Reinhard Blickhan | Friedrich-Schiller-Universität Jena (original) (raw)

Papers by Reinhard Blickhan

Journal of Biomechanics, Nov 1, 2009

To recognise and classify movement patterns correctly can be a difficult task. Nevertheless, move... more To recognise and classify movement patterns correctly can be a difficult task. Nevertheless, movement analysts are working on it on a daily basis. Therefore, we have developed and evaluated a method to do the classification by using contact forces during hopping in a sledge system. Here, experiments showed that reaction-forces of different subjects on a sliding sledge could be divided into four major types. These types are symmetric single-modal (type I), positive mono-modal (type II), negative mono-modal (type III), and multi-modal associated with plateau formation (type IV). Up until now, an exact determination of these types was not possible. However, the new method helps to approximate those four types with well established mathematical functions. With this approach, the measured reaction-force will be reproduced by particular coefficients. Subsequently, the coefficients are subjected to a discriminant-analysis. The result is a three-dimensional functioncoefficient, which allows the classification of the actual force-pattern on the one of the four types.

Springer eBooks, 2012

In this paper, the possible advantages of using grounded running, a running gait without aerial p... more In this paper, the possible advantages of using grounded running, a running gait without aerial phases, for fast robot locomotion are discussed. Extended fields of fixed points for grounded running, obtained using dimensionless numerical modeling, are presented. They can be used to find global parameters for the design and operation of robots. The use of self-stable (for steady-state locomotion) or weak unstable (to improve maneuverability-stability trade-off) gait-modes as a design parameter could represent a strategy to reduce the still existing gap between animals and mimicking machines with respect to elegance and performance.

Springer eBooks, 2012

Underwater undulatory swimming describes one of the fastest modes of human aquatic locomotion. Th... more Underwater undulatory swimming describes one of the fastest modes of human aquatic locomotion. The human swimmer can be considered as natural paradigm for technical segmented linkage systems used in robotics that must compensate its anatomical limitations through sophisticated kinetics. In order to reveal and evaluate such mechanisms the flow around and behind the swimmer was measured by time-resolved particle image velocimetry (TR-2D-PIV) and simulated by computational fluid dynamics (CFD). In comparison to fish, despite of joint asymmetries the swimmers used undulatory waves characterized by very similar absolute amplitude distributions along the body but at much higher Strouhal numbers. The observed 3D-patterns revealed in the CFD helps us to newly interpret experimental findings. Both the experimental flow field as well as that obtained from CFD document the effect of flow preformation and vortex recapturing. We propose that the use of high Strouhal numbers facilitates the recapture of vortices unavoidable due the disadvantageous geometry of the human swimmer.

Journal of Biomechanics, 1986

The stiffness of the tibia-metatarsus joint of several spider-species was determined for differen... more The stiffness of the tibia-metatarsus joint of several spider-species was determined for different loading conditions. The relationship between force applied to the metatarsus tip and deflection is linear and free of hysteresis during axial loading (2'). but not under lateral (_/)and dorsoventral (x') loading, where joint stitTness increases with amplitude and deflection rate. Up to lateral deflections of 2.5' relaxation and nonlinearity of the joint-stilfness can be described by power functions. Considering strain induced in the spider's tibia, the viscoelastic properties of the joint result in a nonlinear amplitude transfer and a high-pass filtering of mechanical vibrations applied to the spider's leg However, this contributes only slightly to the corresponding transfer characteristics measured for the electrical response of biological strain receptors.

Human Movement Science, Oct 1, 2011

To maximize swimming speed athletes copy fish undulatory swimming during the underwater period af... more To maximize swimming speed athletes copy fish undulatory swimming during the underwater period after start and turn. The anatomical limitations may lead to deviations and may enforce compensating strategies. This has been investigated by analyzing the kinematics of two national female swimmers while swimming in a still water pool. Additionally, the flow around and behind the swimmers was measured with the aid of time-resolved particle image velocimetry (TR-2D-PIV). As compared to fish, the swimmers used undulatory waves characterized by much higher Strouhal numbers but very similar amplitude distributions along the body and Froude efficiencies. Vortices generated in the region of strongly flexing joints are suitable to be used pedally to enhance propulsion (vortex recapturing). Complementing studies using numerical and technical modeling will help us to probe the efficiency of observed mechanisms and further improvements of the human strategy.

International Journal of Applied Mechanics, Mar 1, 2011

The change in length of a muscle usually leads to a change in segment position. If a muscle is sh... more The change in length of a muscle usually leads to a change in segment position. If a muscle is shortened the joint is being flexed. However bi-articular muscles are able to bend a joint as well as to stretch it by its contraction because of varied geometrical conditions. This phenomenon has already been analyzed in different surveys on M. biceps femoris. Until today such a consideration cannot be found for M. gastrocnemius, which is a bi-articular muscle, too. The weight- and speed-dependent activities of this muscle during fast and slow hopping, and also during running and walking, suggest that a movement-relevant function exists. This behavior pattern was the reason for a closer look on the actions of M. gastrocnemius from a geometrical point of view. The results clearly show a knee angle-dependent function. Moreover, a so-called critical angle area of the knee joint, from which the shortening of M. gastrocnemius leads from joint flexion to joint stretching, can be specified for the first time. At this, the position of the ankle joint seems to have little influence on the function of M. gastrocnemius.

PLOS ONE, Dec 27, 2017

Though the effects of imposed trunk posture on human walking have been studied, less is known abo... more Though the effects of imposed trunk posture on human walking have been studied, less is known about such locomotion while accommodating changes in ground level. For twelve able participants, we analyzed kinematic parameters mainly at touchdown and toe-off in walking across a 10-cm visible drop in ground level (level step, pre-perturbation step, stepdown, step-up) with three postures (regular erect,~30˚and~50˚of trunk flexion from the vertical). Two-way repeated measures ANOVAs revealed step-specific effects of posture on the kinematic behavior of gait mostly at toe-off of the pre-perturbation step and the stepdown as well as at touchdown of the step-up. In preparation to step-down, with increasing trunk flexion the discrepancy in hip−center of pressure distance, i.e. effective leg length, (shorter at toe-off versus touchdown), compared with level steps increased largely due to a greater knee flexion at toe-off. Participants rotated their trunk backwards during step-down (2-to 3-fold backwards rotation compared with level steps regardless of trunk posture) likely to control the angular momentum of their whole body. The more pronounced trunk backwards rotation in trunk-flexed walking contributed to the observed elevated center of mass (CoM) trajectories during the step-down which may have facilitated drop negotiation. Ablebodied individuals were found to recover almost all assessed kinematic parameters comprising the vertical position of the CoM, effective leg length and angle as well as hip, knee and ankle joint angles at the end of the step-up, suggesting an adaptive capacity and hence a robustness of human walking with respect to imposed trunk orientations. Our findings may provide clinicians with insight into a kinematic interaction between posture and locomotion in uneven ground. Moreover, a backward rotation of the trunk for negotiating step-down may be incorporated into exercise-based interventions to enhance gait stability in individuals who exhibit trunk-flexed postures during walking.

Journal of Comparative Physiology A-neuroethology Sensory Neural and Behavioral Physiology, 1985

to the tibia-metatarsus joint. 4. Influence of mechanical parameters: The strain amplitude, calcu... more to the tibia-metatarsus joint. 4. Influence of mechanical parameters: The strain amplitude, calculated for the organ HS8 (4th leg) from the ground reaction force using the proper mechanical sensitivities is in good agreement with the measured value (Figs. 34, 35; Table 2) and can be mainly attributed to muscle forces.

The Journal of Experimental Biology, Jul 1, 1987

Terrestrial locomotion involving appendages has evolved independently in vertebrates and arthropo... more Terrestrial locomotion involving appendages has evolved independently in vertebrates and arthropods. Differences in the mechanical design of the locomotor apparatus could impose constraints on the energetics of locomotion. The mechanical energy fluctuations of the centre of mass of an arthropod, the ghost crab Ocvpode quadrata (Fabricius), were examined by integrating the ground reaction forces exerted during sideways locomotion. Crabs used a pendulum-type energy exchange mechanism during walking, analogous to an egg rolling end over end, with the same effectiveness as birds and mammals. Moreover, ghost crabs were found to have two running gaits. A switch from a slow to a fast run occurred at the same speed and stride frequency predicted for the trot-gallop transition of a quadrupedal mammal of the same body mass. In addition, the mass-specific mechanical energy developed over a unit distance was independent of speed and was within the limits measured for birds and mammals. Despite the obvious differences in mechanical design between crabs and mammals, energy-conserving mechanisms and the efficiency of locomotion were remarkably similar. These similarities may result from the fact that the muscles that generate forces during terrestrial locomotion have relatively conservative mechanical and energetic properties.

Journal of Theoretical Biology, Sep 1, 2000

In the present paper, a finite-element model for simulating muscle mechanics is described. Based ... more In the present paper, a finite-element model for simulating muscle mechanics is described. Based on nonlinear continuum mechanics an algorithm is proposed that includes the contractile active and passive properties of skeletal muscle. Stress in the muscle is assumed to result from the superposition of a passive and an active part. The passive properties are described by a hyperelastic constitutive material law whereas the active part depends on the fibre length, shortening velocity and an activation function. The constraint of approximate incompressibility of the muscle element is satisfied as a property of the constitutive equations. Because of the nonlinear behaviour of the material and the highly dynamical performance an incremental procedure including iterative methods is used. The advantage of the model over previous formulations is the possibility to integrate the element into an engineering standard finite-element programme ANSYS using advanced numerical tools. The model allows simulations of muscle recruitment, calculations of stress and strain distributions and predictions of muscle shape. Other possible applications are studies of the muscle architecture, the effect of inertia and impacts. First, simple examples are presented.

Flow Measurement and Instrumentation, Dec 1, 2022

Archive of Applied Mechanics, Jun 18, 2011

In this study, we checked experimentally whether anterior-posterior accelerations of the head dur... more In this study, we checked experimentally whether anterior-posterior accelerations of the head during quiet human stance are usually below or above known thresholds of the otolith sensor. Thereto, we measured head kinematics with high spatial resolution. Furthermore, we used both these experimental data and computer simulations of two double inverted pendulum (DIP) models in order to verify the validity of DIP models in general. The results are clear cut. First, not only are acceleration thresholds regularly exceeded about once a second but also are velocity thresholds exceeded, albeit probably less frequently. Second, COM and head movement predicted by interwoven DIP model dynamics can not reproduce the mean measured amplitudes at once. Thus, neither the formerly promoted single inverted pendulum nor any DIP model can causally explain the dynamics of quiet human stance. Instead, we suggest to factor in at least three mechanical degrees of freedom. Due to a couple of reasons discussed, the triple inverted pendulum (TIP) model seems to be a promising abstraction implying potential to better understand the dynamics of quiet human stance.

Gait & Posture, Apr 1, 2008

The mechanism of two-legged quiet stance is unclear. This study specifically investigated biomech... more The mechanism of two-legged quiet stance is unclear. This study specifically investigated biomechanical parameters characterising the mechanisms of rotation around the longitudinal axis (parallel to gravitational acceleration, i.e. in the transverse plane parallel to the ground). Subjects (10) were examined while standing quietly on two force platforms which measured the transverse component of the ground reaction torque (GRT). In addition, right and left hip kinematics were acquired by tracking markers in the sagittal plane. The pelvic rotation in the transverse plane (pelvic angle) was then calculated from the anterior-posterior coordinates of the hip markers. We verified the hypothesis that the pelvis generally may be coupled to the ground by a rotational stiffness provided by both legs. Thus, we asked whether the transverse GRT component may be proportional to the pelvic angle. This hypothesis was rejected. However, the transverse GRT component could be identified as one rotational stabilising mechanism which drove the higher-frequency (>1 Hz) deflections of the pelvic angle back to its lowerfrequency fraction. The respective stiffness coefficient between transverse GRT component and relative displacement between higher-and lower-frequency pelvic angular fraction was about 2.4 N m/8. Implications for the character and the localisation of active control of body rotation around the longitudinal axis are discussed.

Human Movement Science, Dec 1, 2014

Human Movement Science jou rn al homepage: www.elsevier.com /locate/humo v the hip marker shows a... more Human Movement Science jou rn al homepage: www.elsevier.com /locate/humo v the hip marker shows almost the same average swimming speed in both half-cycles.

The Journal of Experimental Biology, 2014

In order to better understand the strategies of locomotion in small insects, we have studied cont... more In order to better understand the strategies of locomotion in small insects, we have studied continuous level locomotion of the wood ant species Formica polyctena. We determined the three-dimensional centre of mass kinematics during the gait cycle and recorded the ground reaction forces of single legs utilising a self-developed test site. Our findings show that the animals used the same gait dynamics across a wide speed range without dissolving the tripodal stride pattern. To achieve higher velocities, the ants proportionally increased stride length and stepping frequency. The centre of mass energetics indicated a bouncing gait, in which horizontal kinetic and gravitational potential energy fluctuated in close phase. We determined a high degree of compliance especially in the front legs, as the effective leg length was nearly halved during the contact phase. This leads to only small vertical oscillations of the body, which are important in maintaining ground contact. Bouncing gaits without aerial phases seem to be a common strategy in small runners and can be sufficiently described by the bipedal spring-loaded inverted pendulum model. Thus, with our results, we provide evidence that wood ants perform 'grounded running'.

European Spine Journal, Jul 27, 2005

Biological Cybernetics, Jul 1, 2003

Under normal conditions human walking or running consists of stable cyclic movements. Minor pertu... more Under normal conditions human walking or running consists of stable cyclic movements. Minor perturbances such as a stone or a pothole do not disrupt the cycle, and the system returns to its prescribed trajectory. We investigated whether a pair of antagonistic muscles is able to stabilize the movement without neuronal feedback. The human is represented by a model consisting of a massless two-segment linkage system (leg) topped by a point mass. Both the extensor and flexor muscles are described by a Hill-type muscle model. Conditions for stability are calculated analytically based on the Ljapunov Theory and the results are illustrated by numerical examples. The activation functions of both the extensor and flexor muscles can be calculated for a prescribed trajectory to maintain the self-stabilizing ability of such a system. Experimental evidence supports the prediction. Our investigation shows that a moving center of rotation of the kneejoint, a biarticular flexor muscle group, the force-velocity relation, and the ascending limb of the force-length relation improves the self-stabilizing ability of human movement.

Journal of Biomechanics, Nov 1, 2009

To recognise and classify movement patterns correctly can be a difficult task. Nevertheless, move... more To recognise and classify movement patterns correctly can be a difficult task. Nevertheless, movement analysts are working on it on a daily basis. Therefore, we have developed and evaluated a method to do the classification by using contact forces during hopping in a sledge system. Here, experiments showed that reaction-forces of different subjects on a sliding sledge could be divided into four major types. These types are symmetric single-modal (type I), positive mono-modal (type II), negative mono-modal (type III), and multi-modal associated with plateau formation (type IV). Up until now, an exact determination of these types was not possible. However, the new method helps to approximate those four types with well established mathematical functions. With this approach, the measured reaction-force will be reproduced by particular coefficients. Subsequently, the coefficients are subjected to a discriminant-analysis. The result is a three-dimensional functioncoefficient, which allows the classification of the actual force-pattern on the one of the four types.

Springer eBooks, 2012

In this paper, the possible advantages of using grounded running, a running gait without aerial p... more In this paper, the possible advantages of using grounded running, a running gait without aerial phases, for fast robot locomotion are discussed. Extended fields of fixed points for grounded running, obtained using dimensionless numerical modeling, are presented. They can be used to find global parameters for the design and operation of robots. The use of self-stable (for steady-state locomotion) or weak unstable (to improve maneuverability-stability trade-off) gait-modes as a design parameter could represent a strategy to reduce the still existing gap between animals and mimicking machines with respect to elegance and performance.

Springer eBooks, 2012

Underwater undulatory swimming describes one of the fastest modes of human aquatic locomotion. Th... more Underwater undulatory swimming describes one of the fastest modes of human aquatic locomotion. The human swimmer can be considered as natural paradigm for technical segmented linkage systems used in robotics that must compensate its anatomical limitations through sophisticated kinetics. In order to reveal and evaluate such mechanisms the flow around and behind the swimmer was measured by time-resolved particle image velocimetry (TR-2D-PIV) and simulated by computational fluid dynamics (CFD). In comparison to fish, despite of joint asymmetries the swimmers used undulatory waves characterized by very similar absolute amplitude distributions along the body but at much higher Strouhal numbers. The observed 3D-patterns revealed in the CFD helps us to newly interpret experimental findings. Both the experimental flow field as well as that obtained from CFD document the effect of flow preformation and vortex recapturing. We propose that the use of high Strouhal numbers facilitates the recapture of vortices unavoidable due the disadvantageous geometry of the human swimmer.

Journal of Biomechanics, 1986

The stiffness of the tibia-metatarsus joint of several spider-species was determined for differen... more The stiffness of the tibia-metatarsus joint of several spider-species was determined for different loading conditions. The relationship between force applied to the metatarsus tip and deflection is linear and free of hysteresis during axial loading (2'). but not under lateral (_/)and dorsoventral (x') loading, where joint stitTness increases with amplitude and deflection rate. Up to lateral deflections of 2.5' relaxation and nonlinearity of the joint-stilfness can be described by power functions. Considering strain induced in the spider's tibia, the viscoelastic properties of the joint result in a nonlinear amplitude transfer and a high-pass filtering of mechanical vibrations applied to the spider's leg However, this contributes only slightly to the corresponding transfer characteristics measured for the electrical response of biological strain receptors.

Human Movement Science, Oct 1, 2011

To maximize swimming speed athletes copy fish undulatory swimming during the underwater period af... more To maximize swimming speed athletes copy fish undulatory swimming during the underwater period after start and turn. The anatomical limitations may lead to deviations and may enforce compensating strategies. This has been investigated by analyzing the kinematics of two national female swimmers while swimming in a still water pool. Additionally, the flow around and behind the swimmers was measured with the aid of time-resolved particle image velocimetry (TR-2D-PIV). As compared to fish, the swimmers used undulatory waves characterized by much higher Strouhal numbers but very similar amplitude distributions along the body and Froude efficiencies. Vortices generated in the region of strongly flexing joints are suitable to be used pedally to enhance propulsion (vortex recapturing). Complementing studies using numerical and technical modeling will help us to probe the efficiency of observed mechanisms and further improvements of the human strategy.

International Journal of Applied Mechanics, Mar 1, 2011

The change in length of a muscle usually leads to a change in segment position. If a muscle is sh... more The change in length of a muscle usually leads to a change in segment position. If a muscle is shortened the joint is being flexed. However bi-articular muscles are able to bend a joint as well as to stretch it by its contraction because of varied geometrical conditions. This phenomenon has already been analyzed in different surveys on M. biceps femoris. Until today such a consideration cannot be found for M. gastrocnemius, which is a bi-articular muscle, too. The weight- and speed-dependent activities of this muscle during fast and slow hopping, and also during running and walking, suggest that a movement-relevant function exists. This behavior pattern was the reason for a closer look on the actions of M. gastrocnemius from a geometrical point of view. The results clearly show a knee angle-dependent function. Moreover, a so-called critical angle area of the knee joint, from which the shortening of M. gastrocnemius leads from joint flexion to joint stretching, can be specified for the first time. At this, the position of the ankle joint seems to have little influence on the function of M. gastrocnemius.

PLOS ONE, Dec 27, 2017

Though the effects of imposed trunk posture on human walking have been studied, less is known abo... more Though the effects of imposed trunk posture on human walking have been studied, less is known about such locomotion while accommodating changes in ground level. For twelve able participants, we analyzed kinematic parameters mainly at touchdown and toe-off in walking across a 10-cm visible drop in ground level (level step, pre-perturbation step, stepdown, step-up) with three postures (regular erect,~30˚and~50˚of trunk flexion from the vertical). Two-way repeated measures ANOVAs revealed step-specific effects of posture on the kinematic behavior of gait mostly at toe-off of the pre-perturbation step and the stepdown as well as at touchdown of the step-up. In preparation to step-down, with increasing trunk flexion the discrepancy in hip−center of pressure distance, i.e. effective leg length, (shorter at toe-off versus touchdown), compared with level steps increased largely due to a greater knee flexion at toe-off. Participants rotated their trunk backwards during step-down (2-to 3-fold backwards rotation compared with level steps regardless of trunk posture) likely to control the angular momentum of their whole body. The more pronounced trunk backwards rotation in trunk-flexed walking contributed to the observed elevated center of mass (CoM) trajectories during the step-down which may have facilitated drop negotiation. Ablebodied individuals were found to recover almost all assessed kinematic parameters comprising the vertical position of the CoM, effective leg length and angle as well as hip, knee and ankle joint angles at the end of the step-up, suggesting an adaptive capacity and hence a robustness of human walking with respect to imposed trunk orientations. Our findings may provide clinicians with insight into a kinematic interaction between posture and locomotion in uneven ground. Moreover, a backward rotation of the trunk for negotiating step-down may be incorporated into exercise-based interventions to enhance gait stability in individuals who exhibit trunk-flexed postures during walking.

Journal of Comparative Physiology A-neuroethology Sensory Neural and Behavioral Physiology, 1985

to the tibia-metatarsus joint. 4. Influence of mechanical parameters: The strain amplitude, calcu... more to the tibia-metatarsus joint. 4. Influence of mechanical parameters: The strain amplitude, calculated for the organ HS8 (4th leg) from the ground reaction force using the proper mechanical sensitivities is in good agreement with the measured value (Figs. 34, 35; Table 2) and can be mainly attributed to muscle forces.

The Journal of Experimental Biology, Jul 1, 1987

Terrestrial locomotion involving appendages has evolved independently in vertebrates and arthropo... more Terrestrial locomotion involving appendages has evolved independently in vertebrates and arthropods. Differences in the mechanical design of the locomotor apparatus could impose constraints on the energetics of locomotion. The mechanical energy fluctuations of the centre of mass of an arthropod, the ghost crab Ocvpode quadrata (Fabricius), were examined by integrating the ground reaction forces exerted during sideways locomotion. Crabs used a pendulum-type energy exchange mechanism during walking, analogous to an egg rolling end over end, with the same effectiveness as birds and mammals. Moreover, ghost crabs were found to have two running gaits. A switch from a slow to a fast run occurred at the same speed and stride frequency predicted for the trot-gallop transition of a quadrupedal mammal of the same body mass. In addition, the mass-specific mechanical energy developed over a unit distance was independent of speed and was within the limits measured for birds and mammals. Despite the obvious differences in mechanical design between crabs and mammals, energy-conserving mechanisms and the efficiency of locomotion were remarkably similar. These similarities may result from the fact that the muscles that generate forces during terrestrial locomotion have relatively conservative mechanical and energetic properties.

Journal of Theoretical Biology, Sep 1, 2000

In the present paper, a finite-element model for simulating muscle mechanics is described. Based ... more In the present paper, a finite-element model for simulating muscle mechanics is described. Based on nonlinear continuum mechanics an algorithm is proposed that includes the contractile active and passive properties of skeletal muscle. Stress in the muscle is assumed to result from the superposition of a passive and an active part. The passive properties are described by a hyperelastic constitutive material law whereas the active part depends on the fibre length, shortening velocity and an activation function. The constraint of approximate incompressibility of the muscle element is satisfied as a property of the constitutive equations. Because of the nonlinear behaviour of the material and the highly dynamical performance an incremental procedure including iterative methods is used. The advantage of the model over previous formulations is the possibility to integrate the element into an engineering standard finite-element programme ANSYS using advanced numerical tools. The model allows simulations of muscle recruitment, calculations of stress and strain distributions and predictions of muscle shape. Other possible applications are studies of the muscle architecture, the effect of inertia and impacts. First, simple examples are presented.

Flow Measurement and Instrumentation, Dec 1, 2022

Archive of Applied Mechanics, Jun 18, 2011

In this study, we checked experimentally whether anterior-posterior accelerations of the head dur... more In this study, we checked experimentally whether anterior-posterior accelerations of the head during quiet human stance are usually below or above known thresholds of the otolith sensor. Thereto, we measured head kinematics with high spatial resolution. Furthermore, we used both these experimental data and computer simulations of two double inverted pendulum (DIP) models in order to verify the validity of DIP models in general. The results are clear cut. First, not only are acceleration thresholds regularly exceeded about once a second but also are velocity thresholds exceeded, albeit probably less frequently. Second, COM and head movement predicted by interwoven DIP model dynamics can not reproduce the mean measured amplitudes at once. Thus, neither the formerly promoted single inverted pendulum nor any DIP model can causally explain the dynamics of quiet human stance. Instead, we suggest to factor in at least three mechanical degrees of freedom. Due to a couple of reasons discussed, the triple inverted pendulum (TIP) model seems to be a promising abstraction implying potential to better understand the dynamics of quiet human stance.

Gait & Posture, Apr 1, 2008

The mechanism of two-legged quiet stance is unclear. This study specifically investigated biomech... more The mechanism of two-legged quiet stance is unclear. This study specifically investigated biomechanical parameters characterising the mechanisms of rotation around the longitudinal axis (parallel to gravitational acceleration, i.e. in the transverse plane parallel to the ground). Subjects (10) were examined while standing quietly on two force platforms which measured the transverse component of the ground reaction torque (GRT). In addition, right and left hip kinematics were acquired by tracking markers in the sagittal plane. The pelvic rotation in the transverse plane (pelvic angle) was then calculated from the anterior-posterior coordinates of the hip markers. We verified the hypothesis that the pelvis generally may be coupled to the ground by a rotational stiffness provided by both legs. Thus, we asked whether the transverse GRT component may be proportional to the pelvic angle. This hypothesis was rejected. However, the transverse GRT component could be identified as one rotational stabilising mechanism which drove the higher-frequency (>1 Hz) deflections of the pelvic angle back to its lowerfrequency fraction. The respective stiffness coefficient between transverse GRT component and relative displacement between higher-and lower-frequency pelvic angular fraction was about 2.4 N m/8. Implications for the character and the localisation of active control of body rotation around the longitudinal axis are discussed.

Human Movement Science, Dec 1, 2014

Human Movement Science jou rn al homepage: www.elsevier.com /locate/humo v the hip marker shows a... more Human Movement Science jou rn al homepage: www.elsevier.com /locate/humo v the hip marker shows almost the same average swimming speed in both half-cycles.

The Journal of Experimental Biology, 2014

In order to better understand the strategies of locomotion in small insects, we have studied cont... more In order to better understand the strategies of locomotion in small insects, we have studied continuous level locomotion of the wood ant species Formica polyctena. We determined the three-dimensional centre of mass kinematics during the gait cycle and recorded the ground reaction forces of single legs utilising a self-developed test site. Our findings show that the animals used the same gait dynamics across a wide speed range without dissolving the tripodal stride pattern. To achieve higher velocities, the ants proportionally increased stride length and stepping frequency. The centre of mass energetics indicated a bouncing gait, in which horizontal kinetic and gravitational potential energy fluctuated in close phase. We determined a high degree of compliance especially in the front legs, as the effective leg length was nearly halved during the contact phase. This leads to only small vertical oscillations of the body, which are important in maintaining ground contact. Bouncing gaits without aerial phases seem to be a common strategy in small runners and can be sufficiently described by the bipedal spring-loaded inverted pendulum model. Thus, with our results, we provide evidence that wood ants perform 'grounded running'.

European Spine Journal, Jul 27, 2005

Biological Cybernetics, Jul 1, 2003

Under normal conditions human walking or running consists of stable cyclic movements. Minor pertu... more Under normal conditions human walking or running consists of stable cyclic movements. Minor perturbances such as a stone or a pothole do not disrupt the cycle, and the system returns to its prescribed trajectory. We investigated whether a pair of antagonistic muscles is able to stabilize the movement without neuronal feedback. The human is represented by a model consisting of a massless two-segment linkage system (leg) topped by a point mass. Both the extensor and flexor muscles are described by a Hill-type muscle model. Conditions for stability are calculated analytically based on the Ljapunov Theory and the results are illustrated by numerical examples. The activation functions of both the extensor and flexor muscles can be calculated for a prescribed trajectory to maintain the self-stabilizing ability of such a system. Experimental evidence supports the prediction. Our investigation shows that a moving center of rotation of the kneejoint, a biarticular flexor muscle group, the force-velocity relation, and the ascending limb of the force-length relation improves the self-stabilizing ability of human movement.