Susanne W. Lipfert

Effect of gender and defensive opponent on the biomechanics of sidestep cutting

PURPOSE Anterior cruciate ligament (ACL) injuries often occur in women during cutting maneuvers to evade a defensive player. Gender differences in knee kinematics have been observed, but it is not known to what extent these are linked to abnormal neuromuscular control elsewhere in the kinetic chain. Responses to defense players, which may be gender-dependent, have not been included in previous studies. This study determined the effects of gender and defense player on entire lower extremity biomechanics during sidestepping. METHODS Eight male and eight female subjects performed sidestep cuts with and without a static defensive opponent while 3D motion and ground reaction force data were recorded. Peak values of eight selected motion and force variables were, as well as their between-trial variabilities, submitted to a two-way (defense x gender) ANOVA. A Bonferroni-corrected alpha level of 0.003 denoted statistical significance. RESULTS Females had less hip and knee flexion, hip and knee internal rotation, and hip abduction. Females had higher knee valgus and foot pronation angles, and increased variability in knee valgus and internal rotation. Increased medial ground reaction forces and flexion and abduction in the hip and knee occurred with the defensive player for both genders. CONCLUSIONS A simulated defense player causes increased lower limb movements and forces, and should be a useful addition to laboratory protocols for sidestepping. Gender differences in the joint kinematics suggest that increased knee valgus may contribute to ACL injury risk in women, and that the hip and ankle may play an important role in controlling knee valgus during sidestepping. Consideration of the entire lower extremity contributes to an understanding of injury mechanisms and may lead to better training programs for injury prevention.

Upright human gait did not provide a major mechanical challenge for our ancestors.

Habitual bipedalism is considered as a major breakthrough in human evolution and is the defining feature of hominins. Upright posture is presumably less stable than quadrupedal posture, but when using external support, for example, toddlers assisted by their parents, postural stability becomes less critical. In this study, we show that humans seem to mimic such external support by creating a virtual pivot point (VPP) above their centre of mass. A highly reduced conceptual walking model based on this assumption reveals that such virtual support is sufficient for achieving and maintaining postural stability. The VPP is experimentally observed in walking humans and dogs and in running chickens, suggesting that it might be a convenient emergent behaviour of gait mechanics and not an intentional locomotion behaviour. Hence, it is likely that even the first hominis may have already applied the VPP, a mechanism that would have facilitated the development of habitual bipedalism.

Swing leg control in human running

Humans can run within a wide range of speeds without thinking about stabilizing strategies. The leg properties seem to be adjusted automatically without need for sensory feedback. In this work, the dynamics of human running are represented by the planar spring mass model. Within this framework, for higher speeds, running patterns can be stable without control strategies. Here, potential strategies that provide stability over a broader range of running patterns are considered and these theoretical predictions are compared to human running data. Periodic running solutions are identified and analyzed with respect to their stability. The control strategies are assumed as linear adaptations of the leg parameters-leg angle, leg stiffness and leg length-during the swing phase. To evaluate the applied control strategies regarding their influence on landing behavior, two parameters are introduced: the velocity of the foot relative to the ground (ground speed matching) and the foots angle of approach. The results show that periodic running solutions can be stabilized and that control strategies, which guarantee running stability, are redundant. For any swing leg kinematics (adaptation of the leg angle and the leg length), running stability can be achieved by adapting the leg stiffness in anticipation of the ground contact.

Functional gait asymmetry of unilateral transfemoral amputees

The aim of prosthetic devices is to mimic the function of biological systems. Numerous investigations have demonstrated significant asymmetries in unilateral amputee gait. The underlying interactions of prosthetic and intact leg are not widely discussed, so far. To get more insight into the functionality of asymmetries, we investigated temporal and kinetic parameters of walking transfemoral amputees wearing the computerized C-Leg and the non-computerized 3R80. Experiments were conducted on an instrumented treadmill at four different walking speeds (0.5, 0.8, 1.1, 1.4m/s) measuring vertical and horizontal ground reaction forces. Single support, double support and contact times, vertical and horizontal impulses as well as their asymmetry factors were calculated. Gait patterns were similar for both prosthetic knee joints, manifesting in (i) reduced stance times of the prosthetic leg, (ii) prolonged load transfer during double support from intact to prosthetic leg at lower speeds, (iii) reduced vertical and horizontal impulses of the prosthetic leg, (iv) net accelerating horizontal impulses during contact of the prosthetic leg, (v) missing impacts at touch-down of the prosthetic leg. Our results suggest that deficits of the prosthetic leg like missing active knee extension and ankle push-off are compensated by the intact leg. The altered touch-down configuration for the prosthetic leg enables it to provide forward propulsion while load bearing is largely shifted to the intact leg.

Effective leg stiffness in running

Leg stiffness is a common parameter used to characterize leg function during bouncing gaits, like running and hopping. In the literature, different methods to approximate leg stiffness based on kinetic and kinematic parameters are described. A challenging point in estimating leg stiffness is the definition of leg compression during contact. In this paper four methods (methods A-D) based on ground reaction forces (GRF) and one method (method E) relying on temporal parameters are described. Leg stiffness calculated by these five methods is compared with running patterns, predicted by the spring mass model. The best and simplest approximation of leg stiffness is method E. It requires only easily accessible parameters (contact time, flight time, resting leg length, body mass and the legs touch down angle). Method D is of similar quality but additionally requires the time-dependent progression of the GRF. The other three methods show clear differences from the model predictions by over- or underestimating leg stiffness, especially at slow speeds. Leg stiffness is derived from a conceptual model of legged locomotion and does not exist without this model. Therefore, it is important to prove which experimental method is suited best for approximating the stiffness in a specific task. This will help to interpret the predictions of the conceptual model in comparison with experimental data.

Impulsive ankle push-off powers leg swing in human walking.

Rapid unloading and a peak in power output of the ankle joint have been widely observed during push-off in human walking. Model-based studies hypothesize that this push-off causes redirection of the body center of mass just before touch-down of the leading leg. Other research suggests that work done by the ankle extensors provides kinetic energy for the initiation of swing. Also, muscle work is suggested to power a catapult-like action in late stance of human walking. However, there is a lack of knowledge about the biomechanical process leading to this widely observed high power output of the ankle extensors. In our study, we use kinematic and dynamic data of human walking collected at speeds between 0.5 and 2.5 m s−1 for a comprehensive analysis of push-off mechanics. We identify two distinct phases, which divide the push-off: first, starting with positive ankle power output, an alleviation phase, where the trailing leg is alleviated from supporting the body mass, and second, a launching phase, where stored energy in the ankle joint is released. Our results show a release of just a small part of the energy stored in the ankle joint during the alleviation phase. A larger impulse for the trailing leg than for the remaining body is observed during the launching phase. Here, the buckling knee joint inhibits transfer of power from the ankle to the remaining body. It appears that swing initiation profits from an impulsive ankle push-off resulting from a catapult without escapement.

Foot Function in Spring Mass Running

The leg function in human running can be characterized by spring-like behaviour. The human leg itself has several segments, which influence the leg function. In this paper a simple model based on spring-mass-running but with with a compliant ankle joint is introduced to investigate the influence of a rigid foot segment. The predicted force-length-curve explains changes in leg stiffness as well as changes in leg length during stance phase similar to what is observed in human running.

Diverging times in movement analysis

In movement analysis, more than one measuring system is often used to record biomechanical variables. Usually, it is desired to assign the occurring events to a common time line, which can be accomplished by synchronizing data acquisition, i.e. using a pulse to trigger a sample on all systems. However, this method is not supported by every system. Alternatively, the measurements of different systems can be started by a common trigger signal with no further synchronization of their sampling clocks during acquisition. With that, two systematic errors may be introduced, namely time lag and time drift. The extents of these errors not only depend on the individual system properties, but also depend on the set of systems combined. In this study, we introduce a simple method to determine time lag and time drift for two systems including cameras and force plates. Our results show that both parameters are present and dependent on chosen sampling frequencies. We conclude that in order to avoid misinterpretation of recorded signals the identified time lag and time drift need to be taken into account for trials of all durations.

Variable Joint Elasticities in Running

In this paper we investigate how spring-like leg behavior in human running is represented at joint level. We assume linear torsion springs in the joints and between the knee and the ankle joint. Using experimental data of the leg dynamics we compute how the spring parameters (stiffness and rest angles) change during gait cycle. We found that during contact the joints reveal elasticity with strongly changing parameters and compare the changes of different parameters for different spring arrangements. The results may help to design and improve biologically inspired spring mechanisms with adjustable parameters.

KNEE JOINT STIFFNESS FOR SELF-STABLE RUNNING

INTRODUCTION Humans and animals with spring-like leg behavior can stabilize running at different speeds using simple leg strategies, such as adjustment of leg stiffness k, angle of attack 0, or leg length l0 [1,2] as predicted by the simple spring-mass model. However, this model does not provide specific predictions on joint properties in segmented legs as found in nature. Here, we investigate how elastic joint behavior should be tuned with running speed in order to maintain self-stability.