Anton J. van den Bogert

Direct dynamics simulation of the impact phase in heel-toe running

The influence of muscle activation, position and velocities of body segments at touchdown and surface properties on impact forces during heel-toe running was investigated using a direct dynamics simulation technique. The runner was represented by a two-dimensional four- (rigid body) segment musculo-skeletal model. Incorporated into the muscle model were activation dynamics, force-length and force-velocity characteristics of seven major muscle groups of the lower extremities: mm. glutei, hamstrings, m. rectus femoris, mm. vasti, m. gastrocnemius, m. soleus and m. tibialis anterior. The vertical force-deformation characteristics of heel, shoe and ground were modeled by a non-linear visco-elastic element. The maximum of a typical simulated impact force was 1.6 times body weight. The influence of muscle activation was examined by generating muscle stimulation combinations which produce the same (experimentally determined) resultant joint moments at heelstrike. Simulated impact peak forces with these different combinations of muscle stimulation levels varied less than 10%. Without this restriction on initial joint moments, muscle activation had potentially a much larger effect on impact force. Impact peak force was to a great extent influenced by plantar flexion (85 N per degree of change in foot angle) and vertical velocity of the heel (212 N per 0.1 m s-1 change in velocity) at touchdown. Initial knee flexion (68 N per degree of change in leg angle) also played a role in the absorption of impact. Increased surface stiffness resulted in higher impact peak forces (60 N mm-1 decrease in deformation).(ABSTRACT TRUNCATED AT 250 WORDS)

Muscle coordination and function during cutting movements.

PURPOSE The objectives of this study were to: 1) establish a database of kinematic and EMG data during cutting movements, 2) describe normal muscle function and coordination of 12 lower extremity muscles during cutting movements susceptible to ankle sprains, and 3) identify potential muscle coordination deficiencies that may lead to ankle sprain injuries. METHODS Kinematic, EMG, and GRF data were collected from 10 recreationally active male subjects during both a side-shuffle and v-cut movement. RESULTS The data showed that muscles functioned similarly during both movements. The primary function of the hip and knee extensors was to decelerate the center-of-mass during landing and to provide propulsion during toe-off. The hip add/abductors functioned primarily to stabilize the hip rather than provide mechanical power. The ankle plantar flexors functioned to provide propulsion during toe-off, and the gastrocnemius had an additional burst of activity to plantarflex the foot before touchdown during the side-shuffle to help absorb the impact. The tibialis anterior functioned differently during each movement: to dorsiflex and supinate the foot after toe-off in preparation for the next step cycle during the side-shuffle and to dorsiflex the foot before impact to provide the heel-down landing and ankle stability in the stance phase during the v-cut. CONCLUSIONS The muscles crossing the ankle joint, especially the tibialis anterior and peroneus longus, may play an important role to prevent ankle sprain injuries. Both muscles provided stability about the subtalar joint by preventing excessive joint rotations. Future theoretical studies with forward dynamic simulations incorporating individual muscle actuators are needed to quantify the segment accelerations induced by active muscles which may prevent or lead to ankle sprain injuries.

A method for inverse dynamic analysis using accelerometry

A method was developed to calculate total resultant force and moment on a body segment, in three dimensions, from accelerometer data. The method was applied for an analysis of intersegmental loading at the hip joint during the single support phase of working and running, using four triaxial accelerometers mounted on the upper body. Results were compared to a conventional analysis using simultaneously recorded kinematics and ground reaction forces. The loading patterns obtained by both methods were similar, but the accelerometry method systematically underestimated the intersegmental force and moment at the hip by about 20%. This could be explained by the inertial and gravitational forces originating from the swing leg which were neglected in the analysis. In addition, the accelerometry analysis was not not reliable during the impact phase of running, when the upper body and accelerometers did not behave as a rigid body. For applications where these limitations are acceptable, the accelerometry method has the advantage that it does not require a gait laboratory environment and can be used for field studies with a completely body-mounted recording system. The method does not require differentiation or integration, and therefore provided the possibility of real-time inverse dynamics analysis.

Computer simulation of landing movement in downhill skiing: Anterior cruciate ligament injuries

Anterior cruciate ligament (ACL) injuries typically occur in high-speed downhill skiing during the landing phase following a jump. A direct dynamics simulation model was developed which allows investigation of possible ACL injury mechanisms without the need to use actual skiers in a potentially dangerous environment. The model included multibody dynamics, muscle dynamics and a model for ski-snow interaction. The models ability to reproduce an actual landing movement was investigated by minimizing the differences between measured and stimulated landing movements as a function of constant muscle stimulation levels. The remaining difference was mainly due to noise in the measurements. A small balance disturbance was induced to simulate an injury condition. This disturbance caused the modeled skier to fall slightly backwards. A recovery attempt was made by maximal activation of the quadriceps and iliopsoas muscles. Peak resultant shear force at the knee joint in ACL direction was substantially higher in the injury simulation (1001 N) when compared to the simulated normal landing movement (589 N). Taking into account quadriceps contraction and orientation of the ACL with respect to tibial plateau, peak ACL force during the injury simulation was estimated to be 1350 N, which is within the range of failure loads for this ligament. The external forces were mainly (75%) responsible for this loading. The contribution of the fully activated quadriceps muscles was only 25%. It was concluded that the model could reproduce a typical landing movement and is therefore considered to be sufficiently realistic. Second, the simulation results suggest that external forces are the main cause for ACL injuries during landing movements in downhill skiing.

Modelling of force production in skeletal muscle undergoing stretch

Many human movements involve eccentric contraction of muscles. Therefore, it is important that a theoretical model is able to represent the kinetic response of activated muscle during lengthening if it is to be applied to dynamic simulation of such movements. The so-called Hill and Distribution Moment models are two commonly used models of skeletal muscle. The Hill model is a phenomenological model based on experimental observations; the Distribution Moment model is based on the cross-bridge theory of muscle contraction. The ability of each of these models to predict the force-velocity relation has been considered previously; however, few attempts have been made to evaluate the force response of each model with respect to time during stretches at different velocities. The purpose of this study was to compare the predicted force-time responses of the Hill and Distribution Moment models to the actual force produced by the cat soleus during experimental iso-velocity stretches at maximal activation. Two stretch velocities were simulated: 7.2 and 400 mm s-1. Model parameters were derived from the literature where possible. In addition, model parameters were optimized to provide the best possible fit between model force predictions and experimental results at each velocity. The results of the study showed that using the Hill model, it was possible to describe qualitatively the force-time response of the muscle at both velocities of stretch using parameters derived from the literature. It was also possible to optimize a set of parameters for the Hill model to provide a quantitative description of the force-time response at each velocity. Using the Distribution Moment model, it was not possible to describe the force-time response of the muscle for both velocities using a single set of rate constants, suggesting that the cross-bridge theory, upon which the model is based, may have to be further evaluated for lengthening muscle. Further research is required to determine if the model results can be generalized to other muscles and other velocities of stretch.

INFLUENCE OF ANKLE LIGAMENTS ON TIBIAL ROTATION : AN IN VITRO STUDY

The purpose of this study was to clarify the role of the ankle ligaments in controlling the tibial rotation for different foot positions. A 6 degrees of freedom device was constructed for in vitro simulation of this movement transfer during the support phase of gait. Tibia rotation angle was measured for different foot positions and vertical loads, while the ligament integrity was modified. Data were collected from eight legs of four different cadavers. The results showed that vertical loading is unimportant to influence tibial rotation, while the lateral ankle ligaments have significant influence, especially during eversion. It was concluded that chronic partial or total lateral ankle instability may contribute to knee and foot injuries through abnormal tibial rotation.

Direct dynamics simulation of FES-assisted locomotion

Using functional electrical stimulation (FES), muscles of spinal-cord injured patients can be activated by externally generated electrical currents in order to restore function. As for gait, the question arises when during the gait cycle and two what extent individual muscles should be stimulated. Computer simulation provides the designer with a tool to evaluate the performance of different muscle stimulation patterns without the need to test patients at every stage of system development. The goals of this paper are: first, to identify, using computer simulation, multi-channel stimulation patterns that are capable of reproducing normal gait kinematics for a full gait cycle, without relying on sensory feedback (open-loop control); second, to briefly assess the stability of the gait obtained. A two-dimensional musculo-skeletal model was developed, based on mathematical representations of muscle properties (including force-length and force velocity characteristics and muscle activation dynamics). A visco-elastic model, including non-linear heel-pad properties, was used to describe the foot-ground interaction. A seven segment skeletal model was actuated by 8 major muscle groups in each leg. Rectangular muscle stimulation patterns were defined by 3 parameters: onset, termination and level of stimulation. Thus, the minimization of the differences between simulated and measured normal gait kinematics was a 24 (3 by 8) parameter optimization problem. Although a good agrement was found between simulated and measured kinematics (rms difference equals 6.5 degrees), stable cyclic locomotion was not achieved. At this point it is concluded that muscle properties do not provide sufficient stability to permit cyclic locomotion with sixteen channels of muscle stimulation, and that incorporation of sensory feedback control will be necessary to achieve this goal.