by fatigue. The goal was to measure changes in lower extremity mechanics during an exhaustive run in individuals with and without a history of iliotibial band syndrome (ITBS). Sixteen recreational runners ran to voluntary exhaustion on a treadmill at a self-selected pace. Eight runners had a history of ITBS. Twenty-three reflective marker positions were recorded by an eight-camera 120 Hz motion capture system. Joint angles during stance phase were exported to a musculoskeletal model (SIMM) with the iliotibial band (ITB) modeled as a passive structure to estimate strain in the ITB. For ITBS runners, at the end of the run: (1) knee flexion at heel-strike was higher than control (20.6 degrees versus 15.3 degrees, p=0.01); (2) the number of knees with predicted ITB impingment upon the lateral femoral epicondyle increased from 6 to 11. Strain in the ITB was higher in the ITBS runners throughout all of stance. Maximum foot adduction in the ITBS runners was higher versus control at the start of the run (p=0.003). Maximum foot inversion (p=0.03) and maximum knee internal rotation velocity (p=0.02) were higher versus control at the end of the run. In conclusion, ITB mechanics appear to be related to changes in knee flexion at heel-strike and internal rotation of the leg. These observations may suggest kinematic discriminators for clinical assessment.

Sex-specific kinetic and kinematic indicators of medial tibiofemoral force during walking and running.

BACKGROUND: Our aims were to (1) Evaluate sex-specific contributions of peak knee flexion moment (pKFM) and peak knee adduction moment (pKAM) in medial tibiofemoral joint (TFJ) force during walking and running; (2) identify kinematic variables to estimate peak medial TFJ force. METHODS: Eighty-seven runners participated (36 females, 51 males; age=23.0±3.8years (1 standard deviation)). Kinematics and kinetics data were collected during treadmill walking (1.3m/s) and running (3.0±0.4m/s). Peak medial TFJ contact force was estimated using a musculoskeletal model. Linear regression analyses were used to assess the contribution of pKFM, pKAM and kinematic indicators to estimated joint forces. RESULTS: During walking and running, pKAM and pKFM accounted for 74.9% and 64.5% of peak medial TFJ force variance (P<0.001), respectively. Similar pKAM contribution was found between males and females during walking (51.8% vs. 47.9%), as opposed to running (50.4% vs. 26.8%). Kinematic indicators during walking were peak knee flexion and adduction angles, regardless of sex. During running, indicators were ankle dorsiflexion at foot strike and center of mass (COM) vertical displacement in females (R2=0.364, P=0.012), and peak knee abduction angle and step length in males (R2=0.508, P=0.019). CONCLUSION: We conclude from these results that pKAM and pKFM make significant but potentially sex-specific contributions to peak medial TFJ force during walking and running. Clinically, peak medial TFJ force during walking can be estimated using peak knee flexion and adduction angles in both sexes. During running, ankle dorsiflexion at foot strike and COM oscillation are best indicators among females, while knee abduction and step length are best among males.

Sex differences in running mechanics and patellofemoral joint kinetics following an exhaustive run.

Patellofemoral joint pain (PFP) is a common running-related injury that is more prevalent in females and thought to be associated with altered running mechanics. Changes in running mechanics have been observed following an exhaustive run but have not been analyzed relative to the sex bias for PFP. The purpose of this study was to test if females demonstrate unique changes in running mechanics associated with PFP following an exhaustive run. For this study, 18 females and 17 males ran to volitional exhaustion. Peak PFJ contact force and stress, PFJ contact force and stress loading rates, hip adduction excursion, and hip and knee joint frontal plane angular impulse were analyzed between females and males using separate 2 factor ANOVAs (2 (male/female)×2 (before/after exhaustion)). We observed similar changes in running mechanics among males and females over the course of the exhaustive run. Specifically, greater peak PFJ contact force loading rate (5%, P=.01), PFJ stress loading rate (5%, P<.01), hip adduction excursion (1.3°, P<.01), hip abduction angular impulse (4%, P<.01), knee abduction angular impulse (5%, P=.03), average vertical ground reaction force loading rate (10%, P<.01) and step length (2.1cm, P=.001) were observed during exhausted running. These small changes in suspected PFP pathomechanical factors may increase a runner׳s propensity for PFP. However, unique changes in female running mechanics due to exhaustion do not appear to contribute to the sex bias for PFP.

Bone stress in runners with tibial stress fracture.

BACKGROUND: Combinations of smaller bone geometry and greater applied loads may contribute to tibial stress fracture. We examined tibial bone stress, accounting for geometry and applied loads, in runners with stress fracture. METHODS: 23 runners with a history of tibial stress fracture & 23 matched controls ran over a force platform while 3-D kinematic and kinetic data were collected. An elliptical model of the distal 1/3 tibia cross section was used to estimate stress at 4 locations (anterior, posterior, medial and lateral). Inner and outer radii for the model were obtained from 2 planar x-ray images. Bone stress differences were assessed using two-factor ANOVA (α=0.05). Key contributors to observed stress differences between groups were examined using stepwise regression. FINDINGS: Runners with tibial stress fracture experienced greater anterior tension and posterior compression at the distal tibia. Location, but not group, differences in shear stress were observed. Stepwise regression revealed that anterior-posterior outer diameter of the tibia and the sagittal plane bending moment explained >80% of the variance in anterior and posterior bone stress. INTERPRETATION: Runners with tibial stress fracture displayed greater stress anteriorly and posteriorly at the distal tibia. Elevated tibial stress was associated with smaller bone geometry and greater bending moments about the medial-lateral axis of the tibia. Future research needs to identify key running mechanics associated with the sagittal plane bending moment at the distal tibia as well as to identify ways to improve bone geometry in runners in order to better guide preventative and rehabilitative efforts.

Effect of step width manipulation on tibial stress during running.

Narrow step width has been linked to variables associated with tibial stress fracture. The purpose of this study was to evaluate the effect of step width on bone stresses using a standardized model of the tibia. 15 runners ran at their preferred 5k running velocity in three running conditions, preferred step width (PSW) and PSW±5% of leg length. 10 successful trials of force and 3-D motion data were collected. A combination of inverse dynamics, musculoskeletal modeling and beam theory was used to estimate stresses applied to the tibia using subject-specific anthropometrics and motion data. The tibia was modeled as a hollow ellipse. Multivariate analysis revealed that tibial stresses at the distal 1/3 of the tibia differed with step width manipulation (p=0.002). Compression on the posterior and medial aspect of the tibia was inversely related to step width such that as step width increased, compression on the surface of tibia decreased (linear trend p=0.036 and 0.003). Similarly, tension on the anterior surface of the tibia decreased as step width increased (linear trend p=0.029). Widening step width linearly reduced shear stress at all 4 sites (p<0.001 for all). The data from this study suggests that stresses experienced by the tibia during running were influenced by step width when using a standardized model of the tibia. Wider step widths were generally associated with reduced loading of the tibia and may benefit runners at risk of or experiencing stress injury at the tibia, especially if they present with a crossover running style.

Effects of step length on patellofemoral joint stress in female runners with and without patellofemoral pain.

BACKGROUND: Patellofemoral pain is common among runners and is frequently attributed to increased patellofemoral joint stress. The purpose of our study was to examine the effects of changing step length during running on patellofemoral joint stress per step and stress per mile in females with and without patellofemoral pain. METHODS: Ten female runners with patellofemoral pain and 13 healthy female runners performed running trials at 3.7m/s in three conditions: preferred step length, at least +10% step length, and at least -10% step length. Knee flexion angles and internal knee extension moments served as inputs for a biomechanical model to estimate patellofemoral joint stress per step. We also estimated total patellofemoral joint stress per mile based on the number of steps necessary to run a mile during each condition. FINDINGS: Patellofemoral joint stress per step increased 31% in the long step length condition (P<.001) and decreased 22.2% in the short step length condition (P<.001). Despite the inverse relationship between step length and number of steps required to run a mile, patellofemoral joint stress per mile increased 14% in the long step length condition (P<.001) and decreased 7.5% in the short step length condition (P<.001). INTERPRETATION: These results suggest a direct relationship between step length and patellofemoral joint loads. Total stress per mile experienced at the patellofemoral joint decreased with a short step length despite the greater number of steps necessary to cover this distance. These findings may have relevance with respect to both prevention and treatment of patellofemoral joint pain.

Running injury and stride time variability over a prolonged run.

Locomotor variability is inherent to movement and, in healthy systems, contains a predictable structure. In this study, detrended fluctuation analysis (DFA) was used to quantify the structure of variability in locomotion. Using DFA, long-range correlations (α) are calculated in over ground running and the influence of injury and fatigue on α is examined. An accelerometer was mounted to the tibia of 18 runners (9 with a history of injury) to quantify stride time. Participants ran at their preferred 5k pace±5% on an indoor track to fatigue. The complete time series data were divided into three consecutive intervals (beginning, middle, and end). Mean, standard deviation (SD), coefficient of variation (CV) and α of stride times were calculated for each interval. Averages for all variables were calculated per group for statistical analysis. No significant interval, group or interval×group effects were found for mean, SD or CV of stride time. A significant linear trend in α for interval occurred with a reduction in α over the course of the run (p=0.01) indicating that over the run, stride times of runners became more unpredictable. This was likely due to movement errors associated with fatigue necessitating frequent corrections. The injured group exhibited lower α (M=0.79, CI(95)=0.70, 0.88) than the non-injured group (p=0.01) (M=0.96, CI(95)=0.88, 1.05); a reduction hypothesized to be associated with altered complexity. Overall, these findings suggest injury and fatigue influence neuromuscular output during running.

The use of external transducers for estimating bone strain at the distal tibia during impact activity.

Noninvasive methods for monitoring the in vivo loading environment of human bone are needed to determine osteogenic loading patterns that reduce the potential for bone injury. The purpose of this study was to determine whether the vertical ground reaction impact force (impact force) and leg acceleration could be used to estimate internal bone strain at the distal tibia during impact activity. Impact loading was delivered to the heels of human-cadaveric lower extremities. The effects of impact mass and contact velocity on peak bone strain, impact force, leg acceleration, and computed impact force (leg acceleration *impact mass) were investigated. Regression analysis was used to predict bone strain from six different models. Apart from leg acceleration, all variables responded to impact loading similarly. Increasing impact mass resulted in increased bone strain, impact force, and computed impact force, but decreased leg acceleration. The best models for bone strain prediction included impact force and tibial cross-sectional area (R(2)=0.94), computed impact force and tibial cross-sectional area (R(2)=0.84), and leg acceleration and tibial cross-sectional area (R(2)=0.73). Results demonstrate that when attempting to estimate bone strain from external transducers some measure of bone strength must be considered. Although it is not recommended that the prediction equations developed in this study be used to predict bone strain in vivo, the strong relationship between bone strain, impact force, and computed impact force suggested that force platforms and leg accelerometers can be used for a surrogate measure of bone strain.