Sit-to-stand motor strategies investigated in able-bodied young and elderly subjects

For the execution of a certain motor task, a motor strategy is chosen by each individual among those that are consistent with the structural and functional constraints of his/her locomotor system, and that tends to maximise the effectiveness of the motor act. The identification of this strategy allows for the assessment of the individual's functional status. This study aimed at identifying the motor strategies adopted for the execution of the sit-to-stand motor task, at different speeds and initial postures, in a sample of 35 community-dwelling elders and in a sample of 16 young able-bodied individuals. This was done using a method, least perceivable to the test subject and "economical" for the experimenter, which entailed the recording of external forces only. A musculo-skeletal system model, based on a telescopic inverted-pendulum (TIP) moved by a linear and two rotational muscle-equivalent actuators, was then used. Parameters describing the kinematics and dynamics of these actuators were extracted and submitted to statistical analysis. Different motor strategies were identified in the two age groups, as well as associated with both a different initial posture (ankle dorsiflexion angle) and speed of execution of the motor task. In particular, the elder group, as compared with the young group, prior to seat-off tended to flex the trunk more, thus bringing the CM closer to the base of support, and at a higher velocity, thus gaining a higher momentum. After seat-off, elders rotated the body forward and, only after having brought their CM over the base of support, effectively started elevation. Both global muscular effort and coordination effort associated with the achievement of balance and raising were lower. However, maximal speed was also lower. The above results indicated that the elders could count on a lower functional reserve than the young individuals and, from the methodological viewpoint, that the TIP approach is a good candidate for subject-specific functional evaluation in a clinical context.     

Task failure during fatiguing contractions performed by humans.

By comparing the physiological adjustments that occur when two similar fatiguing contractions are performed to failure, it is possible to identify mechanisms that limit the duration of the more difficult task. This approach has been used to study two fatiguing contractions, referred to as the force and position tasks, which differed in the type of feedback given to the subject and the amount of support provided by the surroundings. Even though the two tasks required a similar net muscle torque during submaximal isometric contractions, the duration that the position task could be sustained was consistently much briefer than that for the force task. The position task involved a greater rate of increase in EMG activity and more marked changes in motor unit recruitment and rate coding compared with the force task. These observations are consistent with the hypothesis that the motor unit pool was recruited more rapidly during the position task. The difference in motor unit behavior appeared to be caused by variation in synaptic input, likely involving heightened sensitivity of the stretch reflex during the position task. Upon repeat performances of the two fatiguing contractions, some subjects were able to increase the time to failure for the force task but not the position task. Furthermore, the time to failure for the position task could be influenced by the postural demands associated with maintaining the position of the limb, and the difference in the two durations was enhanced when the postural activity evoked a pressor response. These observations indicate that the difference in the duration of the two fatiguing contractions was attributable to differences in the control strategy used to sustain the tasks and the magnitude of the associated postural activity.     

Minimum measured-input models for the assessment of motor ability

The problem of assessing the physical functional limitation of a given individual and establishing the relationship between impairment/s and disability using a biomechanical approach is addressed. This endeavour was pursued with reference to the locomotor system and in order to address the following specific clinical issues: prognosis, eligibility for health services, measure of the outcome of a therapy, and therapeutic programming. A thorough biomechanical analysis of selected motor tasks would be effective but awkward to apply for subject-specific evaluation in clinical practice by reason of the complexity of both instrumentation and experimental protocols. In addition, as illustrated in the paper, the adequacy of the accuracy with which this type of analysis provides relevant information may be argued. Therefore, different methods were devised in the attempt to join objectivity with field applicability. These entailed the measurement of a minimum number of biomechanical variables during the execution of the selected motor task and these quantities were acquired using a low cost experimental apparatus least perceivable to the test subject, that is a dynamometric plate. However, since data thus obtained do not necessarily lend themselves to straightforward interpretation in terms of function assessment, models of the musculo-skeletal system that embodied the invariant aspects of both the modelled system and the specific motor task were devised. Using such "minimum measured-input models", physiology-related, and thus easier to interpret, information was obtained. Two different sets of mathematical models are presented: one deals with the lowest level of detail and normally aims at assessing a global physical performance score, the other discloses joint function and segmental mechanics and therefore contributes to establishing a relationship between impairment and disability. The validation of these models, carried out in the laboratory, has shown that they possess a potential for application in clinical practice.     

Impedance response characteristics of the primate Mucaca mulatta exposed to seated whole-body gz vibration.

A mathematical model was used to quantify and describe the variability in the mechanical impedance response of the Rhesus monkey subjected to vibrations in the range 3-20 Hz at 0.5 g peak acceleration. Due to the similarities in response, a two-mass, one-degree-of-freedom (DOF) model was selected and the associated mechanical parameters determined using a nonlinear least-squares optimization program. For the six tests conducted on each of the four subjects, appreciable parameter variations were observed within a subject; however, the majority of the mean parameter values among different subjects and among the repeated tests on the population were within +/- 1 S.D. of each other. Significant differences were observed in the stiffness coefficient and the total mass among different subjects, and in the mass ratio (between inert and sprung masses) among the repeated tests. Variations in the profile shapes following resonance were described and limited by changes in the mass ratio and the damping factor. Higher mass ratios (greater than 1.0) were associated with lower damping factors (less than 0.50). The impedance response beyond resonance approached the response described by the impedance of the inert mass and the damper elements of the model combined in parallel, and supported the assumption that the lower torso was rigidly attached to the seat. Physically, the reactive force produced by the upper torso increasingly diminished following resonance, due to the load transmission/attenuation characteristics of the spinal structures at 0.5 g peak acceleration. The impedance measured at the seat becomes dominated by the transmitted damping force associated with the spine and the force generated by the rigid lower-torso mass.     

Slow motor neuron stimulation of locust skeletal muscle: model and measurement

The isometric force response of the locust hind leg extensor tibia muscle to stimulation of a slow extensor tibia motor neuron is experimentally investigated, and a mathematical model describing the response presented. The measured force response was modelled by considering the ability of an existing model, developed to describe the response to the stimulation of a fast extensor tibia motor neuron and to also model the response to slow motor neuron stimulation. It is found that despite large differences in the force response to slow and fast motor neuron stimulation, which could be accounted for by the differing physiology of the fibres they innervate, the model is able to describe the response to both fast and slow motor neuron stimulation. Thus, the presented model provides a potentially generally applicable, robust, simple model to describe the isometric force response of a range of muscles.     

A Wavelet-Based Method to Predict Muscle Forces From Surface Electromyography Signals in Weightlifting

The purpose of this study was to develop a wavelet-based method to predict muscle forces from surface electromyography (EMG) signals in vivo.The weightlifting motor task was implemented as the case study.EMG signals of biceps brachii,triceps brachii and deltoid muscles were recorded when the subject carried out a standard weightlifting motor task.The wavelet-based algorithm was used to process raw EMG signals and extract features which could be input to the Hill-type muscle force models to predict muscle forces.At the same time,the musculoskeletal model of subject's weightlifting motor task was built and simulated using the Computed Muscle Control (CMC) method via a motion capture experiment.The results of CMC were compared with the muscle force predictions by the proposed method.The correlation coefficient between two results was 0.99(p＜0.01).However,the proposed method was easier and more efficiency than the CMC method.It has potential to be used clinically to predict muscle forces in vivo.     

Measuring dynamic stability requirements during sitting pivot transfers using stabilizing and destabilizing forces in individuals with complete motor paraplegia

Dynamic stability requirements have never been quantified when long-term manual wheelchair users transfer themselves in a seated position from an initial surface to a target surface, a functional task commonly referred to as sitting pivot transfers (SPTs). Ten individuals with spinal cord injury (SCI), who rely on a manual wheelchair for mobility, underwent a comprehensive biomechanical SPT assessment. SPTs performed toward a target seat of same height (even) and a seat 10cm higher than the initial seat (uneven), repeated three times for each task, were assessed. A dynamic equilibrium model, continuously measuring the theoretical forces required to move the center of pressure to the limit of the base of support (destabilizing force) and to neutralize the kinetic energy and stop the displacement of the center of mass at the limit of the base of support (stabilizing force) at each instance during the performance of SPTs, was used to identify the phases of greatest instability during the SPT tasks. The greatest levels of instability were reached around the time the buttocks lost contact with the initial seat and around the time the buttocks landed on the target seat (pre- and post-lift transition phases). These transition periods, characterized by the lowest destabilizing force (424.7-487.1N) and the greatest stabilizing force (24.2-33.2N), confirmed the greatest level of instability. The height of the target seat had no significant effect (p=0.278-0.739) on dynamic postural stability requirements during the SPTs. During SPTs towards even and uneven target seats, the greatest postural instability occurs during the transition phases in individuals with complete motor thoracic SCI.     

Postural dynamics in maximal isometric ramp efforts

 A global biomechanical model of transient push efforts is proposed where transient efforts are taken into consideration, with the aim to examine in greater depth the postural adjustments associated with voluntary efforts. In this context, the push effort is considered as a perturbation of balance, and the other reaction forces as a counter-perturbation which is necessary for the task to be performed efficiently. The subjects were asked to exert maximal horizontal two-handed isometric pushes on a dynamometric bar, as rapidly as possible. They were seated on a custom-designed device which measured global and partitive dynamic quantities. The results showed that the horizontal reaction forces and the horizontal displacement of the centre of pressure increased quasi-proportionally with the perturbation. In addition, it was established that vertical reaction forces increased at seat level whereas they decreased at foot level, resulting in minor vertical acceleration and displacement of the centre of gravity. On the contrary, the anteroposterior reaction forces increased both at foot and at seat levels. Based on a detailed examination of the various terms of the model, it is concluded that transient muscular effort induces dynamics of the postural chain. These observations support the view that there is a postural counter-perturbation which is associated with motor activity. More generally, the model helped to specify the effect of postural dynamic phenomena. It makes it possible to stress the importance of adherence at the contact level between the subject and the seat in the course of transient efforts. Received: 1 February 2001 / Accepted in revised form: 20 February 2002     

Consolidation of dynamic motor learning is not disrupted by rTMS of primary motor cortex

Motor skills, once learned, are often retained over a long period of time. However, such learning first undergoes a period of consolidation after practice. During this time, the motor memory is susceptible to being disrupted by the performance of another motor-learning task. Recently, it was shown that repetitive transcranial magnetic stimulation (rTMS) over the primary motor cortex could disrupt the retention of a newly learned ballistic task in which subjects had to oppose their index finger and thumb as rapidly as possible. Here we investigate whether the motor cortex is similarly involved during the consolidation that follows learning novel dynamics. We applied rTMS to primary motor cortex shortly after subjects had either learned to compensate for a dynamic force field applied to their index finger or learned a ballistic finger abduction task. rTMS severely degraded the retention of the learning for the ballistic task but had no effect on retention of the dynamic force-field learning. This suggests that, unlike learning of simple ballistic skills, learning of dynamics may be stored in a more distributed manner, possibly outside the primary motor cortex.     

Postural dynamics in the standing human

The purpose of this study was to develop a mathematical model of the linkage dynamics in upright standing, and to use this model to study output principles for postural control. The standing human was modelled in the sagittal plane as a three-segment linkage. Mechanical disturbances were simulated as forces which could be applied at various points in this linkage. An iterative approach was used to find joint torque combinations which would restore balance within 80 ms of these mechanical disturbances. The model predicted that a specific proportional relationship was necessary between the hip, knee and ankle torques in order for balance to be restored. This proportional relationship was shown to be a function of the model structure, but independent of the location, direction and amplitude of the disturbance. These predictions were tested experimentally. A disturbance apparatus was designed to apply an impulsive force to the subjects. The joint torque responses of the subjects were in quantitative agreement with the predictions of the model. The results suggest that a fixed relationship between joint torques may be required to restore balance, and this fixed relationship may make the task of postural control simpler for the nervous system.     

双任务事件中脑电信号的熵计算

采用复杂性分析中的样品熵算法,计算并分析了受试者在单任务事件以及双任务事件活动过程中的神经电生理数据.在利用样品熵算法对短时程（秒）脑电数据的复杂度和规则度进行计算之前,首先应用了代替数据分析法,以排除所分析的实验数据是由线性加随机部分构成.所有的实验数据分别在单任务和双任务等不同的生理条件下采集.其中单任务为一个听觉辨别任务;双任务有两种形式,分别为听觉任务和不同的震动任务的结合.计算结果显示,任何一种双任务过程中脑电信号的熵值都明显的低于单任务状态时脑电信号的熵值（P＜0.05～0.001）.研究表明对应于受试者仅仅进行单任务工作而言,当受试者处于双任务工作状态时大脑的神经信息传递可能会受到某种程度的削弱,神经信息流通的范围也可能更为孤立.结果进一步说明对于短时程（秒）脑电信号分析,样品熵算法是有效的非线性分析方法.     

Dependence of the motor response time in discrimination task on the character of visual information available

The interaction of visual and proprioceptive afferentation were studied in the motor task for discrimination of weights of falling objects. The availability of visual information reduced the time of motor response; however, the degree of shortening depended on the type of this information. The decrease in the response time was significantly greater when the subject saw the beginning of the real falling of object instead of having only visual information about the beginning of the fall. Thus, a subject solves the motor task for discrimination of weights of falling objects more efficiently when he sees the real beginning of the fall, rather than in the case when the subject receives only a visual signal at the moment when an electromagnet releases the object. This may be due to the fact that seeing the initial part of a real trajectory instead of an abstract signal about the beginning of the fall allows the subject to better predict the moment of the impact.     

A computational simulated control system for a high-force pneumatic muscle actuator: system definition and application as an augmented orthosis

High-force pneumatic muscle actuators (PMAs) are used for force assistance with minimal displacement applications. However, poor control due to dynamic nonlinearities has limited PMA applications. A simulated control system is developed consisting of: (1) a controller relating an input position angle to an output proportional pressure regulator voltage, (2) a phenomenological model of the PMA with an internal dynamic force loop (system time constant information), (3) a physical model of a human sit-to-stand task and (4) an external position angle feed-back loop. The results indicate that PMA assistance regarding the human sit-to-stand task is feasible within a specified PMA operational pressure range.     

Adaptive rehabilitation games

In conventional neuromuscular rehabilitation, patients are required to perform biomechanical exercises to recover their neuromotor abilities. These physiotherapeutic tasks are defined by the physiotherapist, according to his estimate of the patient’s pathologic neuromotor function. The definition of the task is mainly qualitative and it is often merely demonstrated to the patient as a gesture to reproduce. Success of the treatment relies then on the accuracy and repetition of the motor training.We propose a novel approach to neuromotor training by combining the advantages of a virtual reality platform with biofeedback information on the training subject from biometric equipment and with the computational power of artificial neural networks. In a calibration stage, the subject performs motor training on a known task to train the network. Once trained, the tuned network generates a new patient-specific task, based on the definition of the subject’s expected performance dictated by the therapist. The system was tested for upper limb rehabilitation on healthy subjects. We measured a 33% improvement in the triceps performance (p = 0.027). The novelty of the proposed approach lies in its use of learning systems to the estimation of biological models.     

A computational simulated control system for a high-force pneumatic muscle actuator: system definition and application as an augmented orthosis

High-force pneumatic muscle actuators (PMAs) are used for force assistance with minimal displacement applications. However, poor control due to dynamic nonlinearities has limited PMA applications. A simulated control system is developed consisting of: (1) a controller relating an input position angle to an output proportional pressure regulator voltage, (2) a phenomenological model of the PMA with an internal dynamic force loop (system time constant information), (3) a physical model of a human sit-to-stand task and (4) an external position angle feed-back loop. The results indicate that PMA assistance regarding the human sit-to-stand task is feasible within a specified PMA operational pressure range.     

Effect of muscle fatigue on internal model formation and retention during reaching with the arm.

The motor system adapts to novel dynamic environments by forming internal models that predict the muscle forces needed to move skillfully. The goal of this study was to determine how muscle fatigue affects internal model formation during arm movement and whether an internal model acquired while fatigued could be recalled accurately after rest. Twelve subjects adapted to a viscous force field applied by a lightweight robot as they reached to a target. They then reached while being resisted by elastic bands until they could no longer touch the target. This protocol reduced the strength of the muscles used to resist the force field by approximately 20%. The bands were removed, and subjects adapted again to the viscous force field. Their adaptive ability, quantified by the amount and time constant of adaptation, was not significantly impaired following fatigue. The subjects then rested, recovering approximately 70% of their lost force-generation ability. When they reached in the force field again, their prediction of the force field strength was different than in a nonfatigued state. This alteration was consistent with the use of a higher level of effort than normally used to counteract the force field. These results suggest that recovery from fatigue can affect recall of an internal model, even when the fatigue did not substantially affect the motor system's ability to form the model. Recovery from fatigue apparently affects recall because the motor system represents internal models as a mapping between effort and movement and relies on practice to recalibrate this mapping.     

Modular neuromuscular control of human locomotion by central pattern generator

The central pattern generators (CPG) in the spinal cord are thought to be responsible for producing the rhythmic motor patterns during rhythmic activities. For locomotor tasks, this involves much complexity, due to a redundant system of muscle actuators with a large number of highly nonlinear muscles. This study proposes a reduced neural control strategy for the CPG, based on modular organization of the co-active muscles, i.e., muscle synergies. Four synergies were extracted from the EMG data of the major leg muscles of two subjects, during two gait trials each, using non-negative matrix factorization algorithm. A Matsuoka׳s four-neuron CPG model with mutual inhibition, was utilized to generate the rhythmic activation patterns of the muscle synergies, using the hip flexion angle and foot contact force information from the sensory afferents as inputs. The model parameters were tuned using the experimental data of one gait trial, which resulted in a good fitting accuracy (RMSEs between 0.0491 and 0.1399) between the simulation and experimental synergy activations. The model׳s performance was then assessed by comparing its predictions for the activation patterns of the individual leg muscles during locomotion with the relevant EMG data. Results indicated that the characteristic features of the complex activation patterns of the muscles were well reproduced by the model for different gait trials and subjects. In general, the CPG- and muscle synergy-based model was promising in view of its simple architecture, yet extensive potentials for neuromuscular control, e.g., resolving redundancies, distributed and fast control, and modulation of locomotion by simple control signals.     

Normalized Index of Synergy for Evaluating the Coordination of Motor Commands

Humans perform various motor tasks by coordinating the redundant motor elements in their bodies. The coordination of motor outputs is produced by motor commands, as well properties of the musculoskeletal system. The aim of this study was to dissociate the coordination of motor commands from motor outputs. First, we conducted simulation experiments where the total elbow torque was generated by a model of a simple human right and left elbow with redundant muscles. The results demonstrated that muscle tension with signal-dependent noise formed a coordinated structure of trial-to-trial variability of muscle tension. Therefore, the removal of signal-dependent noise effects was required to evaluate the coordination of motor commands. We proposed a method to evaluate the coordination of motor commands, which removed signal-dependent noise from the measured variability of muscle tension. We used uncontrolled manifold analysis to calculate a normalized index of synergy. Simulation experiments confirmed that the proposed method could appropriately represent the coordinated structure of the variability of motor commands. We also conducted experiments in which subjects performed the same task as in the simulation experiments. The normalized index of synergy revealed that the subjects coordinated their motor commands to achieve the task. Finally, the normalized index of synergy was applied to a motor learning task to determine the utility of the proposed method. We hypothesized that a large part of the change in the coordination of motor outputs through learning was because of changes in motor commands. In a motor learning task, subjects tracked a target trajectory of the total torque. The change in the coordination of muscle tension through learning was dominated by that of motor commands, which supported the hypothesis. We conclude that the normalized index of synergy can be used to evaluate the coordination of motor commands independently from the properties of the musculoskeletal system.     

The influence of different lower seat height on the muscular stress during squatting for a ground level job

Work requiring extremely body flexion is strongly associated with a high incidence of musculoskeletal injuries often reported during adopting squatting. In this study, the influence of different lower seat heights on the muscular stress in squatting on a stool (SS) were examined in comparison with fully squatting (FS). Fourteen healthy Indonesian males were recruited in the experiment. Two-dimensional body kinematics, ground reaction force (GRF) and electromyography (EMG) data were collected as subjects performed forward movement under four squatting height conditions which were FS and SS at 10 cm, 15 cm and 20 cm seat height. The results demonstrated that the change from FS to SS primarily affected the segmental angular flexions and muscular activities in the upper and lower limbs. GRF data showed that the SS conditions delivered 24% body weight onto the seat. The change of FS to SS showed significantly decrease in muscular load of the rectus femoris and tibialis anterior. In contrast, the soleus and gastrocnemius increased the activities as the seat height increased. The type of task that required the hand to handle the object on the ground level affected the trunk to be more flexed as the seat height increased. The findings of this study suggest that the use of a lower seat stool of a proper height seems to be a sub-optimal solution considering the change of muscular load associated with the discomfort in a squatting posture.     

Prediction precedes control in motor learning

Skilled motor behavior relies on the brain learning both to control the body and predict the consequences of this control. Prediction turns motor commands into expected sensory consequences, whereas control turns desired consequences into motor commands. To capture this symmetry, the neural processes underlying prediction and control are termed the forward and inverse internal models, respectively. Here, we investigate how these two fundamental processes are related during motor learning. We used an object manipulation task in which subjects learned to move a hand-held object with novel dynamic properties along a prescribed path. We independently and simultaneously measured subjects' ability to control their actions and to predict their consequences. We found different time courses for predictor and controller learning, with prediction being learned far more rapidly than control. In early stages of manipulating the object, subjects could predict the consequences of their actions, as measured by the grip force they used to grasp the object, but could not generate appropriate actions for control, as measured by their hand trajectory. As predicted by several recent theoretical models of sensorimotor control, our results indicate that people can learn to predict the consequences of their actions before they can learn to control their actions.