Sex differences in the neural control of muscle

Sex-differences in muscle strength have been linked to differences in muscle size, involved limb, and daily activities. Early work has shown that sex-differences are greater in the upper compared to lower limb, making the upper limb an ideal model to investigate the best statistical approaches for sex comparison. Large differences in the upper limb reveals how biomechanical factors may impact neural control. Since males and females are more comparable with respect to strength in the lower limb, it allows for a determination of whether potential sex-differences in neural control exist without large differences in biomechanics. Understanding sex-differences allows for prescription of rehabilitation and training modalities, taking into account potential specificities in sex-related neuromuscular and musculoskeletal factors. The overall purpose was to examine neural and biomechanical differences that would account for sex-differences in neural control of muscle. Manuscript 1 examined normalization versus an ANCOVA to assess sex-differences. Sex-differences were seen in elbow flexor strength and rate of force development (RFD). Normalization by either maximum strength or neural factors couldn’t account for all sex-differences in RFD, resulting in an ambiguous interpretation. In contrast, both variables were able to be incorporated in an ANCOVA to determine their relative contribution. Manuscript 2 examined the effect of task familiarization and the contribution of maximum strength, twitch contraction time, muscle fiber condition velocity, and rate of muscle activation to sex-differences in the RFD during dorsiflexion. There were no significant differences between the sexes in muscle properties, but there were differences in neural control. Additionally, across days females exhibited a neural adaptation leading to an improvement in the RFD. Manuscript 3 directly assessed potential sex-differences in neural control during force gradation by recording motor unit activity during maximal and submaximal contractions. Females had less force steadiness (FS), which may have resulted from neural compensation for a less optimal pennation angle or a tendency towards greater joint laxity. Higher motor unit discharge rates and incidence of doublets may increase twitch force summation leading to a reduction in FS. Thus, biomechanical, not inherent sex-differences in neural drive led to neural compensation strategies manifesting as a difference in FS

Mechanical factors affecting the estimation of tibialis anterior force using an EMG-driven modelling approach

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel UniversityThe tibialis anterior (TA) muscle plays a vital role in human movement such as walking and running. Overuse of TA during these movements leads to an increased susceptibility of injuries e.g. chronic exertional compartment syndrome. TA activation has been shown to be affected by increases in exercise, age, and the external environment (i.e. incline and footwear). Because activation parameters of TA change with condition, it leads to the interpretation that force changes occur too. However,activation is only an approximate indicator of force output of a muscle. Therefore, the overall aim of this thesis was to investigate the parameters affecting accurate measure of TA force, leading to development of a subject-specific EMG-driven model, which takes into consideration specific methodological issues. The first study investigated the reasons why the tendon excursion and geometric method differ so vastly in terms of estimation of TA moment arm. Tendon length changes during the tendon excursion method, and location of the TA line of action and irregularities between talus and foot rotations during the geometric method, were found to affect the accuracy of TA moment arm measurement. A novel, more valid, method was proposed. The second study investigated the errors associated with methods used to account for plantar flexor antagonist co-contraction. A new approach was presented and shown to be, at worse, equivalent to current methods, but allows for accounting throughout the complete range of motion. The final study utilised the outputs from studies one and two to directly measure TA force in vivo. This was used to develop, and validate, an EMG-driven TA force model. Less error was found in the accuracy of estimating TA force when the contractile component length changes were modelled using the ankle, as opposed to the muscle. Overall, these findings increase our understanding of not only the mechanics associated with TA and the ankle, but also improves our ability to accurately monitor these.Headley Court Trust and the Defence Medical Rehabilitation Centre

Mechanical factors affecting the estimation of tibialis anterior force using an EMG-driven modelling approach

The tibialis anterior (TA) muscle plays a vital role in human movement such as walking and running. Overuse of TA during these movements leads to an increased susceptibility of injuries e.g. chronic exertional compartment syndrome. TA activation has been shown to be affected by increases in exercise, age, and the external environment (i.e. incline and footwear). Because activation parameters of TA change with condition, it leads to the interpretation that force changes occur too. However,activation is only an approximate indicator of force output of a muscle. Therefore, the overall aim of this thesis was to investigate the parameters affecting accurate measure of TA force, leading to development of a subject-specific EMG-driven model, which takes into consideration specific methodological issues. The first study investigated the reasons why the tendon excursion and geometric method differ so vastly in terms of estimation of TA moment arm. Tendon length changes during the tendon excursion method, and location of the TA line of action and irregularities between talus and foot rotations during the geometric method, were found to affect the accuracy of TA moment arm measurement. A novel, more valid, method was proposed. The second study investigated the errors associated with methods used to account for plantar flexor antagonist co-contraction. A new approach was presented and shown to be, at worse, equivalent to current methods, but allows for accounting throughout the complete range of motion. The final study utilised the outputs from studies one and two to directly measure TA force in vivo. This was used to develop, and validate, an EMG-driven TA force model. Less error was found in the accuracy of estimating TA force when the contractile component length changes were modelled using the ankle, as opposed to the muscle. Overall, these findings increase our understanding of not only the mechanics associated with TA and the ankle, but also improves our ability to accurately monitor these

Intramuscular Pressure of Human Tibialis Anterior Muscle Reflects in vivo Muscular Activity

Intramuscular pressure (IMP) is the fluid hydrostatic pressure generated within a muscle and reflects the mechanical forces produced by a muscle. By providing accurate quantification of interstitial fluid pressure, the measurement of IMP may be useful to detect changes in skeletal muscle function not identified with established techniques. However, the relationship between IMP and muscle activity has never been studied in vivo in healthy human muscles. To determine if IMP is able to evaluate electromechanical performance of muscles in vivo, we tested the following hypotheses on the human tibialis anterior (TA) muscle: (i) IMP increases in proportion to muscle activity as measured by electrical (Compound Muscle Action Potential (CMAP)) and mechanical (ankle torque) responses to activation by nerve stimulation and (ii) the onset delay of IMP (IMPD) is shorter than the ankle torque electromechanical delay (EMD). Twelve healthy adults (six females; mean (SD) = 28.1 (5.0) years old) were recruited. Ankle torque, TA IMP, and CMAP responses were collected during maximal stimulation of the fibular nerve at different intensity levels of electrical stimulation, and at different frequencies of supramaximal stimulation, i.e., at 2, 5, 10, and 20 Hz. The IMP response at different stimulation intensities was correlated with the CMAP amplitude (r2 = 0.94). The area of the IMP response at different stimulation intensities was also significantly correlated with the area of the CMAP (r2 = 0.93). Increasing stimulation intensity resulted in an increase of the IMP response (P < 0.001). Increasing stimulation frequency caused torque (P < 0.001) as well as the IMP (P < 0.001) to increase. The ankle torque EMD (median (interquartile range) = 41.8 (14.4) ms) was later than the IMPD (33.0 (23.6) ms). These findings support the hypotheses and suggest that IMP captures active mechanical properties of muscle in vivo and can be used to detect muscular changes due to drugs, diseases, or aging

Markers of physical functioning and neuromuscular fatigue for the post-discharge follow-up of subjects already assisted in intensive care for COVID-19 and non-communicable diseases.

L'insorgenza di malattie non trasmissibili con incidenza neuromuscolare e il prolungato ricovero in terapia intensiva comportano implicazioni negative sulla capacità funzionale e l'autonomia dei pazienti. Queste alterazioni sono causate da fattori come l'immobilità prolungata, l'infiammazione sistemica, le disfunzioni neuromuscolari e gli effetti diretti della patologia stessa. Ciò porta a fatica e debolezza muscolare, contribuendo a un significativo declino nelle abilità motorie, ostacolando il recupero e peggiorando la qualità della vita dei pazienti. Le principali disfunzioni motorie si manifestano con l’alterata deambulazione, limitando l'autonomia nelle attività quotidiane. Tali disfunzioni sono causate dalla compromissione delle vie motorie del sistema nervoso e si manifestano già durante la fase acuta della malattia, peggiorando successivamente con il suo evolversi. Tale quadro clinico è aggravato ulteriormente dalla conseguente inattività fisica, favorita dalle condizioni psicofisiche dei singoli. Di conseguenza, tutto ciò incide notevolmente sulle proprietà contrattili dei muscoli, portando ad alterazioni critiche che influenzano la capacità del sistema nervoso centrale e periferico di reclutare e modulare l'attività delle unità motorie. Date le complessità associate a queste considerazioni, diventa fondamentale identificare marcatori in grado di quantificare e caratterizzare l’alterazione della capacità funzionale e l’insorgenza della debolezza muscolare e della fatica. Ciò faciliterebbe una diagnosi precoce e, in modo cruciale, il monitoraggio continuo di tali problematiche. Pertanto, l'obiettivo primario di questa ricerca di Dottorato è identificare e analizzare marcatori in grado di supportare efficacemente gli operatori sanitari nella progettazione e nell'implementazione di approcci terapeutici personalizzati per accelerare il recupero di questi individui. Nonostante, infatti, la pratica clinica attualmente in uso negli ospedali offra continui miglioramenti, questa presenta ancora delle limitazioni. Sebbene le valutazioni attualmente impiegate riescano ad identificare la presenza di fatica e debolezza muscolare nei pazienti post-ricovero in terapia intensiva o in soggetti affetti da malattie non trasmissibili, non riescono tuttavia ad indagare a fondo su quali siano le effettive cause che innescano la perdita della forza muscolare o ad esaminare in modo esaustivo i fattori centrali e/o periferici che contribuiscono all'insorgenza della fatica. Per colmare in modo esaustivo questa lacuna, lo studio ha condotto un'ampia ricerca combinando l'elettromiografia di superficie con la capacità di generare forza muscolare in diverse condizioni patologiche, includendo attivazioni muscolari sia volontarie che indotte elettricamente. Il muscolo oggetto di studio è stato il tibiale anteriore, scelto per il suo ruolo cruciale nella biomeccanica della deambulazione e quindi fondamentale per il mantenimento dell’autonomia motoria. I risultati hanno mostrato che le variazioni nella forza e nei parametri delle unità motorie possono servire da indicatori per le alterazioni neuromuscolari e il recupero progressivo, facilitando il monitoraggio a breve e lungo termine. Questo studio ha quindi un'importanza fondamentale per le popolazioni coinvolte e può suggerire approcci più ampi per la gestione delle alterazioni neuromuscolari in diversi contesti clinici. In particolare, sottolinea l'importanza di programmi di riabilitazione personalizzati e soggettivati alle esigenze specifiche di ciascun individuo.The onset of non-communicable neuromuscular diseases and prolonged stays in the intensive care unit have deep implications for physical functioning and neuromuscular health. These repercussions arise from muscle deconditioning, systemic inflammation, and the direct impact of the pathology. Moreover, resulting fatigue and acquired muscle weakness contribute to reduced muscular performance, significantly hampering recovery and diminishing overall quality of life. The predominant motor impairments observed in these patients primarily manifest in their ability to perform correct walking, substantially limiting their independent execution of daily activities. This compromised excitability in descending motor pathways becomes evident during the acute phase of the disease and intensifies as the condition progresses chronically, exacerbated by prolonged physical inactivity. Consequently, this significantly affects the muscle's contractile properties, leading to critical alterations that influence the nervous systems' capacity to recruit and modulate the activity of motor units, the fundamental functional units responsible for planning, executing, and maintaining motor gestures. Given these considerations, it becomes crucial to identify markers that enable the quantification and characterization of physical functioning impairment, muscle weakness and fatigue. This would facilitate early diagnosis and, crucially, the ongoing monitoring of these issues. Thus, the primary goal of this PhD research is to identify and analyze markers that can effectively support healthcare practitioners in devising and delivering personalized therapeutic approaches to expedite the recovery of these individuals. The overarching objective is to optimize the current clinical practice commonly employed in hospitals. Despite ongoing refinements, these practices still exhibit limitations. While standard assessments succeed in identifying the presence of fatigue and muscle weakness in ICU patients or those afflicted by non-communicable diseases, they fall short of investigating the root causes of muscle strength deterioration or thoroughly probing the central and/or peripheral factors contributing to the emergence of pathological fatigue. To comprehensively bridge this existing gap, the study undertook an extensive exploration by measuring concurrent joint torques and surface electromyography across various pathological conditions, encompassing both voluntary and electrically induced muscle activations. The focal point was the tibialis anterior muscle, chosen for its pivotal role in gait patterns and consequential influence on individual autonomy. The presented results were mainly achieved through the decomposition of signals recorded using the High-Density Surface EMG technique. This technique enabled the analysis of individual motor units recruited during motor tasks administered to patients within the studied populations. The process of data collection and analysis revealed that variations in muscle strength values and motor unit parameters can serve as indicators of neuromuscular system alterations and progressive recovery. These factors are pivotal for subsequent follow-up procedures. Indeed, by establishing a robust framework of markers, is possible to contribute to the development of evidence-based protocols that enhance the post-discharge care of these individuals. This study is not only pivotal for these specific cohorts but also holds the potential to inform broader strategies for managing physical impairment and neuromuscular challenges in diverse clinical settings. Notably, the study highlights that hospitalization in intensive care, as well as the onset of non-communicable pathologies with high motor impact, leads to specific alterations in parameters of both central and peripheral neuromuscular pathways. This underscores the imperative for devising personalized rehabilitation regimens tailored to each patient's needs

Ankle Muscle Activation and Mechanics during the Ebbet\u27s Foot Drills

Introduction: The ankle is a complex structure of three joints that allow multiplanar motion (Brockett & Chapman, 2016). Lateral ankle sprains (LAS) are the most common injury seen today in both the general and athletic populations and have a high recurrence rate. When left untreated or mistreated, it often leads to developing chronic ankle instability or osteoarthritis, which a lower quality of life. Dr. Russ Ebbets created a set of foot drills with the claim that they can strengthen the muscles of the lower leg, lessen lower leg aliments, and the chances of a severe ankle sprain (Ebbets, 2011a). The purpose of this study was to explore the muscle activation of the lower extremity musculature during Ebbets’ foot drills while examining the sEMG of the tibialis anterior, tibialis posterior, peroneus longus, soleus, and during normal walking. Methods: Twenty-two college students (11 males, 11 females avg age 23.76) participated in the study with one female being excluded; after informed consent, demographics were collected. Next, the Identification of Functional Ankle Instability questionnaire and the Foot and Ankle Ability Measure questionnaires were taken. After balance testing, preparation of sEMG of the tibialis anterior, tibialis posterior, peroneus longus, and soleus. The sEMG was collected during strength testing and all the walking trials, including normal walking and each Ebbets’ foot drill. Mean RMS was calculated for each trial and was used for comparison. Results: Results found that compared to normal walking, Ebbets’ foot drills increased all the selected muscles\u27 muscle activity compared to normal walking. The tibialis anterior saw a significant increase during all the drills. The tibialis posterior saw a significant increase during the last three drills. The peroneus longus saw a significant increase during all but one drill. The soleus saw a significant increase during all Ebbets’ drills. Conclusion: Dr. Ebbets’ foot drills have revealed that they generate greater muscle activity than regular walking, which means the drills may strengthen the tibialis anterior, peroneus longus, and tibialis posterior and soleus. These results build evidence on Dr. Ebbets’ theory and indicate that these foot drills may be used to rehabilitate and prevent LAS and CAI development

Continuous Proportional Myoelectric Control of an Experimental Powered Lower Limb Prosthesis During Walking Using Residual Muscles.

Current robotic lower limb prostheses rely on intrinsic sensing and finite state machines to control ankle mechanics during walking. State-based controllers are suitable for stereotypical cyclic locomotor tasks (e.g. walking on level ground) where joint mechanics are well defined at specific gait phases (i.e. states) and state transitions are easily detected. However, state-based controllers are not ideal for non-stereotypical acyclic tasks (e.g. freestyle dancing) where joint mechanics cannot be predefined and transitions are unpredictable. An alternative to state-based control is to utilize the amputee's nervous system for myoelectric control. A robotic lower limb prosthesis that uses continuous proportional myoelectric control would allow the amputee to adapt their ankle mechanics freely. One potential source for myoelectric control is the amputee’s residual muscles. I conducted four studies to examine the feasibility of using residual muscles for continuous myoelectric control during walking. In my first study, I demonstrated that it is possible to record residual electromyography from amputees during walking that are viable for continuous myoelectric control. My results showed that the stride-to-stride variability of residual and intact muscle activation patterns was similar. However, residual muscle activation patterns were significantly different across amputee subjects and significantly different than corresponding muscles in intact subjects. In my second study, I built and tested an experimental powered transtibial prosthesis and demonstrated that an amputee subject was able to walk using continuous proportional myoelectric control to alter prosthetic ankle mechanics. In my third study, I showed that five amputee subjects were able to adapt their residual muscles to walk using continuous proportional myoelectric control. With visual feedback of their control signal, amputees were able to generate higher peak ankle power walking with the experimental powered prosthesis compared to their prescribed prosthesis. In my fourth study, I conducted a user experience study and found that despite challenges with the device user interface, walking with continuous proportional myoelectric control gave amputees a sense of empowerment and embodiment. The results of my studies demonstrated the advantages and disadvantages of using continuous proportional myoelectric control for a powered transtibial prosthesis and suggest how next generation prostheses can build upon these findings.PHDBiomedical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/110412/1/shuangz_1.pd

Electromechanical Delay of the Dorsiflexors in Young and Old Women

PURPOSE: The aim of the current study was to examine the effect on electromechanical delay (EMD) in the dorsiflexors of young and old women during maximal isometric voluntary and electrically evoked contractions, and after a bout of lengthening contractions. METHODS: Nine young (25.1±1.3 years) and nine old (68.3±6.1 years) women performed baseline isometric contractions with evoked twitches followed by a series of dynamic lengthening contractions using a Biodex multi-joint dynamometer. Maximal isometric voluntary and evoked contractions were measured to assess EMD. Time points were recorded at baseline, mid-point of the intervention, post-task termination, and during recovery at 0.5, 2, 10, and 30 minutes. RESULTS: The EMD of the evoked twitches and voluntary contractions were not different in the young and old at baseline. Following the lengthening contractions the EMD of the evoked contractions at the midpoint of fatigue were shorter in the young compared to old, but not different between groups at task termination, or during recovery. No differences in the EMD measured from the voluntary maximal isometric contractions in the young and old at any fatigue or recovery time points. CONCLUSION: Shorter evoked EMD in the young during the midpoint of the intervention was possibly a result of potentiation which dissipated by task-termination as fatigue developed. This did not occur in old women. Results indicate that in the dorsiflexors EMD is not affected by age in women and overall is not affected by fatigue in either group. Recovery in both measures of EMD was not differently affected by age

BIOMECHANICAL ADAPTATIONS FOLLOWING A LATERAL ANKLE SPRAIN INJURY: AN EXPLANATION FOR CHRONIC ANKLE INSTABILITY?

The purpose of this study was to determine whether biomechanical adaptations play a clinically significant role in chronic ankle instability following lateral ankle sprain injury. Synchronised 3D motion analysis was conducted on 32 grade 11 lateral ankle sprain patients (grouped by functional stability score into capers and non-copers) during a dynamic cutting maneuvre. Simultaneous EMG and force data were collected and compared for the injured and non-injured limbs. Copers could be distinguished from noncopers by certain EMG and ground reaction force parameters. Other distinctions could also be made between the injured and non-injured limbs. However these variables did not show significant group- by-side interactions to explain the symptoms of unilateral functional instability experienced by the non-coper group