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Simulation of equinus gait and effects of ankle stiffness compensation with a neuromusculoskeletal model

Master thesis (2020) - F. Santini, W. Mugge, Jurriaan H. De Groot, M. Stijntjes, K.E. Rodriguez Hernandez, L. Peternel, Mohammad J. Mirzaali, A.C. Schouten

Introduction: Pes equinus with increased ankle joint stiffness is a common impairment in stroke and Cerebral Palsy (CP) patients with structural and/or neural deficits of the ankle muscles. Equinus gait is characterized by toe-strike, abnormal ankle plantarflexion and decreased a ...

Introduction: Pes equinus with increased ankle joint stiffness is a common impairment in stroke and Cerebral Palsy (CP) patients with structural and/or neural deficits of the ankle muscles. Equinus gait is characterized by toe-strike, abnormal ankle plantarflexion and decreased ankle range of motion (ROM). A new ankle-foot orthosis (AFO) with negative stiffness was previously developed to compensate for the increased ankle joint stiffness and improve equinus gait, reducing plantarflexion and increasing ankle ROM. The goal of this project was to predict the effects of ankle stiffness compensation on equinus gait simulations.

Methods: Forward simulations of unimpaired and equinus gait were generated in
the sagittal plane with a musculoskeletal model implemented in OpenSim and a
neural controller implemented in SCONE. After validation of the unimpaired gait
simulation, a sensitivity analysis on ankle kinematics was performed, introducing
structural and/or neural alterations of the ankle muscles to achieve an equinus gait. Consecutively, equinus gait simulations were validated with previously collected data of CP patients. Finally, an AFO model was developed in OpenSim to simulate AFO-assisted gait.

Results: The unimpaired gait simulation yielded realistic results, and was robust to all alterations, generating stable gaits. Shorter Gastrocnemius fiber length and/or increased plantarflexor muscles activations, with Tibialis Anterior weakness, had the largest effect on the ankle joint kinematics and resulted in realistic CP equinus gaits. Simulations of AFO-assisted gait resulted in reduced abnormal plantarflexion, and increased ankle ROM in the condition with shorter Gastrocnemius.

Discussion and Conclusion: This study presented realistic simulations of equinus
gait, by modelling structural and/or neural alterations of the ankle muscles and
predicted improved ankle function when compensating ankle stiffness with an external force. Although higher stiffness compensation and more simulations should be obtained, this study provides a solid ground for further investigation of the AFO effects. These results can ultimately assist in the prescription and tuning of the new AFO for patients with equinus.

A Trade-Off Analysis Between Random Noise Attenuation and Muscle State Preservation

A Simulation Study on Stretch Reflex Responses

Master thesis (2019) - I. Heijmans, A.C. Schouten, K.E. Rodriguez Hernandez

The surface electromyographic (sEMG) signals that originate from skeletal muscle electrical activity, are used clinically and experimentally to determine muscular behaviour, e.g. amplitude, area under the curve and onset of activity. Surface EMG signals are inevitably contaminate ...

The surface electromyographic (sEMG) signals that originate from skeletal muscle electrical activity, are used clinically and experimentally to determine muscular behaviour, e.g. amplitude, area under the curve and onset of activity. Surface EMG signals are inevitably contaminated by noise and artefacts from the site between the skin and electrodes, non-target muscles and recording hardware. After recording, signal processing methods like filtering, are used in an attempt to determine the underlying active state of the muscle, portrayed by the motoneuron pool firing. As EMG is in fact a deformed representation of the actual muscle activity, processing is used to extract a more veracious description of the active muscle states.

This study investigated the effects of random noise - which in practice resembles transducer noise -, and filtering on the simulation accuracy of short and long latency muscle stretch responses, extracted from simulated EMG signals. To obtain the deviation from the noiseless signals, a fiber potential model was developed to simulate the EMG surface potentials that used an existing motoneuron pool firing model by Schuurmans et al. 2009. The resulting EMGs were the muscle responses to stretch perturbations at different velocities and amplitudes combinations (1.5, 2, 3, 5 rad/s and 0.06, 0.10, 0.14 rad). Consecutively, the EMG signals were contaminated with different noise intensities (SNR: -1, 2, 5, 7, 9 dB) and then filtered with a \nth{3} order Butterworth low-pass filter, with cut-off frequencies between [1-200Hz]. Finally, the short- and long latency stretch responses areas were calculated and compared between the filtered noiseless and filtered noisy EMG signals, by calculating the difference between the values as a fraction of the value from the noiseless simulated signal. It was found that a signal-to-noise ratio of at least 5 dB with a 85Hz cut-off low-pass filter was necessary to keep the error below 10\% maintaining M1 and M2 characteristics. It was also seen that M1 was more affected than M2 under the same amount of contamination, suggesting different spectral frequency contents between the stretch responses, and different underlying neuronal firing behaviour. The described signal-to-noise ratio thresholds and proposed cut-off frequencies resulting in acceptable signal error, can be used as a reference on accuracy of latency response simulations. The error courses provide information about the way error and signal are attenuated or preserved. Besides, the differences in error course comparing the two latency responses provides an insight into the difference in behaviour between the underlying reflex mechanisms. Apart from the findings the combination of adapted and developed model can be used in future research where noise-free surface potentials are required, and can be further developed to produce veracious EMG signals.

Separating background activity from noise in EMG using a neuromuscular model

Master thesis (2019) - B.P. van der Ploeg, W. Mugge, K.E. Rodriguez Hernandez, Jurriaan De Groot, A.C. Schouten, J.J. van den Dobbelsteen

To objectively diagnose the severity of spasticity, it is important to measure the muscle activity accurately. Filtered EMG has an offset above zero which is part noise and part background activity. The goal of this study is to separate these two parts, using a non-linear neuromu ...

To objectively diagnose the severity of spasticity, it is important to measure the muscle activity accurately. Filtered EMG has an offset above zero which is part noise and part background activity. The goal of this study is to separate these two parts, using a non-linear neuromuscular ankle model. The model uses positional and EMG data as inputs to make a prediction of the output torque around the ankle, optimizing several muscle parameters. It was assumed model error was partly caused by the lack of separation of background activity and noise in the EMG offset. The correlation between model error and the force-length relationship of the muscle was investigated, as it was hypothesized that the force-length relationship could be used to separate background activity from noise in EMG.Twelve subjects (11 male, 1 female, mean age 25.21.7 years) participated in the study, which was approved by the TU Delft Human Research Ethics Committee. The Achilles ankle perturbator and the TMSi Porti7 EMG system were used to obtain positional and EMG data around the ankle. The subjects performed several trials containing multiple ramp-and-hold phases. The trials were performed while relaxed, and while performing co-contraction at 5% and 10% of maximum voluntary co-contraction. Model validity was judged on robustness and fit. Model fit shows how close the predicted torque resembles the measured torque. High robustness is defined as a low variance of the model parameters per subject, between trials. To improve robustness, two model configurations, one using averaged data and one simplified model configuration, were compared with the original model. The most robust model configuration was used as a basis for an alternative model, which added a parameter for each EMG signal, that subtracted part of the EMG offset. This EMG offset subtraction parameter was added to remove the constant part of the noise in the EMG offset. The correlation between model error and the force-length relationship of the model was calculated for the model with the best robustness, and the alternative model.Model robustness was not improved in the averaged data model configuration or the simplified model configuration, compared to the original model. The original model was therefore used for the rest of the study. The EMG offset subtraction parameter did not yield to desired reduction of noise in the EMG offset. This was likely a result of difficulties with determining the EMG offset. Only weak correlations (R<<0.3) were found between the model error and the force-length relationship of the muscles, for both the original model and the alternative model configuration.

Quantifying directed nonlinear coupling between brain and muscle activity

A NARX model-based approach

Master thesis (2018) - N.A. van der Helm, A.C. Schouten, K.E. Rodriguez Hernandez

Relevance: To enhance our understanding of motor impairments (e.g. post-stroke or due to Parkinson's Disease), objective measures for communication in the nervous system are required. By applying system identification techniques to oscillations in brain and muscle activity, we ca ...

Using ultrasound muscle imaging to assess the proportionality between ankle angle and contractile element length

A feasibility study to test the assumption with plane-wave ultrasound and system identification of joint dynamics

Master thesis (2018) - B.W. Ossenkoppele, A.C. Schouten, W. Mugge, K.E. Rodriguez Hernandez, N. de Jong, V. Daeichin

Ultrasound gives the opportunity to look at muscles and observe their change in length. This tool has increased the knowledge about muscle-tendon dynamics and sometimes revealed surprising muscle stretch behaviour. System identification experiments use robots to disturb the ankle ...

Resting period induces time and mechanical history-dependence in muscle thixotropy around the human ankle joint

Master thesis (2018) - Anindito Anindito Kusumojati, A.C. Schouten, W. Mugge, K.E. Rodriguez Hernandez, D. Dodou

Subjectivity, lack of precision, and accuracy are the challenges in the spasticity clinical measurement. One of the causes is the unwell standardization of the measurements that cause temporary changes in muscle resistance termed as muscle thixotropy. This temporary changes can ...

Subjectivity, lack of precision, and accuracy are the challenges in the spasticity clinical measurement. One of the causes is the unwell standardization of the measurements that cause temporary changes in muscle resistance termed as muscle thixotropy. This temporary changes can affect the accuracy of the result that may lead to a wrong diagnosis. Unfortunately, resting period between the measurements, as one of the factors generating the thixotropic effect, is still not well considered. The present study investigates the effect of resting period on the short-range stiffness and the resting torque in muscle thixotropy around the human ankle joint. Twenty participants were seated on a chair with the foot placed onto the footplate connected to the Achilles Rehabilitation Device(ARD). The experimental procedure was done using segmental passive slow movements (v=5° /s) applied to the ankle with each segment consisting of multiple dorsi-plantarfexion cycles (Amplitude 15° in dorsiflexion direction) controlled by the ARD. Six different resting period durations (1s, 2s, 4s, 8s, 16s, 32s) were applied to the segments with Balance Latin Square randomization rule. The results clearly showed the time dependency of the Short-range stiffness (SRS) (RM ANOVA, F(1.702,32.399)=210.776, p<0.0005) and the resting torque (RM ANOVA, F(1.709,32.472)=210.776, p<0.0005) with a negative exponential pattern. We suggested that the most responsible structures for SRS and resting torque were the cross-bridges. We found that the resting torque was significantly affected by the multiple movements in the primary procedure (Wilcoxon Signed-Rank test, Z=-3920, p<0.0005) which had not recovered to its initial value. However, the insignificant effect was found in SRS (dependent t-test, t(19)=1.512, and p=0.147). We suggested that the SRS was generated as long as having enough resting period whereas resting torque was not. Our interpretation is that the resting torque was influenced by the mechanical-history of both dorsi- and plantar flexor muscles. The compliance of the tendon and the parallel connective tissue of plantarflexors, as well as the stiffness recovery of the dorsiflexor muscles, caused the slow resting torque recovery even though the cross-bridges in plantarflexors already started reforming.

The effects of different control methods on human biomechanics during exoskeleton walking

Master thesis (2017) - W.F. de Boer, H. van der Kooij, K.E. Rodriguez Hernandez, I. Horvath

Ever since human ancestors picked up rocks and used them as tools, technology has enhanced human capabilities. Over the last decades more and more research has been done in trying to enhance strength and endurance by means of an exoskeleton. Reducing the metabolic cost of human w ...

Karen Rodriguez Hernandez

Authored

Ankle-Foot-Orthosis “Hermes” Compensates Pathological Ankle Stiffness of Chronic Stroke—A Proof of Concept

System identification of ankle joint dynamics based on plane-wave ultrasound muscle imaging

Passive ankle joint stiffness compensation by a novel Ankle-Foot-Orthosis

Contributed