Losing the battle but winning the war: game theoretic analysis of the competition between motoneurons innervating a skeletal muscle

The fibers in a skeletal muscle are divided into groups called “muscle units” whereby each muscle unit is innervated by a single neuron. It was found that neurons with low activation thresholds have smaller muscle units than neurons with higher activation thresholds. This results in a fixed recruitment order of muscle units, from smallest to largest, called the “size principle.” It is thought that the size principle results from a competitive process—taking place after birth—between the neurons innervating the muscle. The underlying mechanism of the competition was not understood. Moreover, the results in the majority of experiments that manipulated the activity during the competition period seemed to contradict the size principle. Experiments at the isolated muscle fibers showed that the competition is governed by a Hebbian-like rule, whereby neurons with low activation thresholds have a competitive advantage at any single muscle fiber. Thus neurons with low activation thresholds are expected to have larger muscle units in contradiction to what is seen empirically. This state of affairs was termed “paradoxical.” In the present study we developed a new game theoretic framework to analyze such competitive biological processes. In this game, neurons are the players competing to innervate a maximal number of muscle fibers. We showed that in order to innervate more muscle fibers, it is advantageous to win (as the neurons with higher activation thresholds do) later competitions. This both explains the size principle and resolves the seemingly paradoxical experimental data. Our model establishes that the competition at each muscle fiber may indeed be Hebbian and that the size principle still emerges from these competitions as an overall property of the system. Thus, the less active neurons “lose the battle but win the war.” Our model provides experimentally testable predictions. The new game-theoretic approach may be applied to competitions in other biological systems

Photobiomodulation effects on fatigue and muscle damage induced by neuromuscular electrical stimulation in healthy individuals

A presente dissertação teve como objetivo avaliar os efeitos agudos da fotobiomodulação (FBM) e da FBM-sham em indivíduos saudáveis, aplicadas antes de um protocolo de fadiga evocada por estimulação elétrica neuromuscular (EENM). Esses efeitos agudos foram avaliados em parâmetros neuromusculares de fatigabilidade (torque isométrico máximo, torque do abalo supramáximo, atividade elétrica, trabalho total da integral da curva torque-tempo, redução do torque evocado), no dano muscular (ecointensidade, dor muscular de início tardio), no desconforto (escala visual analógica – EVA, algometria) e na funcionalidade (Single Hop Test - SHT, Teste de subir e descer escadas). A dissertação está dividida em três capítulos a saber: Capítulo I: Revisão da literatura referente à EENM e à FBM, tendo conceitos básicos, parâmetros de tratamentos, aplicação na prática clínica, e efeitos terapêuticos. Também abordamos tópicos como os tipos de fadiga existentes e as evidências encontradas na literatura dos estudos que utilizaram FBM para prevenir a fadiga e o dano muscular produzidos pela EENM. Capítulo II: Descrição dos Métodos: Os participantes visitaram o laboratório em nove dias, sendo que foram submetidos à familiarização com as técnicas e protocolos no primeiro dia, avaliação do primeiro membro nos quatro dias seguintes, e os quatro dias finais para a avaliação do segundo membro. A familiarização dos sujeitos foi efetuada com o ultrassom (US), com a contração voluntária máxima isométrica (CVMI), com os testes funcionais e com a EENM. No segundo dia foi realizado o protocolo de fatiga; porém, antes do protocolo de fadiga evocada pela EENM, se aplicou FBM ou FBM-sham (de forma randomizada) nos extensores do joelho de cada um dos membros. Um intervalo de sete dias foi observado entre os momentos de intervenção nos dois membros (período de “wash out”). Foram utilizados 8 pontos de aplicação da FBM e cada ponto foi tratado por 30s, com uma dose de 6J por diodo (sonda com 5 diodos), ou seja, 30J por local, totalizando 240J no membro inferior que recebeu FBM. A seguir, os sujeitos foram submetidos a um protocolo de fadiga de EENM (20 minutos de duração, duração de pulso = 1 ms, tempos de contração-repouso de 5s:10s, e uma frequência de 100 Hz). Após o protocolo de fadiga evocada, foram avaliadas as variáveis de dor, desconforto e desempenho nos testes funcionais SHT e teste de subir e descer escadas. Para avaliar o dano muscular, foram coletadas nove imagens de ultrassom do músculo reto femoral (RF) e 3 do vasto lateral (VL). O nível de desconforto foi obtido por meio da EVA e do algômetro, o qual foi aplicado em todos os três extensores do joelho. Para os músculos RF e VL, o algômetro foi aplicado a 50% da linha entre a espinha ilíaca anterossuperior e a borda superior da patela, enquanto para o vasto medial (VM) o ponto de aplicação foi a 50% da linha entre a borda superior da patela e o ventre do músculo. A força voluntária máxima foi avaliada por meio de três CVMIs dos extensores do joelho pré protocolo de fadiga e uma CVMI pós protocolo de fadiga para avaliação do índice de fadiga. Além disso, os participantes executaram os testes funcionais pré e pós protocolo de fadiga a fim de avaliar os efeitos da fadiga e da FBM e FBM-sham na funcionalidade. Todas as variáveis mencionadas acima foram mensuradas novamente após o protocolo de fadiga evocada e pós 24h, 48h e 72h do protocolo de fadiga. Todas as mesmas etapas foram realizadas no outro membro. Na análise dos dados, inicialmente foi realizada estatística descritiva e avaliação da normalidade dos dados por meio do teste de Shapiro-Wilk. As comparações envolvendo os fatores membro (FBM e FBM-sham) e também os momentos (Pré, Pós-imediato, Pós-24h, Pós-48h e Pós-72h) foram realizadas utilizando o modelo de Equações de Estimação Generalizadas com um gama log link para resposta de escala. Quando foi identificada diferença estatisticamente significativa entre os momentos, foi utilizado o teste post hoc LSD (Least Significant Difference). Para comparações envolvendo as variáveis apenas com os momentos Pré e Pós-Imediato, foi utilizado o teste U de Mann-Whitney. Todas as informações quantitativas foram apresentadas como média e erro padrão, exceto para as variáveis de caracterização onde foram utilizados valores de média e desvio padrão. O nível de significância para todas as análises estatísticas foi fixado em p0,05). Em relação ao desconforto, também não foram observadas diferenças significativas entre os membros (p>0,05), porém houve aumento do desconforto imediatamente após o protocolo de fadiga tanto para FBM quanto para FBM-Sham (p0,05), porém sim entre os momentos, tendo uma queda do desempenho imediatamente após o protocolo de fadiga (p0.05). Regarding discomfort, no between-limbs differences were found (p>0.05), but there were differences between the moments (p0.05), but there were differences between the moments (p0.05). However, the performance in the SHT did not decrease after the fatigue protocol in the PBM limb, while it did in the PBM-sham limb. Conclusion: No significant differences were observed among the limbs for discomfort, muscle damage, and functional test performance. These findings suggest that PBM does not have a significant impact over the deleterious NMES effects, which reduces its use in increasing performance as well as its applicability in clinical practice to reduce fatigue and muscle damage

The role of noise in sensorimotor control

Goal-directed arm movements show stereotypical trajectories, despite the infinite possible ways to reach a given end point. This thesis examines the hypothesis that this stereotypy arises because movements are optimised to reduce the consequences of signal-dependent noise on the motor command. Both experimental and modelling studies demonstrate that signal-dependent noise arises from the normal behaviour of the muscle and motor neuron pool, and has a particular distribution across muscles of different sizes. Specifically, noise decreases in a systematic fashion with increasing muscle strength and motor unit number. Simulations of obstacle avoidance performance in the presence of signal-dependent noise demonstrate that the optimal trajectory for reaching the target accurately and without collision matches the observed trajectories. Isometric force generation is also shown to have systematic changes in variability with posture, which can be explained by the presence of signal-dependent noise in the muscles of the arm. These results confirm the tested hypothesis and imply that consideration of the statistics of action is crucial to human movement planning. To investigate the importance of feedback in the motor system, the impact of static position on motor excitability was examined using transcranial magnetic stimulation and systematic changes in motor evoked potentials were observed. Force generated at the wrist following stimulation was analysed in terms of different possible movement representations, and the differences between force fields arising from stimulation over the cervical spinal cord and from stimulation over primary motor cortex are determined. These results demonstrate the structured influence of proprioceptive feedback on the human motor system. All the experiments are discussed in relation to current theories describing the control of human movements and the impact of noise in the motor system

Feasibility of a surface electromyography-based compression garment for monitoring internal player load in professional basketball

The psychophysiological demands placed on professional athletes nowadays is greater than ever. In fact, professional basketball players can compete up to three time per week in addition to frequent and regular training sessions. Thus, adequately prescribing and monitoring athletes’ loads is important to maintain player well-being, reduce fatigue while optimising performance. Therefore, sport science research is saturated with different internal and external load monitoring approaches to help teams achieve these goals. Expansion of the global wearable technology market in sport is ever growing as practitioners seek a competitive advantage to their competitors. One such technology which has clinically and extensively been used for decades but has entered a new era into the wearable technology field in sport is surface electromyography (sEMG). However, little research reports on this technology in sport and the internal load metrics which representative companies claim it can report. The purpose of this doctoral thesis was to comprehensively examine internal load experienced by professional basketball players in the British Basketball League (BBL), while investigating a wearable sEMG technology for reporting a novel sEMG-based internal load metric (“Training Load”) during controlled lab-based exercise protocols, as well as determine the feasibility of the wearable sEMG-based internal load monitoring system in the professional basketball environment. The first observational study assessed the internal load experienced by professional basketball players during an entire season in the BBL. The research used the session-rating of perceived exertion (sRPE) method for quantifying load in professional basketball players following training sessions and competition. Results show that players experience greater Weekly Load (training only) during preseason compared to the in-season phase. Weekly Load is greater in 1-game weeks compared to 2-game weeks, while Total Weekly Load (training and competition) is higher during 2-game weeks compared to 1-game weeks. In addition, starting players experience a moderately higher Total Weekly Load compared to bench players, yet playing status did not result in differences in Weekly Load. The results show variances in internal load depending on weekly game fixtures, training schedules and phases of the season. While the sRPE method provides a valid global measurement of the training session or competition, the nature of retrieving RPE’s from players by asking a question prevents deeper investigation of internal load from specific phases of play. The second investigative study explores the possibility of using a novel wearable sEMG garment for capturing internal load (Training Load). The research investigated the sEMG derived Training Load during a 3-speed treadmill test and its relationship with oxygen consumption (V̇O2) during an exhaustive ramp incremental running treadmill test to determine maximal oxygen uptake (V̇O2max). Findings demonstrate sEMG-derived Training Load is a sensitive measure in detecting small changes in work rate during dynamic exercise, and while a moderate positive correlation between %V̇O2 max is shown, 80% of participants’ Training Loads show a very strong positive correlation at the individual level. The findings conclude that wearable sEMG technology may provide an alternative and new approach to capturing players internal load during sport and dynamic, whole-body exercise. The third study investigates the feasibility, practicality, and acceptability of wearable sEMG technology in the professional basketball environment. Results show a high acceptance rate (seventy-five percent) of the sEMG technology amongst professional basketball players, who report they would use the wearable sEMG technology again during team basketball training. A minority of players (twenty-five percent) report they would not use the wearable sEMG technology again due to negative experiences such as, comfortability issues and perceived negative effects on performance. While the wearable sEMG technology is relatively feasible in the environment, a few practical implications are considered important for coaches to understand before use. In particular, the time taken for downloading data to report to coaching staff or players takes longer than other load monitoring systems, such as GPS. In addition, the technology is more suited to the professional environment where a kit manager takes care of the handling procedures associated with the shorts. Lastly, the Core unit attached to the shorts can interrupt training practice. The current thesis contributes original research to the field of wearable sEMG for monitoring internal load. Findings provide important implications for practitioners endeavouring to use wearable sEMG in a professional sport context or research to further extent. Most research in basketball is conducted internationally, within Europe and America. The thesis is one of the first studies to identify internal loads in professional male basketball players within the United Kingdom. The thesis was the first to investigate an sEMG-derived Training Load during specific running tests. Lastly, the thesis was the first to assess professional athletes’ perceptions on wearable sEMG technology, highlighting reasons for and against using the technology

Size principle and information theory

The motor units of a skeletal muscle may be recruited according to different strategies. From all possible recruitment strategies nature selected the simplest one: in most actions of vertebrate skeletal muscles the recruitment of its motor units is by increasing size. This so-called size principle permits a high precision in muscle force generation since small muscle forces are produced exclusively by small motor units. Larger motor units are activated only if the total muscle force has already reached certain critical levels. We show that this recruitment by size is not only optimal in precision but also optimal in an information theoretical sense. We consider the motoneuron pool as an encoder generating a parallel binary code from a common input to that pool. The generated motoneuron code is sent down through the motoneuron axons to the muscle. We establish that an optimization of this motoneuron code with respect to its information content is equivalent to the recruitment of motor units by size. Moreover, maximal information content of the motoneuron code is equivalent to a minimal expected error in muscle force generation