About

William Rymer is a Professor of Physical Medicine and Rehabilitation and a Professor of Biomedical Engineering (by courtesy) at Northwestern University. His research focuses on the neural control and mechanics of movement in both normal and neurologically impaired human subjects. He conducts studies to understand skeletomotor reflex function in patients with disorders of muscle tone and investigates the physiological effects of spinal cord injury. Using electrophysiological, pharmacological, and morphological techniques, his work aims to identify the key transmitters and neuromodulators responsible for altered motoneuronal responses in spinal segments below partial or complete spinal cord transection, with the goal of developing compounds to counteract these abnormalities in human subjects.

Research topics

• Medicine
• Psychology
• Physical medicine and rehabilitation
• Physical therapy
• Risk analysis (engineering)
• Neuroscience
• Engineering ethics
• Surgery
• Mechanical engineering
• Medical education
• Engineering

Selected publications

• Safety and potential benefits of acute intermittent hypoxia in people with chronic traumatic brain injury

  medRxiv · 2025-10-07

  preprintOpen access

  Abstract Background Acute intermittent hypoxia (AIH) was recently demonstrated to improve motor and cognitive function in several patient populations, including incomplete spinal cord injury, stroke, multiple sclerosis and mild cognitive impairment. Objective Our clinical trial aimed to establish if AIH can be safely administered in patients with traumatic brain injury (TBI) and to collect preliminary data about its potential efficacy for treating motor, cognitive and affective sequelae of TBI. Methods Twelve volunteers with chronic TBI underwent four AIH sessions conducted on separate days, in which they were exposed to fifteen 30-60-s hypoxic episodes interspersed with 60-90 s of breathing ambient air. Inspired oxygen (O 2 ) concentration during hypoxic episodes was gradually reduced from 21% (equal to ambient air, sham), to 17%, 13%, and 9%, over the course of four sessions. Neuropsychological and motor tests were administered on days before and after AIH, as well as 60 min after each AIH session. In addition, transcranial magnetic stimulation (TMS) was applied to the hand motor area 45 min after the first (21% O 2 ) and last (9% O 2 ) AIH session. Results All participants tolerated the AIH sessions well and were able to complete the entire protocol. No significant improvement in cognition or mood was noted after the AIH intervention. Motor performance gradually improved over the course of the study, but no significant changes in response to TMS were found in corticospinal excitability. Conclusions AIH dosage as low as 9% O 2 appears safe to use in chronic TBI, but its potential benefits remain to be investigated.

  PublisherOA PDFDOI

• Investigation of Interelectrode Distance for Surface Recording of Electrical Responses From a Single Motor Unit: A Simulation Study

  IEEE Sensors Journal · 2025-09-08

  article

  Computer simulation studies of physiologically informed mathematical models have successfully revealed the dependence of the surface electromyogram (sEMG) on the functional and structural properties of the neuromuscular system. The surface recording of the propagating motor unit action potential (MUAP) is influenced by the montage of the recording electrode placed on the skin during differential sEMG recording. In this context, along with several other topographical factors of the motor units (MU), the appropriate inter-electrode distance (IED) along the directions of the muscle fiber is of vital importance. Here, we have proposed and implemented a physiologically relevant three-dimensional in-silico model of activated muscle fibers associated with a single motor unit to investigate the effect of the IED exclusively under several conditions. Based on the model output, we found the optimal IED (OIED) that records the maximum peak-to-peak (P-P) amplitude of sEMG signals. The OIEDs were found to vary from 6 to 13 mm according to the selected muscle fiber parameters (i.e., fiber length, fiber density, distribution of innervation zone, fiber distribution, fiber alignment etc.). We have reported that the OIED values are positively correlated with fiber depth, while a millimeter increase in MU territory results in 8% reduction of the OIED (p < 0.01). The concentrically distributed fiber density resulted 14% lower (p < 0.05) OIEDs compared to the randomly distributed fibers. Finally, this paper provides a method of IED optimization to potentially improve the EMG signal that may be usefully combined with more complex models in future studies.

  PublisherDOI

• Noninvasive spatiotemporal spinal neuromodulation targeting dorsal roots improves paretic leg motor control during walking in persons with stroke

  Journal of Neurophysiology · 2025-09-28 · 1 citations

  articleOpen access

  Applying spatially and temporally controlled transcutaneous spinal cord stimulation, combined with constraint force applied to the nonparetic leg during swing, may induce greater improvements in step length and step height of the paretic leg, and reduced foot path variability in the paretic leg compared with the sham stimulation, combined with targeted constraint force during walking. These findings support the development of targeted spinal cord neuromodulation strategies to enhance paretic leg motor control in individuals post-stroke.

  PublisherDOI

• Motor variability predicts motor learning of improved trunk postural control through repeated trunk perturbations during walking in children with cerebral palsy

  Journal of Neurophysiology · 2025-11-20 · 1 citations

  articleOpen access

  We examined the effect of providing lateral perturbation force at the trunk during walking on improving trunk control in children with cerebral palsy (CP). We found that repeated motor adaptation to trunk perturbation force during walking may promote motor learning of improved trunk control in children with CP. In particular, we found that motor variability of trunk movement can predict motor learning ability of trunk postural control in children with CP.

  PublisherDOI

• Increasing motor variability facilitates motor learning of trunk postural control during sitting in children with cerebral palsy

  Journal of Neurophysiology · 2025-10-30

  articleOpen access

  Many children with cerebral palsy (CP) exhibit impairments in trunk postural control, particularly the second level of postural control. Results from this study suggest that applying repeated pelvis perturbations during sitting astride may induce an improvement in reactive trunk postural control in children with CP. Further, increasing motor variability by applying repeated VARIED force perturbations may be more effective than the CONSTANT perturbations in promoting motor learning of improved head stability and reactive trunk postural control in children with CP.

  PublisherDOI

• Tracking spasticity dynamics in hemiparetic stroke survivors following cyproheptadine administration: a pilot study using controlled varying tendon indentation depths

  Frontiers in Stroke · 2025-04-23

  articleOpen access

  This study evaluates the potential of the Linmot ® tapper as a precise tool for tracking spasticity changes in hemiparetic stroke survivors following cyproheptadine HCl administration. Spasticity, a significant health concern among stroke survivors, is characterized by increased muscle tone due to upper motor neuron dysfunction. Conventional clinical assessments, such as the Modified Ashworth Scale (MAS), often lack the sensitivity to accurately monitor treatment. In this study, we utilized the Linmot ® tapper to assess the stretch reflex threshold (SRT) in three stroke survivors and one control subject by progressively altering tendon indentation to change muscle length. The SRT was defined as the indentation depth at which consistent reflex responses of the biceps brachii were observed, as indicated by reflex force or rectified integrated EMG (RIEMG) signals. Measurements were taken at baseline and at 2, 4, and 6 h after drug administration. Results showed significant increases in SRT following cyproheptadine administration, indicating reduced motor neuron excitability and highlighting the drug's effect on spasticity. Both reflex force and RIEMG data consistently captured these changes, while MAS grades remained unchanged. The high correlation between SRTs derived from force and EMG further supports the tool's accuracy in detecting subtle neuromuscular changes. These findings highlight that the Linmot ® tapper offers a precise, quantitative method for monitoring spasticity dynamics, providing a more accurate alternative to conventional clinical assessments and demonstrating potential for enhancing stroke rehabilitation strategies.

  PublisherOA PDFDOI

• Acute Intermittent Hypoxia in People Living With Chronic Stroke: A Case Series

  Stroke · 2025-02-25 · 4 citations

  articleOpen accessSenior author

  BACKGROUND: Acute intermittent hypoxia (AIH) is a novel therapeutic intervention that facilitates recovery of function, but the tolerability and effectiveness have not been tested in people living with chronic stroke. The purpose here was to examine whether AIH is tolerable and effective in this population. METHODS: Ten participants with a unilateral, hemispheric stroke were assessed before and after 4 sessions of AIH separated by ≥48 hours in a case series at Shirley Ryan AbilityLab (Chicago). Physician-assessed signs and symptoms (assessed via: repeated symptom reviews, National Institutes of Health Stroke Scale, cranial nerve assessment, a muscle strength test, the Brunnstrom scale, sensory changes, reflexes, assessment of heart and lung status, Fugl-Meyer test, Chedoke-McMaster Stroke Assessment, Modified Ashworth Scale for Spasticity, and Delis Kaplan Executive Function System Color-Word Interference Test) and bilateral upper limb strength (grip and elbow flexion) were assessed before, ≈15 to 30 minutes, and ≈60 minutes after the intervention. RESULTS: AIH was well-tolerated and there were no adverse events observed. After AIH, grip strength (12.91% and 16.53% improvement at 30 and 60 minutes post-AIH, respectively) and elbow flexion force (5.87% and 7.01% improvement at 30 and 60 minutes post-AIH, respectively) improved in the more-affected limb. CONCLUSIONS: AIH is potentially safe and effective for improving strength in the more-affected limb in people living with hemiparetic stroke. Future work should explore the use of AIH to enhance task-specific training-induced plasticity. REGISTRATION: URL: https://www.clinicaltrials.gov; Unique identifier: NCT04019522.

  PublisherDOI

• Increased motor variability facilitates motor learning of improved trunk postural control during sitting in children with cerebral palsy

  2024-11-12

  preprint

  The goal of this study was to determine whether increasing motor variability by applying varied perturbation forces to the pelvis during sitting astride would facilitate motor learning of improved trunk postural control in children with cerebral palsy (CP). Fourteen children with spastic CP were tested in two conditions: CONSTANT and VARIED force perturbations in two training sessions with each session lasted for 15 minutes. A custom robotic system was used to deliver repetitive perturbation forces to the pelvis while participants sat astride. Postural reactions to the unpredicted perturbations in the anterior and posterior directions were tested before and after each training session. Kinematics of head, trunk, and pelvis movement, and muscle electromyography signals were recorded. One session of pelvis perturbation training with either the CONSTANT (P = 0.028) or VARIED force magnitudes (P = 0.009) induced an earlier onset of trunk extensor under unpredicted perturbations. In addition, one session of pelvis perturbation training with VARIED force magnitudes induced a significant decrease in peak angle of head extension (P = 0.016), and an improvement in recruitment order of the neck and trunk muscles under unpredicted perturbations (P = 0.020). These results suggest that repeated pelvis perturbations during sitting astride may induce improvement in muscle activation onset under unpredicted perturbations in children with CP. Moreover, repeated pelvis perturbations with varied force magnitudes, which may increase motor variability, may lead to improvements in head stability and muscle activation sequence of trunk and neck muscles in response to unpredicted perturbations in children with CP.

  PublisherDOI

• BiLSTM-Based Joint Torque Prediction From Mechanomyogram During Isometric Contractions: A Proof of Concept Study

  IEEE Transactions on Neural Systems and Rehabilitation Engineering · 2024-01-01 · 7 citations

  articleOpen accessSenior author

  Quantifying muscle strength is an important measure in clinical settings; however, there is a lack of practical tools that can be deployed for routine assessment. The purpose of this study is to propose a deep learning model for ankle plantar flexion torque prediction from time-series mechanomyogram (MMG) signals recorded during isometric contractions (i.e., a similar form to manual muscle testing procedure in clinical practice) and to evaluate its performance. Four different deep learning models in terms of model architecture (based on a stacked bidirectional long short-term memory and dense layers) were designed with different combinations of the number of units (from 32 to 512) and dropout ratio (from 0.0 to 0.8), and then evaluated for prediction performance by conducting the leave-one-subject-out cross-validation method from the 10-subject dataset. As a result, the models explained more variance in the untrained test dataset as the error metrics (e.g., root-mean-square error) decreased and as the slope of the relationship between the measured and predicted joint torques became closer to 1.0. Although the slope estimates appear to be sensitive to an individual dataset, >70% of the variance in nine out of 10 datasets was explained by the optimal model. These results demonstrated the feasibility of the proposed model as a potential tool to quantify average joint torque during a sustained isometric contraction.

  PublisherOA PDFDOI

• Author response for "Motor interference on lateral pelvis shifting towards the paretic leg during walking and its cortical mechanisms in persons with stroke"

  2024-05-28

  peer-review
  PublisherDOI

Recent grants

• NIH Grant R21NS075463

  NIH · $418k · 2014

• Origins of Increased Motoneuron Excitability in Hemispheric Stroke

  NIH · $1.3M · 2016–2023

• NIH Grant P01NS017489

  NIH · $3.3M · 1995

• NIH Grant R01NS019331

  NIH · $2.0M · 1999

• Origins of Increased Motoneuron Excitability in Hemispheric Stroke

  NIH · $317k · 2016–2021

Frequent coauthors

• Ping Zhou

  140 shared

• Nina L. Suresh

  118 shared

• Arun Jayaraman

  Shirley Ryan AbilityLab

  108 shared

• Ming Wu

  University of Illinois Chicago

  79 shared

• Xiaogang Hu

  Pennsylvania State University

  62 shared

• M.M. Mirbagheri

  Tehran University of Medical Sciences

  55 shared

• Derek G. Kamper

  55 shared

• Maria Knikou

  City University of New York

  49 shared

Labs

• Single Motor Unit LaboratoryPI