Human locomotion is a topic that has been studied for many years in biomechanics. To perform athletic tasks or everyday tasks, balance and symmetry is needed. Symmetry is the perfect balance and correspondence of the body or parts of the body. This concept has often been used to evaluate the normality of movements. Limb symmetry, specifically, is the equal actions of the lower limbs during movement. This is needed to perform tasks safely and efficiently without injury. Gait and movement symmetry has been used to predict lower limb injury risk for many populations and improve performance for athletes. It has also been used in assessment for rehabilitation processes and return to sport processes following injury or surgery. For many years, healthy gait was considered to be symmetrical for simplification purposes. However, many studies have contradicted that conclusion showing that even for has asymmetrical patterns. Deficits in symmetry can reduce quality of life for some individuals and can have detrimental health effects. Many measures have been used to assess symmetry in various tasks that have important implications on gait patterns. Another component of gait and movement that affects performance and injury risk is limb stiffness. Limb stiffness is the body's resistance to deformation when moments and forces are applied to it. The body has been shown to be modeled as a spring mass system that can restore and reuse energy. This is associated with the stretch shortening cycle during cyclic movements, such as running and walking. Limb stiffness is also associated with musculoskeletal loading that impacts performance and injury. Therefore, optimizing limb stiffness is important to improve utilization of elastic energy for athletic performance and reduce injuries associated with high and low limb stiffness values. Imbalances in limb stiffness have been shown to increase injury risk during walking and other tasks. Studying these imbalances using symmetry indices could give insight into the injury risk associated with this metric. In addition, limb stiffness in humans has been shown to change with the type of contact surface. This is associated with compensation methods used by humans when contacting different surfaces. Studying the relationship between limb stiffness symmetry and different surfaces during walking is important to observe how humans adjust and how it impacts injury risk. The purpose of this research was to assess the impact that surface stiffness has on limb stiffness symmetry during walking in healthy adults. To assess limb stiffness differences when transitioning to different surface stiffnesses anteriorly and posteriorly, the Normalized Symmetry Index (NSI) was determined for the two transition conditions and the control. The results showed that limb stiffness NSI was significant between the conditions (p=0.012). More specifically, a difference was seen between the stiff to compliant transition and the control (p=0.020) and the compliant to stiff transition and the control (p=0.032). These results show that humans do compensate when transitioning onto different surfaces. This is essential for understanding how humans adjust during real world walking and what patterns are used to maintain stability. To assess limb stiffness symmetry, when surface stiffness is different between limbs, the limb stiffness NSI was compared between two conditions. This included the side-to-side stiffness difference condition and the control condition. The results revealed that surface stiffness was not significant between conditions (p=0.244). Based on these results, limb stiffness symmetry is not significantly impacted when the surface stiffness is different between limbs. This contradicts prior studies that observed changes limb stiffness and symmetry depending on the surface stiffness. This may be due to overcompensation or the ability of the healthy adult population to quickly adjust to the surface stiffness changes before the measurements were taken. Simulating uneven surfaces is important to understand how humans compensate to maintain stability on surfaces in real world walking and for imbalances due to disorders. Further research is needed to study the changes in limb stiffness symmetry on different surfaces during walking to improve injury prevention methods. / Master of Science / Humans perform many daily tasks and athletic tasks that have been observed in human movement analysis. To perform these tasks safely and efficiently, many factors must be considered. One of the important factors in performing tasks is symmetry. Symmetry is the perfect balance between parts of the body, such as the lower limbs during walking or gait. Gait in healthy adults was considered to be symmetrical for simplification purposes. However, studies have revealed that gait asymmetry is present in the healthy adult population during walking and other movements. Gait symmetry has been used to assess normality of gait patterns in healthy individuals and in clinical populations. Asymmetrical gait patterns can lead to injury and have detrimental effects on health. Therefore, limb symmetry has been an important metric to assess lower limb injury risk and improving injury prevention methods to correct asymmetrical patterns in healthy adults and other populations. Another aspect of human movement that impacts injury is limb stiffness. Limb stiffness is the body's resistance to deformation under applied forces. High limb stiffness values have been associated with bony injuries due to increased loading. However, low stiffness values have been associated with soft tissue injuries. Therefore, regulating limb stiffness is important to reduce injuries in the long term. The type of contact surface during walking and other tasks has been shown to change limb stiffness values. Humans often encounter changes to surfaces when walking. For example, hikers who encounter uneven terrain or everyday walking on uneven pavement. Uneven surfaces have been shown to require more energy and work to move forwards during walking. Therefore, simulating uneven surfaces in the real world is important to understand how humans compensate on different surfaces. This could be important for understanding how limb stiffness imbalances on different surfaces affect injury. To quantify these imbalances, the metric of limb stiffness symmetry will be used. Limb stiffness imbalances due to surface stiffness are essential to assess how humans adapt to instability during real world walking. Therefore, this study aims to determine how humans adjust when transitioning to different surface stiffnesses and when surface stiffness is different between limbs. To determine how humans adjust when transitioning to different surfaces of different stiffnesses, the limb stiffness symmetry was calculated using the Normalized Symmetry Index (NSI). This was calculated for three different surface stiffness conditions, consisting of a stiff to compliant transition, a compliant to stiff transition, and the control condition. The results showed that there was a significant difference between the NSI values of the three conditions. However, there was no difference between the two transition conditions. This indicated that there was no difference between the transition order. Based on the results, limb stiffness symmetry does change when transitioning to different surface stiffness conditions. This agrees with previous literature that suggests that surface stiffness has an impact on limb stiffness. This information is beneficial to understand the patterns humans use to compensate to maintain stability. To determine how limb stiffness symmetry is impacted when surface stiffness is different between limbs, the limb stiffness NSI was calculated for two surface conditions. This included the side-to-side condition and the control condition. The results showed that there was no statistical difference between the limb stiffness NSI values of the two conditions. This shows that limb stiffness symmetry doesn't change when the surface stiffness is different between limbs, which disagrees with previous literature. Overall, this information is important to understand how humans compensate when transitioning on different surfaces or walking on uneven surfaces. This is important to understand how stability is maintained despite imbalances for improvement of injury prevention methods.

Anterior cruciate ligament (ACL) injuries are common orthopaedic injuries among athletes who participate in sports that involve cutting and changing directions. Many of these adolescent athletes intend to return to sports (RTS), and therefore undergo ACL reconstruction (ACLR). These athletes exhibit unfavorable landing biomechanics from muscle atrophy and asymmetrical neuromuscular control post-ACLR, putting them at a higher risk of re-injury. Thus, rehabilitation following ACLR is important to improve kinetic and kinematic outcomes and reduce re-injury risk. Biofeedback during rehabilitation is thought to be one way to potentially restore neuromuscular control deficits of athletes recovering from ACLR. Therefore, understanding the effectiveness of a biofeedback intervention on factors associated with re-injury among post-ACLR patients is essential in successful RTS. The purpose of this study is to analyze the effect of a 6-week biofeedback intervention on eccentric knee joint power (ECCKP), limb stiffness, and limb stiffness symmetry (using normalized symmetry index, NSI), in addition to secondary lower extremity outcomes that are associated with these metrics, during landing among patients following ACLR. This study used data collected from an ACL-Biofeedback Trial (ClinicalTrials.gov: AR069865) where participants were randomized into a biofeedback (BF) or control group (C). The BF group received visual and tactile feedback during a series of controlled squats while the C group participated in several online and in-person educational sessions. Participants completed 10 stop-jump tasks before (pre), after (post), and 6 weeks after (ret) the intervention. Kinetic, kinematic, and ground reaction forces (GRF) were collected from embedded force plates and 3D motion capture. Partaking in a biofeedback intervention did not improve ECCKP, limb stiffness, or limb stiffness NSI compared to controls. A group-by-time interaction was found for hip excursion (p=0.035), and a main effect of time was found for ECCKP, with this variable increasing by 18.5% from pre to ret (p=0.001). In addition, when considering surgical versus non-surgical limbs, this cohort exhibited interlimb asymmetries in stiffness, peak resultant GRF (rGRF), and time to reach peak rGRF (p<0.009). Further, a group-by-limb interaction (p=0.005) and a 7.1% reduction in peak rGRF were found from post to ret (p=0.02). Participants in this study also exhibited limb stiffness asymmetry greater than 10%, which supports existing literature that observed interlimb asymmetries in athletes following ACLR around the typical RTS time (9-12 months post-ACLR). The results from this analysis demonstrated that the current biofeedback intervention was inadequate in improving ECCKP, limb stiffness, and limb stiffness NSI, but additional biofeedback studies with larger sample sizes that investigate task dependencies are needed to better understand the effectiveness of biofeedback interventions. / Master of Science / Anterior cruciate ligament (ACL) injuries are common orthopaedic injuries among athletes who participate in sports that involve cutting and changing directions. Many of these adolescent athletes intend to return to their pre-injury level, therefore undergo a surgical procedure called ACL reconstruction (ACLR). However, following this procedure, athletes display unsafe and stiff landing patterns due to muscle weakness and asymmetrical neuromuscular, or mind-body, control post-ACLR, which increases their risk of re-injury once they return to sport (RTS) following recovery. Rehabilitation for patients following ACLR is of the utmost importance in improving unsafe movement patterns to reduce the risk of re-injury. Biofeedback training refers to receiving external signals that can be processed and transferred to the muscles in the body. This technique aims to restore the neuromuscular deficits of athletes following ACLR and could potentially be helpful during ACLR rehabilitation. Therefore, understanding the effectiveness of a biofeedback intervention on outcomes associated with an increased risk of re-injury in patients following ACLR is important to safely RTS. The purpose of this study is to determine the effect of a 6-week biofeedback intervention on the ability of the knee to absorb impact forces (quantified as eccentric knee joint power, ECCKP), limb stiffness, and limb stiffness symmetry (measured with normalized symmetry index, NSI), along with secondary outcomes related to these variables, among patients following ACLR. This study used data collected from an ACL-Biofeedback Trial (ClinicalTrials.gov: AR069865) where participants were randomized into a biofeedback (BF) or control group (C). The BF group received visual and resisted feedback during a series of controlled squats while the C group participated in several online and in-person educational sessions. Participants completed 10 stop-jump tasks before and after the intervention, and biomechanical data was obtained. The biofeedback intervention did not result in an improved ability for the knee to absorb impact from landing, and it was not able to decrease limb stiffness or limb stiffness asymmetry. It was able to improve hip excursion, which allows for a favorable, less upright posture when landing. ECCKP improved for both groups, indicating that the biofeedback did not add extra benefit to the participant's rehabilitation outside of the study. Asymmetries were observed between the surgical and non-surgical limbs in limb stiffness, peak GRF, and the time it takes to reach this peak GRF. This sample exhibited limb stiffness asymmetry greater than the recommended 10% threshold, raising concern for when these athletes RTS. The results from this analysis demonstrated that the current biofeedback intervention was inadequate in improving ECCKP, limb stiffness, and limb stiffness NSI, but biofeedback in ACLR rehabilitation can still be efficacious in improving hip biomechanics and overall neuromuscular control but may be task-dependent and call for a larger sample size.

Yes / Background: Individuals with lower-limb amputation can use running specific prostheses (RSP) that store and then return elastic energy during stance. However, it is unclear whether varying the stiffness category of the same RSP affects spring-mass behaviour during self-selected, submaximal speed running in individuals with unilateral transtibial amputation. Research question: The current study investigates how varying RSP stiffness affects limb stiffness, running performance, and associated joint kinetics in individuals with a unilateral transtibial amputation. Methods: Kinematic and ground reaction force data were collected from eight males with unilateral transtibial amputation who ran at self-selected submaximal speeds along a 15 m runway in three RSP stiffness conditions; recommended habitual stiffness (HAB) and, following 10-minutes of familiarisation, stiffness categories above (+1) and below (-1) the HAB. Stance-phase centre of mass velocity, contact time, limb stiffness’ and joint/RSP work were computed for each limb across RSP stiffness conditions. Results: With increased RSP stiffness, prosthetic limb stiffness increased, whilst intact limb stiffness decreased slightly (p

Background: Anterior cruciate ligament (ACL) injuries have become commonplace among female athletes in today’s society. With more than 70% of injuries resulting from noncontact mechanisms such as jump landing, the relationship between fatigue and altered movements patterns has become an important topic of research. Purpose: The main purpose of this study was to investigate the influence of residual fatigue on lower extremity kinematics and vertical leg stiffness at landing as experienced by female athletes. Method: The participants in this study were 12 NCAA female intercollegiate soccer players. Participants completed five single-leg drop jumps on their dominant leg every day for 4 days. The first day was completed without intervention to obtain pre-fatigue data and drop jumps on days two through four were completed after a fatigue protocol. Results: A repeated measures MANOVA did not reveal significant differences in post-fatigue peak knee flexion angle, vertical ground reaction forces, or vertical leg stiffness. Despite lack of statistical significance, vertical leg stiffness was increased during post-fatigue testing when compared to pre-fatigue values. Implications: The increased vertical leg stiffness may indicate altered landing techniques in post-fatigue states. If fatigue results in compromised movement patterns, it may explain the increased number of ACL injuries during the end of soccer matches. Suggestions for Future Research: Future research with a larger sample size should include post-fatigue dominant and nondominant leg comparison due to previous conflicting findings regarding which limb is most often injured. Future researchers should also quantify the magnitude of fatigue induced by the fatiguing protocol to document the strength of the independent variable. / text

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Non-contact injuries can occur when athletes use poor or inconsistent mechanics during typical sport-related movements like landing from a jump. Anterior cruciate ligament (ACL) injuries are especially devastating, and certain populations like female athletes and athletes with a previous ACL reconstruction (ACLR) are at greater risk of suffering an ACL injury, with altered biomechanical strategies being one proposed reason. Asymmetric landings where one limb experiences greater landing force can decrease joint stability and place the overloaded limb at greater risk for ACL injury. Additionally, a stiff landing, characterized by increased ground reaction force (GRF), extended joints at initial ground contact, and decreased joint flexion throughout the landing, has been proposed to increase ACL injury risk. While load distribution between limbs is a common landing assessment to determine injury risk, it is unclear what role limb stiffness plays in the likelihood of experiencing an ACL injury. Limb stiffness is simply the deformation of the limb in response to the downward force applied to the lower limb during ground contact, which can be approximated using GRF. Limb stiffness has been commonly used to assess performance in running, hopping, and jumping, however, its relationship with injury risk during landings is relatively unexplored. Past research has revealed that the ACL experiences peak strain prior to initial ground contact when the knee is at or near full extension. Additionally, expert video analyses have determined that ACL injuries most likely occur within 50 milliseconds of ground contact. It is possible that limb stiffness and limb stiffness asymmetry can be used during the early impact phase of landings to reveal ACLR- and sex-specific landing mechanics differences when the ACL appears to be most vulnerable. Moreover, game-like, sport-specific landing tasks with a greater horizontal component that load the ACL and those that divert attention away from landing strategies may uncover differences that do not appear in standard, controlled laboratory tasks. The overall goal of this project was to use limb stiffness, limb stiffness asymmetry, and related measures to analyze the early landing phase mechanics of groups at greater risk for ACL injury during game-like, sport-specific landings. First, in an ACLR cohort, greater knee power and knee work asymmetries were found when compared to healthy recreational athletes, supporting previous literature that found that athletes with an ACLR land unevenly by offloading their surgical limb. However, limb stiffness asymmetry was not different between groups, implying that the groups may have modulated limb stiffness differently between limbs. Second, minimal sex-by-task interactions were determined for landings that varied by horizontal approach prior to initial ground contact. Significant differences were found for most measures across tasks overall, however, male and female athletes displayed similar landing mechanics, indicating that expected sex-specific differences may not exist during the immediate landing phase when ACL injuries are thought to occur. Last a landing task that mimicked a ball in mid-air and diverted attention away from landing mechanics produced a sex-by-task interaction for peak impact force but no other measure. When comparing each sex-task pairing, a trend for greater peak impact force by female athletes during the distracted landing (p=0.098) was found which may indicate that future tasks with additional external focuses or another game-like component will reveal anticipated sex-specific differences. Increased time between limbs for initial ground contact for female athletes also revealed that a time-synchronized assessment of between-limb coordination may be beneficial for future research. / Doctor of Philosophy / Non-contact injuries can occur when athletes use poor or inconsistent mechanics during typical sport-related movements like landing from a jump. Anterior cruciate ligament (ACL) injuries are especially tough, and certain populations like female athletes and athletes with a previous ACL reconstruction surgery (ACLR) are at greater risk of suffering an ACL injury, with different movement techniques being one proposed reason. Uneven landings where one limb has greater landing forces can decrease joint posture and place the overloaded limb at greater risk for ACL injury. Additionally, a stiff landing, defined by larger ground reaction force (GRF), extended joints at initial ground contact, and decreased joint flexion throughout the landing, is thought to increase ACL injury risk. While landing force distribution between limbs is a common way of evaluating landings to determine injury risk, it is unclear what role limb stiffness plays in the likelihood of experiencing an ACL injury. Limb stiffness is simply the deformation of the limb in response to the downward force applied on the lower limb during ground contact, which can be estimated using GRF. Limb stiffness has been commonly used to assess performance in running, hopping, and jumping, however, its relationship with injury risk during landings is pretty limited. Past research has revealed that the ACL experiences maximum stretch prior to initial ground contact when the knee is or is almost completely straight. Additionally, expert video investigations have determined that ACL injuries most likely occur within 50 milliseconds of ground contact. It is possible that limb stiffness and limb stiffness asymmetry can be used during the early impact phase of landings to reveal sex- and ACLR-specific landing mechanics differences when the ACL appears to be most in danger. Additionally, game-like, sport-specific landing tasks with a greater horizontal element that load the ACL and those that redirect attention away from landing strategies may show differences that do not appear in basic laboratory tasks. The overall goal of this project was to use limb stiffness, limb stiffness asymmetry, and related measures to examine the early landing phase techniques of groups at greater risk for ACL injury during game-like, sport-specific landings. First, in a group of athletes with a previous ACLR, greater knee storage differences between limbs were found when compared to healthy recreational athletes, supporting previous research studies that found that athletes with an ACLR land unevenly by offloading their surgical limb. However, limb stiffness asymmetry was not different between groups, implying that the groups may have regulated limb stiffness differently between limbs. Second, only a couple measures were significantly affected by the combined effect of sex and task during landings that were different due to their horizontal element. Significant differences were found for most measures across tasks overall, however, male and female athletes had similar landing techniques, showing that the expected differences between sexes may not happen very early in the landing phase when ACL injuries are thought to happen. Last, a landing task that imitated a ball in mid-air and redirected attention away from landing mechanics produced a larger sex-specific difference for peak impact force compared to a basic landing task. When comparing each sex-task pairing, a trend for greater peak impact force by female athletes during the distracted landing (p=0.098) was found which may show that future tasks with additional distractions or another game-like element will reveal expected differences between sexes. Increased time between limbs for initial ground contact for female athletes also revealed that looking at the coordination of both limbs on the same timescale may be useful for future research.