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Background Limited walking ability is an important problem for patients with multiple sclerosis. A better understanding of how gait impairments lead to limited walking ability may help to develop more targeted interventions. Although gait classifications are available in cerebral palsy and stroke, relevant knowledge in MS is scarce. Objective The aims of this study were: (1) to identify distinctive gait patterns in patients with MS based on a combined evaluation of kinematics, gait features, and muscle activity during walking and (2) to determine the clinical relevance of these gait patterns. Design This was a cross-sectional study of 81 patients with MS of mild-to-moderate severity (Expanded Disability Status Scale [EDSS] median score=3.0, range=1.0–7.0) and an age range of 28 to 69 years. Method The patients participated in 2-dimensional video gait analysis, with concurrent measurement of surface electromyography and ground reaction forces. A score chart of 73 gait items was used to rate each gait analysis. A single rater performed the scoring. Latent class analysis was used to identify gait classes. Results Analysis of the 73 gait variables revealed that 9 variables could distinguish 3 clinically meaningful gait classes. The 9 variables were: (1) heel-rise in terminal stance, (2) push-off, (3) clearance in initial swing, (4) plantar-flexion position in mid-swing, (5) pelvic rotation, (6) arm-trunk movement, (7) activity of the gastrocnemius muscle in pre-swing, (8) M-wave, and (9) propulsive force. The EDSS score and gait speed worsened in ascending classes. Limitations Most participants had mild-to-moderate limitations in walking ability based on their EDSS scores, and the number of walkers who were severely limited was small. Conclusions Based on a small set of 9 variables measured with 2-dimensional clinical gait analysis, patients with MS could be divided into 3 different gait classes. The gait variables are suggestive of insufficient ankle push-off.

BackgroundDynamometry has been used extensively to measure knee extensor strength in individuals with cerebral palsy (CP). However, increased coactivation can lead to underestimation of knee extensor strength and, therefore, reduce validity of strength measurements. It is yet unknown to what extent coactivation occurs during dynamometry testing and whether coactivation is influenced by severity of CP, load levels, and muscle fatigue.ObjectivesThe aims of this study were: (1) to investigate coactivation in adolescents with and without CP during dynamometer tests and (2) to assess the effect of Gross Motor Function Classification System (GMFCS) level, load level, and muscle fatigue on coactivation.DesignA cross-sectional observational design was used.MethodSixteen adolescents with CP (GMFCS levels I and II: n=10/6; age range=13–19 years) and 15 adolescents without CP (n=15; age range=12–19 years) performed maximal isometric contractions (maximal voluntary torque [MVT]) and a series of submaximal dynamic contractions at low (±65% MVT), medium (±75% MVT), and high (±85% MVT) loads until fatigue. A coactivation index (CAI) was calculated for each contraction from surface electromyography recordings from the quadriceps and hamstring muscles.ResultsAdolescents with CP classified in GMFCS level II showed significantly higher CAI values than adolescents classified in GMFCS level I and those without CP during maximal and submaximal contractions. No differences were observed among load levels. During the series of fatiguing submaximal contractions, CAI remained constant in both the CP group and the group with typical development (TD), except for adolescents with TD at the low-load condition, which showed a significant decrease.LimitationsElectromyography tracings were normalized to amplitudes during maximal isometric contractions, whereas previous studies suggested that these types of contractions could not be reliably determined in the CP population.ConclusionCoactivation was higher in adolescents with CP classified in GMFCS level II than in adolescents with TD and those with CP in GMFCS level I at different load levels. Within all groups, coactivation was independent of load level and fatigue. In individuals with CP, coactivation can lead to an underestimation of agonist muscle strength, which should be taken into account while interpreting the results of both maximal and submaximal dynamometer tests.

Aim To investigate the relation between muscle endurance and subjectively reported fatigue, walking capacity, and participation in mildly affected adolescents with cerebral palsy (CP) and peers with typical development. Method In this case–control study, knee extensor muscle endurance was estimated from individual load–endurance curves as the load corresponding to a 15-repetition maximum in 17 adolescents with spastic CP (six males, 11 females; age 12–19y) and 18 adolescents with typical development (eight males, 10 females; age 13–19y). Questionnaires were used to assess subjectively reported fatigue (Pediatric Quality of Life Inventory Multidimensional Fatigue Scale) and participation (Life-Habits questionnaire). Walking capacity was assessed using the 6-minute walk test. Relations were determined using multiple regression analyses. Results Muscle endurance related significantly to subjectively reported fatigue and walking capacity in adolescents with CP, while no relations were found for adolescents with typical development (subjectively reported fatigue: regression coefficient β [95% confidence intervals] for CP=23.72 [6.26 to 41.18], for controls=2.72 [−10.26 to 15.69]; walking capacity β for CP=125m [−87 to 337], for controls=2m [−86 to 89]). The 15-repetition maximum did not relate to participation in adolescents with CP. Interpretation Subjectively reported fatigue and reduced walking capacity in adolescents with CP are partly caused by lower muscle endurance of knee extensors. Training of muscle endurance might contribute to reducing the experience of fatigue and improving walking capacity. Reduced muscle endurance seems to have no effect on participation.

Objective: To quantify dynamic spasticity, i.e. the coupling between muscle-tendon stretch velocity and muscle activity during gait, of the gastrocnemius and soleus muscles in children with spastic cerebral palsy. Design: Prospective, cross-sectional study. Subjects: Seventeen ambulatory children with cerebral palsy with spastic calf muscles, and H matched typically developing children. Methods: The children walked at 3 different speeds. Threedimensional kinematic and electromyographic data were collected. Muscle-tendon velocities of the gastrocnemius medialis and soleus were calculated using musculoskeletal modelling. Results: In typically developing children, muscles were stretched fast in swing without subsequent muscle activity, while spastic muscles were stretched more slowly for the same walking speed, followed by an increase in muscle activity. The mean ratio between peak activity and peak stretch velocity in swing was approximately 4 times higher in spastic muscles, and increased with walking speed. In stance, the stretch of muscles in typically developing children was followed by an increase in muscle activity. Spastic muscles were stretched fast in loading response, but since muscle activity was already built up in swing, no clear dynamic spasticity effect was present. Conclusion: Spastic calf muscles showed increased coupling between muscle-tendon stretch velocity and muscle activity, especially during the swing phase of gait. © 2010 Foundation of Rehabilitation Information.

It was hypothesized that rheumatoid arthritis (RA) patients with a total knee prosthesis that allows axial rotation of the bearing (MB) will show more co-contraction to stabilize the knee joint during a step-up task than RA patients with a fixed bearing total knee prosthesis (FB) where this rotational freedom is absent while having the same articular geometry. Surface EMG, kinematics and kinetics about the knee were recorded during a step-up task of a MB group (n = 5), a FB group (n = 4) and a control group (n = 8). Surface EMG levels of thigh muscles were calibrated to either knee flexion or extension moments by means of isokinetic contractions on a dynamometer. During the step-up task co-contraction indices were determined from an EMG-force model. Controls showed a higher active ROM during the step-up task than the patient group, 96° versus 88° (P = 0.007). In the control group higher average muscle extension, flexion and net moments during single limb support phase were observed than in the patient group. During the 20-60% interval of the single limb support, MB patients showed a significant higher level of flexor activity, resulting in a lower net joint moment, however co-contraction levels were not different. Compared to the control group arthroplasty patients showed a 40% higher level of co-contraction during this interval (P = 0.009). Control subjects used higher extension moments, resulting in a higher net joint moment. Visual analysis revealed a timing difference between the MB and FB group. The FB group seems to co-contract approximately 20% later compared to the MB group. RA patients after total knee arthroplasty show a lower net knee joint moment and a higher co-contraction than controls indicating avoidance of net joint load and an active stabilization of the knee joint. MB and FB patients showed no difference in co-contraction levels, although timing in FB is closer to controls than MB subjects. Since visual analysis revealed a timing difference between the MB and FB group, this may express compensation by coordination. Rehabilitation programs for RA patients should include besides muscle strength training, elements of muscle-coordination training. © 2008 The Author(s).

Accurate estimates of hamstrings lengths are useful, for example, to facilitate planning for surgical lengthening of the hamstrings in patients with cerebral palsy. In this study, three models used to estimate hamstrings length (M1: Delp, M2: Klein Horsman, M3: Hawkins and Hull) were evaluated. This was done by determining whether the estimated peak semitendinosus, semimembranosus and biceps femoris long head lengths, as measured in eight healthy subjects, were constant over a range of hip and knee angles. The estimated peak hamstrings length depended on the model that was used, even with length normalized to length in anatomical position. M3 estimated shorter peak lengths than M1 and M2, showing that more advanced models (M1 and M2) are more similar. Peak hamstrings length showed a systematic dependence on hip angle for biceps femoris in M2 and for semitendinosus in M3, indicating that either the length was not correctly estimated, or that the specific muscle did not limit the movement. Considerable differences were found between subjects. Large inter-individual differences indicate that modeling results for individual subjects should be interpreted with caution. Testing the accuracy of modeling techniques using in vivo data, as performed in this study, can provide important insights into the value and limitations of musculoskeletal models. © 2008 Elsevier Ltd. All rights reserved.

Objective To investigate the effects of total-contact fitted carbon-composite knee-ankle-foot orthoses (KAFOs) on energy cost of walking in patients with former polio who normally wear a conventional leather/metal KAFO or plastic/metal KAFO. Design A prospective uncontrolled study with a multiple baseline and follow-up design. Follow-up measurements continued until 26 weeks after intervention. Participants Twenty adults with polio residuals (mean age 55 years). Intervention Each participant received a new carbon-composite KAFO, fitted according to a total-contact principle, which resulted in a rigid, lightweight and well-fitting KAFO. Outcome measures Energy cost of walking, walking speed, biomechanics of gait, physical functioning and patient satisfaction. Results The energy cost decreased significantly, by 8%, compared with the original KAFO. Furthermore, the incremention energy cost during walking with the carbon-composite KAFO was reduced by 18% towards normative values. An improvement in knee flexion, forward excursion of the centre of pressure, peak ankle moment, and timing of peak ankle power were significantly associated with the decrease in energy cost. Walking speed and physical functioning remained unchanged. Conclusion In patients with former polio, carbon-composite KAFOs are superior to conventional leather/metal and plastic/metal KAFOs with respect to improving walking efficiency and gait, and are therefore important in reducing overuse and maintaining functional abilities in polio survivors.

A practicable EMG-force model is evaluated for muscles about the knee. The model included envelope signal processing and a gain-dependency of knee angle and angular velocity. Six healthy subjects participated in the experiments. For calibration, maximal isokinetic contractions about the knee were performed on a dynamometer with recordings of knee joint movement, net moment and EMG of thigh muscles. The model parameters were fitted on these calibration experiments. For validation, estimated moments from the EMG levels were compared to the actual exerted moments of two independent isokinetic contractions. Averaged RMS values of the difference ranged from 11 to 20% of the actual exerted moment. For isokinetic concentric contractions, the present model is suitable as a method to estimate muscle moments.

Objective. To investigate whether a simple electromyography–force (EMG-force) model can be used to measure different levels of co-contraction about the knee for healthy subjects and patients with an anterior cruciate ligament deficiency. Design. To evaluate an EMG-to-force processing model, two groups of subjects, with and without deficiency of the anterior cruciate ligament, participated in experiments in which surface EMG, kinematics and kinetics about the knee were recorded during isokinetic and functional movements. Background. Clinical and biomechanical evidence supports the hypothesis that higher level of co-contraction of quadriceps and hamstrings provide an active stabilization of the knee to compensate for the lost anterior cruciate ligament. To quantify the level of co-contraction, the contribution of both agonist and antagonist muscles to the net joint moment must be known. Methods. Surface EMG levels were calibrated to moment by means of a limited number of isokinetic contractions about the knee. With these calibration values, an estimate of the muscle moments during a vertical jump were obtained and compared with the net joint moment, calculated with inverse dynamics. Also co-contraction indices were determined. Results. The EMG–force model provided a fair estimate of the net joint moment. The co-contraction index in anterior cruciate ligament deficient subjects was significantly higher (mean 0.54 (SD, 0.04)) compared to healthy subjects (mean 0.25 (SD, 0.07)). Conclusion. Although the EMG-to-force processing model is not perfectly accurate, it is appropriate within a clinical context. Relevance Previous research supports the hypothesis that subjects with an anterior cruciate ligament deficiency compensate the loss of passive stability by developing higher co-activation levels of the knee muscles, i.e. active stabilization. Quantifying co-contraction may serve as a valuable parameter to evaluate clinical interventions and rehabilitation processes. The EMG–force model presented in this study appears to be a useful instrument for this purpose.

To compare muscle endurance in adolescents with spastic cerebral palsy (CP) with typically developing (TD) peers using a submaximal repetitions-to-fatigue (RTF) protocol.Cross sectional.Human motion laboratory.Adolescents with spastic CP (n=16; Gross Motor Function Classification System levels I or II) and TD adolescents (n=18) within the age range of 12 to 19 years old.Not applicable.Each participant performed 3 RTF tests at different submaximal loads, ranging from 50% to 90% of their maximal voluntary knee extension torque. The relation between the number of repetitions (repetition maximum [RM]) and imposed submaximal relative (percent of maximal voluntary torque [%MVT]) and absolute (Nm/kg) torque was quantified. To compare adolescents with CP with TD adolescents, a mixed linear model was used to construct load endurance curves. Surface electromyography of quadriceps muscles was measured to assess changes in normalized amplitude and median frequency (MF) as physiological indicators of muscle fatigue.Adolescents with CP showed a larger decrease in %MVT per RM than TD adolescents (P.05). TD adolescents showed substantial higher absolute (Nm/kg) load endurance curves than adolescents with CP (P.001), but they did not show a difference in slope. Electromyographic normalized amplitude increased significantly (P.05) in the quadriceps muscles in all tests for both groups. Electromyographic MF decreased significantly (P.05) in tests with the low and medium loads. Electromyographic responses did not differ between groups, indicating that similar levels of muscle fatigue were reached.Adolescents with CP show slightly lower muscle endurance compared with TD adolescents on a submaximal RTF protocol, which is in contrast with earlier findings in a maximal voluntary fatigue protocol. Accordingly, adolescents with CP have a reduced capacity to endure activities at similar relative loads compared with TD adolescents.

In guided leg movements (e.g., in cycling or wheelchair propulsion), the kinematics of a limb are determined by the object on which a force is applied. As a consequence, the force direction can vary and may deviate from the movement direction, that is, the effective direction. In the present study, the relation of effective force application and maximal power output was examined. Subjects (n = 5) performed guided leg tasks on a special dynamometer. They were instructed to exert a maximal force against a moving forceplate in the direction of the movement, as if they were pushing the plate away. Three different movement directions were tested: perpendicular to the horizontal, rotated 30 degrees backward, and rotated 30 degrees forward. For each trial, force and position data were recorded. The results of the experiments showed that in the extreme movement directions (both 30 degrees conditions), the force vector deviated significantly from the direction of the movement. Apparently, maximal power output was achieved with a low force effectiveness in these tasks. The background of this phenomenon was revealed by using the kinematics of one of these tasks in a simulation model. The stimulation level of 6 leg muscles was optimized toward a maximal effective force component (a) without a constraint on the direction of the total force or (b) with a constraint on the force component perpendicular to the effective force. The muscle stimulation pattern that resulted in the highest effective force coincided with a low force effectiveness. Apparently, this is a prerequisite for maximal power transfer from the muscles to the plate in these guided movements.

In this study, two different strategies of rising from a chair were compared, using integrated biomechanical and electromyographic analyses. Nine healthy subjects were instructed to rise using two different strategies: natural sit-to-stand transfer (NSTS) and a sit-to-stand transfer with full flexion of the trunk (FSTS). Sagittal kinematics and ground reaction forces were registered. Muscle activity of nine muscles of the right leg were recorded by means of surface EMG. All signals were synchronized at seat-off. The results show that no differences occur between the kinematics of knee and ankle, whereas the hip flexion is, as expected, higher during FSTS. The higher moment about the knee during NSTS is shifted to proportionally higher moments about the hip and ankle during FSTS. It is mainly the differences in the EMG-levels of the biarticular hip and knee muscles which might explain the differences in net moment. These results are in accordance with a theory about a particular role of biarticular muscles. On the other hand, the shift from knee to ankle cannot be associated with a particular increase in activity of the biarticular m.gastrocnemius. It is hypothesized that about the ankle, control of stability is preferred over movement control. An important conclusion for rehabilitation medicine is that a lower net moment about the knee in FSTS does not automatically imply that this reduces the load on the knee extensors.

PURPOSE: To determine the reproducibility and validity of video screen measurement (VSM) of sagittal plane joint angles during gait. METHODS: 17 children with spastic cerebral palsy walked on a 10m walkway. Videos were recorded and 3d-instrumented gait analysis was performed. Two investigators measured six sagittal joint/segment angles (shank, ankle, knee, hip, pelvis, and trunk) using a custom-made software package. The intra- and interrater reproducibility were expressed by the intraclass correlation coefficient (ICC), standard error of measurements (SEM) and smallest detectable difference (SDD). The agreement between VSM and 3d joint angles was illustrated by Bland-Altman plots and limits of agreement (LoA). RESULTS: Regarding the intrarater reproducibility of VSM, the ICC ranged from 0.99 (shank) to 0.58 (trunk), the SEM from 0.81 degrees (shank) to 5.97 degrees (trunk) and the SDD from 1.80 degrees (shank) to 16.55 degrees (trunk). Regarding the interrater reproducibility, the ICC ranged from 0.99 (shank) to 0.48 (trunk), the SEM from 0.70 degrees (shank) to 6.78 degrees (trunk) and the SDD from 1.95 degrees (shank) to 18.8 degrees (trunk). The LoA between VSM and 3d data ranged from 0.4+/-13.4 degrees (knee extension stance) to 12.0+/-14.6 degrees (ankle dorsiflexion swing). CONCLUSION: When performed by the same observer, VSM mostly allows the detection of relevant changes after an intervention. However, VSM angles differ from 3d-IGA and do not reflect the real sagittal joint position, probably due to the additional movements in the other planes

Background The calf muscles of children with cerebral palsy are often spastic, which can lead to an equinus gait pattern. Although spasticity is defined as a velocity-dependent increase in muscle tone, very little is known about the effect of walking speed on muscle–tendon behavior of spastic muscles during gait. The aim of this study was to investigate gastrocnemius and soleus length and lengthening velocity during gait in spastic muscles with and without static contractures compared to non-spastic muscles, as well as the effect of walking speed, and the interacting effect of walking speed and spasticity on muscle–tendon length and lengthening velocity. Methods Seventeen ambulatory children with spastic cerebral palsy and 11 typically developing children, aged 6–12, walked at comfortable, slow, and fast walking speeds. 3D kinematic data were collected and muscle–tendon lengths and velocities were calculated using musculoskeletal modeling. Spasticity and contractures of calf muscles were measured during standardized physical examination. Findings Spastic calf muscles showed a deviating muscle–tendon length pattern with two peaks in stance, which was found to be irrespective of muscle contracture. This deviating pattern became more pronounced as walking speed increased. In swing, spastic calf muscles were stretched approximately one third slower than normal, while in stance, spastic calf muscles were stretched twice as fast as normal, with peak velocity occurring earlier in the gait cycle. Interpretation The increasingly deviating muscle–tendon length pattern at faster walking speed indicates a velocity-dependent spasticity effect. This impairs walking especially at faster speeds, and may therefore limit comfortable walking speed.

Children with cerebral palsy often walk with reduced knee extension in terminal swing, which can be associated with short length or slow lengthening velocity of hamstrings muscles during gait. This study investigated the role of two factors that may contribute to such short and slow hamstrings: walking speed and spasticity. 17 children with spastic cerebral palsy and 11 matched typically developing children walked at comfortable, slow, and fast walking speed. Semitendinosus muscle-tendon length and velocity during gait were calculated using musculoskeletal modeling. Spasticity of the hamstrings was tested in physical examination. Peak hamstrings length increased only slightly with walking speed, while peak hamstrings lengthening velocity increased strongly. After controlling for these effects of walking speed, spastic hamstrings acted at considerably shorter length and slower velocity during gait than normal, while non-spastic hamstrings did not (all P < 0.001). These data are important as a reference for valid interpretation of hamstrings length and velocity data in gait analyses at different walking speeds. The results indicate that the presence of spasticity is associated with reduced hamstrings length and lengthening velocity during gait, even at constant walking speed.

Upper extremity (UX) movement analysis by means of 3D kinematics has the potential to become an important clinical evaluation method. However, no standardized protocol for clinical application has yet been developed, that includes the whole upper limb. Standardization problems include the lack of a single representative function, the wide range of motion of joints and the complexity of the anatomical structures. A useful protocol would focus on the functional status of the arm and particularly the orientation of the hand. The aim of this work was to develop a standardized measurement method for unconstrained movement analysis of the UX that includes hand orientation, for a set of functional tasks for the UX and obtain normative values. Ten healthy subjects performed four representative activities of daily living (ADL). In addition, six standard active range of motion (ROM) tasks were executed. Joint angles of the wrist, elbow, shoulder and scapula were analyzed throughout each ADL task and minimum/maximum angles were determined from the ROM tasks. Characteristic trajectories were found for the ADL tasks, standard deviations were generally small and ROM results were consistent with the literature. The results of this study could form the normative basis for the development of a 'UX analysis report' equivalent to the 'gait analysis report' and would allow for future comparisons with pediatric and/or pathologic movement patterns. © 2007 Elsevier B.V. All rights reserved.

The purpose of this study was to explore how origin-insertion length and lengthening velocity of hamstring and psoas muscle change as a result of crouch gait. The second purpose was to study the effect of changes in walking speed, in crouch, on muscle lengths and velocities. Eight healthy female subjects walked on a treadmill both normally and in crouch. In the crouch condition, subjects walked at three different walking speeds. 3D kinematic data were collected and muscle lengths and velocities were calculated using musculoskeletal modeling. It was found that voluntary walking in crouch resulted in shorter psoas length compared to normal, but not in shorter hamstrings length. Moreover, crouch gait did not result in slower muscle lengthening velocities compared to normal gait. These results do not support the role of hamstrings shortness or spasticity in causing crouch gait. Decreasing walking speed clearly reduced muscle lengths and lengthening velocities. Therefore, patients with short or spastic muscles are more likely to respond by walking slower than by walking in crouch. Also, differences in walking speed should be avoided as a confounding factor when comparing patient groups with controls.

Positions of the shoulder joint are commonly described in terms of degrees of humeral elevation in the principal planes. This approach is inadequate for an accurate and unambiguous description of functional arm movements that are not confined to these planes. In this paper, a general unambiguous method for describing shoulder positions is adopted and visualized in globe graphs. This facilitates the use and interpretation of the method in clinical practice. To illustrate this globe system of description, a healthy subject participated in the experiments. The shoulder position is described for several functional and standardized tasks for the upper limb with three angles: (1) the angle of the plane of elevation, (2) the angle of elevation within the plane of elevation, and (3) the angle of axial rotation. With these parameters, the position of the upper arm can be visualized as a position on a “globe” about the shoulder joint. Although not perfect, the globe system provides the most unambiguous description of functional thoracohumeral positions, which is easy to apply in clinical practice.

M.M. Eken1,2,∗, H. Houdijk2,3, C.A.M. Doorenbosch1,4, H. Dekkers2, J.G. Becher1, A.J. Dallmeijer1 1 Department of Rehabilitation Medicine, MOVE Research Institute Amsterdam, VU University Medical Center, Amsterdam, The Netherlands 2 Heliomare Rehabilitation, Research and Development, Wijk aan Zee, The Netherlands 3 MOVE Research Institute Amsterdam, Faculty of Human Movement Science, VU University, Amsterdam, The Netherlands 4 Academy of Human Kinetic Technology, University of Applied Sciences, The Hague, The Netherlands

A cross-sectional, descriptive study of shoulder movements conducted on nonimpaired subjects.To investigate whether a single functional rotation axis about the shoulder complex can be determined during elevation in the coronal or sagittal planes, and to identify their location.Accurate measurement of isokinetic torques about a joint requires alignment of the dynamometer axis with an assumed rotation axis of the joint. To assess shoulder function on a dynamometer, the location of a single rotation axis is not evident because the shoulder joint motion is based on several anatomical joints. Therefore, the rotation axis where humerothoracic movements occur should be judged as a functional rotation axis.During slow elevation movements in the sagittal and coronal plane, the position of the epicondyle and acromion were recorded with a motion analysis system. The motion trajectory of the elbow coordinates was fitted to a circle and considered an estimate of the functional shoulder joint rotation axis in the specified plane.The fitted trajectory appeared to be very accurate (root-mean-square error2%; N = 7). In the sagittal plane, the estimated functional rotation axis was found at the humeral head; in the coronal plane, it was located about 13 centimeters medial relative to the acromion.The shoulder complex of nonimpaired subjects can act as a hinge joint of the upper arm relative to the thorax during elevation in each measured plane.

From previous inverse dynamic analyses of human leg extensions, it was hypothesized that the underlying processes for the activation of mono- and biarticular muscles are different; the mono-articular muscles being activated when they shortened, whereas the biarticular muscles appeared responsible for the control of the external force direction. In the present study, experiments were performed on a dynamometer which was especially developed to test this hypothesis. Subjects had to exert different prescribed force vectors on a moving force-plate during leg extension, which they had intensively practised prior to the actual experiments. Of each trial, position, force and EMG activity were recorded. Net joint torques were calculated by the method of inverse dynamics and related to the EMG-patterns of the mono- and biarticular upper leg muscles to reveal whether the previously observed different roles in contact tasks might constitute a general principle in motor control. The results showed that although the action of the biarticular m. rectus femoris and hamstrings muscles was consistent in controlling the direction of the external force, the actions of the mono-articular muscles did not agree with their hypothesized role as simple work generators. The generalizability of a different control for mono- and biarticular muscles could thus not be confirmed for these tasks. They might rather reflect one out of more available strategies the CNS can use to control different contact control tasks.

The role of lower leg muscles is investigated during contact control tasks, in which the external force, applied by the foot on the surface, has to be controlled. Force, position and muscle activation were recorded. All subjects showed a stereotyped activation pattern to accomplish the tasks. The torques about hip and knee changed by a reciprocal activation pattern of the upper leg muscles, whereas the ankle torque remained remarkably constant as a result of a strong co-activation of the ankle muscles.

Children with cerebral palsy frequently experience foot dragging and tripping during walking due to a lack of adequate knee flexion in swing (stiff-knee gait). Stiff-knee gait is often accompanied by an overly flexed knee during stance (crouch gait). Studies on stiff-knee gait have mostly focused on excessive knee muscle activity during (pre)swing, but the passive dynamics of the limbs may also have an important effect. To examine the effects of a crouched posture on swing knee flexion, we developed a forward-dynamic model of human walking with a passive swing knee, capable of stable cyclic walking for a range of stance knee crouch angles. As crouch angle during stance was increased, the knee naturally flexed much less during swing, resulting in a 'stiff-knee' gait pattern and reduced foot clearance. Reduced swing knee flexion was primarily due to altered gravitational moments around the joints during initial swing. We also considered the effects of increased push-off strength and swing hip flexion torque, which both increased swing knee flexion, but the effect of crouch angle was dominant. These findings demonstrate that decreased knee flexion during swing can occur purely as the dynamical result of crouch, rather than from altered muscle function or pathoneurological control alone. © 2010 The Author(s).