Gerhard Staude

Onset detection in surface electromyographic signals: a systematic comparison of methods

Various methods to determine the onset of the electromyographic activity which occurs in response to a stimulus have been discussed in the literature over the last decade. Due to the stochastic characteristic of the surface electromyogram (SEMG), onset detection is a challenging task, especially in weak SEMG responses. The performance of the onset detection methods were tested, mostly by comparing their automated onset estimations to the manually determined onsets found by well-trained SEMG examiners. But a systematic comparison between methods, which reveals the benefits and the drawbacks of each method compared to the other ones and shows the specific dependence of the detection accuracy on signal parameters, is still lacking. In this paper, several classical threshold-based approaches as well as some statistically optimized algorithms were tested on large samples of simulated SEMG data with well-known signal parameters. Rating between methods is performed by comparing their performance to that of a statistically optimal maximum likelihood estimator which serves as reference method. In addition, performance was evaluated on real SEMG data obtained in a reaction time experiment. Results indicate that detection behavior strongly depends on SEMG parameters, such as onset rise time, signal-to-noise ratio or background activity level. It is shown that some of the threshold-based signal-power-estimation procedures are very sensitive to signal parameters, whereas statistically optimized algorithms are generally more robust.

Precise onset detection of human motor responses using a whitening filter and the log-likelihood-ratio test

Investigation of the human motor system frequently requires precise determination of the motor response onset indicating the time of movement initiation (e.g., in reaction time experiments). This paper presents a new model-based algorithm for computerized response onset detection in kinematic signals (e.g., joint angle). The response onset is identified as an abrupt change in the (time-varying) parameters of a statistical process model adapted to the measured signal. The accuracy of the algorithm is assessed by statistical simulations, and the performance of the method is compared to the performance of conventional onset detection methods using simulated as well as real kinematic signals. Results show that onset detection can substantially be improved by including a priori knowledge on the physiological background of the measured signals to the decision process.

Steps towards a miniaturized, robust and autonomous measurement device for the long-term monitoring of patient activity: ActiBelt.

Abstract We describe the first steps in the development of a wearable measurement device for measuring a subjects three-dimensional acceleration. The ultimate aim is a standard measurement instrument integrated in a belt buckle that allows objective evaluation of treatment and rehabilitation measures in patients, in particular for disabling chronic diseases such as multiple sclerosis. In a first step we combined standard hardware elements to record test data from healthy volunteers. We then developed algorithms to automatically distinguish between different stages of activity, such as jogging, walking, lying, standing and sitting, and to detect and count steps. Distinction between standing and sitting is the most difficult to accomplish. As a first validation, we calculated the distance traveled from data of 17 experiments and a total of 4.5 h, for which one proband was walking and running for a known distance, and compared the results with two commercially available pedometers. We could show that the relative error for the ActiBelt is only half of that for the two pedometers. Apart from developing much smaller, robust and integrated hardware, we describe ideas on how to develop algorithms that allow extraction of a “baseline step pattern” in analogy to baseline ECG to define and detect clinically relevant deviations.

The discontinuous nature of motor execution

Abstract. Human movement control requires adequate coordination of different movements, which is particularly important when different motor tasks are simultaneously executed by the same effector(s) (e.g. a muscle or a joint). The process of movement execution involves a series of highly nonlinear elements; for instance, a motor unit of a muscle produces force only in the direction of muscle shortening, thus representing a threshold operator that transforms the bipolar (i.e. excitatory or inhibitory) information at its spinal input into a purely unipolar signal (i.e. muscle force). This tripartite research report addresses the contribution of the nonlinearity of neuromuscular elements to the coordination of different motor tasks simultaneously executed by the same limb. In this first part of the series, a new hypothesis for such a single-muscle multiple-task coordination is presented which suggests an essentially threshold-linear coordination mechanism. Control signals generated by the central nervous system for each individual movement independently and feedback information from peripheral receptors are linearly superimposed. This compound control/feedback signal is processed by a nonlinear limiter element reflecting the discontinuous properties of the muscle and its reflex circuitry. It is shown that threshold-linear interaction of descending commands and afferent feedback information can lead to complex interdependent patterns of compound motor action. This includes the possibility of gating (i.e. the ability of one movement pattern to constrain or even impede the execution of another pattern) and of delayed response initiation when simultaneously performing more than one voluntary motor task. A theoretical analysis of the threshold-linear coordination mechanism and an extensive experimental validation of the model is provided in part II and part III of the report.

The discontinuous nature of motor execution II. Merging discrete and rhythmic movements in a single-joint system – the phase entrainment effect

Abstract. Initiation of rapid discrete flexion movements is significantly altered when a secondary rhythmic movement is performed simultaneously with the same limb; the onset of a stimulus-evoked discrete movement tends to occur time-locked to the oscillation: i.e., the rhythmic movement entrains the discrete response. This nonlinear interaction may reflect a specific principle of coordination of motor tasks which are simultaneously executed with the same effector. This part II of a tripartite research report on such single-muscle multiple-task coordination investigates the contribution of the dynamic properties of the muscle and its reflex circuitry to phase entrainment. Assuming a simple threshold-linear relationship between the control signals generated by the central nervous system and the observable kinematic and electromyographic signals, a secondary rhythmic movement will cause an additional phase-dependent delay between the central “go” command and the first observable change in actual kinematics of the compound movement. Several indicators for such threshold-linear interaction are derived and tested on real data obtained in psychophysical experiments. Four healthy subjects performed rapid lateral abductions of the index finger in response to a visual “go” signal. During a portion of the experiments, subjects produced additional low-amplitude oscillatory movements before stimulus presentation with either the same finger (one-handed task), or with the index finger of the other hand (two-handed task). Results showed phase entrainment and modulation of reaction times when the cyclic and the discrete movements were simultaneously executed by the same finger. But there was no entrainment in the bimanual execution of the tasks. The model was capable of reproducing the observed effects. It is concluded that coordination of voluntary movements which are concurrently performed by the same effector involves specific discontinuous operations, which represents an essential part of the mechanism of motor coordination. Phase entrainment reflects this characteristic discontinuous behavior of the lower stages of motor execution and does not necessarily require nonlinear interaction of motor commands at higher levels of motor processing.

A Three-Component Model of the Control Error in Manual Tracking of Continuous Random Signals

Objective: The performance of human operators acting within closed-loop control systems is investigated in a classic tracking task. The dependence of the control error (tracking error) on the parameters display gain, kdisplay, and input signal frequency bandwidth, fg, which alter task difficulty and presumably the control delay, is studied with the aim of functionally specifying it via a model. Background: The human operator as an element of a cascaded human–machine control system (e.g., car driving or piloting an airplane) codetermines the overall system performance. Control performance of humans in continuous tracking has been described in earlier studies. Method: Using a handheld joystick, 10 participants tracked continuous random input signals. The parameters fg and kdisplay were altered between experiments. Results: Increased task difficulty promoted lengthened control delay and, consequently, increased control error. Tracking performance degraded profoundly with target deflection components above 1 Hz, confirming earlier reports. Conclusion: The control error is composed of a delay-induced component, a demand-based component, and a novel component: a human tracking limit. Accordingly, a new model that allows concepts of the observed control error to be split into these three components is suggested. Application: To achieve optimal performance in control systems that include a human operator (e.g., vehicles, remote controlled rovers, crane control), (a) tasks should be kept as simple as possible to achieve shortest control delays, and (b) task components requiring higher-frequency (>1 Hz) tracking actions should be avoided or automated by technical systems.

Dual-tasking: Is manual tapping independent of concurrently executed saccades?

Maintaining both spatial and temporal accuracy of concurrent motor actions is a challenging behavioral requirement in multi-tasking, where possible resource bottlenecks may become apparent when these units are shared between tasks. This study addresses the question of whether periodic self-paced finger movements (tapping) compulsorily interact with concurrently executed saccades, because they share some common neural control pathways. We employed a dual-task paradigm which was previously used to demonstrate strong interference between independent but concurrently conducted bimanual tapping tasks (Wachter, C., Cong, D.K., Staude, G., Wolf, W., 2008. Coordination of a discrete response with periodic finger tapping, additional experimental aspects for a subtle mechanism. J. Motor Behav. 40, 417-432). Instead of the discrete left hand response, the 13 participants now executed a single saccadic eye movement to a fixed visual target in parallel to continuous periodic tapping of the dominant hand. We expected these reactive saccades to act as a strong perturbation event to the continuous tapping, but the experimental data did not reveal a considerable interference in this specific oculo-manual dual-task experiment.

Automatic event detection in surface EMG of rhythmically activated muscles

Precise detection of discrete events in the surface electromyogram (EMG) like the phasic change in the activity pattern associated with the initiation of a rapid motor response is an important issue in the analysis of the human motor system. However, accurate detection is difficult when the muscle is already involved in a secondary motor task, and two superimposed activation patterns have to be separated. This paper describes a method which allows automatic detection of phasic events that arise during simultaneous execution of rhythmical and discrete motor tasks by the same muscle. Based on a nonlinear signal model, events are identified as characteristic changes in the variance of the original EMG signal by using a two-window scheme and the generalized likelihood ratio test. Both, the beginning as well as the end of epochs with prominent EMG activity related to the rhythmical movement and the onset of phasic activity indicating initiation of a discrete contraction can be detected. Problems arising from modelling the EMG by a sequence of independent Gaussian random variables modulated by a deterministic control pattern are discussed.

Coordination of a Discrete Response With Periodic Finger Tapping: Additional Experimental Aspects for a Subtle Mechanism

The authors investigated the coordination of periodic right-hand tapping with single stimulus-evoked discrete lefthand taps to check for task interactions and a possible relationship between phase resetting (see tapping literature; e.g., J. Yamanishi, M. Kawato, & R. Suzuki, 1979) and phase entrainment (see tremor literature; e.g., R. J. Elble, C. Higgins, & L. Hughes, 1994). The experimental paradigm employs a dual-task condition as used by K. Yoshino, K. Takagi, T. Nomura, S. Sato, and M. Tonoike (2002), and it includes normal tapping and isometric tapping with the authors recording finger positions and ground contact forces. Four different types of coordination schemes were observed in tapping behavior: marginal tapping interaction (MTI), periodic tap retardation (PTR), periodic tap hastening (PTH), and discrete tap entrainment (DTE); MTI and PTR correspond to the phase-resetting effect for the coordination of periodic tapping with single discrete taps. The novel aspect of the study described in this article includes the impact of the periodic tapping on the discrete tap timing and the hastening of the periodic tapping due to the discrete tap behaviors resulting in a synchronized execution of the two concurrent tapping tasks. All participants showed a dominant tapping behavior, but they all used the other nondominant forms of the four reported coordination schemes in some trials too, which reflects possible constraints of the sensorimotor system in handling two competing tasks.

A dynamic generator model for motor-related biosignals

Investigation of the human motor system frequently requires the precise determination of motor events indicating transitions between different motor processes currently activated. Frequently, off-line techniques are used for automatic detection of these events from the motor-related biosignals measured in experiments. However, little is known about the accuracy of these methods. This paper describes a new dynamic generator model for motor-related biosignals that fuses knowledge from both biomedical and technical disciplines in order to provide a solid basis for a quantitative assessment of the performance of detection methods. The model comprises higher level control mechanisms of voluntary movement as well as characteristic details of muscle function like its complex nonlinear feedback mechanisms. Further, important morphological and physiological aspects of signal generation and transmission that are peculiar for these signals are considered. The generator is equally suited for modelling surface electromyographic (EMG) signals as well as kinetic signals (e.g. joint angle, muscle force) associated with a particular movement. The performance of the model is demonstrated for the special case of superimposed discrete and rhythmical single-joint movements.