DOES CENTRAL FATIGUE LIMIT MUSCLE FORCE GENERATION CAPACITY DURING FATIGUE?

There is general agreement on peripheral factors of muscle fatigue that develop within the muscle and impair muscle fiber contractile mechanisms and muscle performance. Central factors of muscle fatigue that arise within the central nervous system have also been suggested to influence muscle force during fatigue. However, no direct empirical evidence of their influence on muscle force capacity has yet been reported. We used a force model to investigate whether peripheral factors are sufficient to explain the loss of muscle force generation capacity during fatiguing submaximal voluntary contractions that is commonly attributed to central factors. Our simulations showed that the force behavior during fatigue could be explained solely by peripheral factors. These data raise concerns about the influence of central factors on muscle force generation capacity during fatigue

CONTROL OF MOTOR UNITS DURING VOLUNTARY FORCE-PRODUCTION: IMPLICATIONS FOR EXERCISE

We have recently developed a technology that enables studies of the firing properties of a large set (typically 30 to 40) of concurrently active motor units during isometric voluntary contractions ranging from low force levels to maximal voluntary contractions (MVC). With this technology we have executed studies to investigate the behavior of the firing rates of motor units as a function of their recruitment properties during contractions at various force levels. We found that the firing rates have a hierarchical structure wherein the firing rate value of motor units is inversely related to their recruitment threshold, with earlier recruited motor units having greater firing rates at any time and any force level during a contraction. This relationship does not support the opposite notion that has been generally held for the past five decades. Knowing the structure of the firing behavior of motor units during voluntary contractions provides guidance for understanding the performance of muscles during exercise and sports

Experimental protocol to investigate cortical, muscular and body representation alterations in adolescents with idiopathic scoliosis

Background: Adolescent idiopathic scoliosis (AIS) is the most common form of scoliosis. AIS is a three-dimensional morphological spinal deformity that affects approximately 1-3% of adolescents. Not all factors related to the etiology of AIS have yet been identified. Objective: The primary aim of this experimental protocol is to quantitatively investigate alterations in body representation in AIS, and to quantitatively and objectively track the changes in body sensorimotor representation due to treatment. Methods: Adolescent girls with a confirmed diagnosis of mild (Cobb angle: 10°-20°) or moderate (21°-35°) scoliosis as well as age and sex-matched controls will be recruited. Participants will be asked to perform a 6-min upright standing and two tasks-named target reaching and forearm bisection task. Eventually, subjects will fill in a self-report questionnaire and a computer-based test to assess body image. This evaluation will be repeated after 6 and 12 months of treatment (i.e., partial or full-time brace and physiotherapy corrective postural exercises). Results: We expect that theta brain rhythm in the central brain areas, alpha brain rhythm lateralization and body representation will change over time depending on treatment and scoliosis progression as a compensatory strategy to overcome a sensorimotor dysfunction. We also expect asymmetric activation of the trunk muscle during reaching tasks and decreased postural stability in AIS. Conclusions: Quantitatively assess the body representation at different time points during AIS treatment may provide new insights on the pathophysiology and etiology of scoliosis

Ability-Based Methods for Personalized Keyboard Generation

This study introduces an ability-based method for personalized keyboard generation, wherein an individual's own movement and human-computer interaction data are used to automatically compute a personalized virtual keyboard layout. Our approach integrates a multidirectional point-select task to characterize cursor control over time, distance, and direction. The characterization is automatically employed to develop a computationally efficient keyboard layout that prioritizes each user's movement abilities through capturing directional constraints and preferences. We evaluated our approach in a study involving 16 participants using inertial sensing and facial electromyography as an access method, resulting in significantly increased communication rates using the personalized keyboard (52.0 bits/min) when compared to a generically optimized keyboard (47.9 bits/min). Our results demonstrate the ability to effectively characterize an individual's movement abilities to design a personalized keyboard for improved communication. This work underscores the importance of integrating a user's motor abilities when designing virtual interfaces.Comment: 20 pages, 7 figure

A muscle-force model with physiological bases

Muscle force is regulated by varying two main motor unit properties: the recruitment and the firing rates of motor units. Discrepancies still exist on the mechanisms involved in motor unit control and muscle force generation. This study investigated the behavior of motor unit firing rate during sustained fatiguing contractions and the motor unit parameters that are most likely to influence force fluctuation increase. We also studied the firing rate of motor units during linearly increasing force contractions up to maximum, or near maximum voluntary contraction force, at different rates of force increase, and developed an equation that models the firing rate behavior as a function of increasing excitation to the motor unit pool. Results were used to create a model of muscle force production that is based on verifiable physiological concepts and data. The model also includes the concept of common drive, i.e. of an oscillatory common input received by all motor units in the motor unit pool, the time-dependent changes of motor unit twitches, and a feedback loop to simulate force generation in a target-force tracking mode. Simulations showed that the model is able to mimic the force and firing rate patterns which have been experimentally observed during repeated contractions sustained to exhaustion: the excitation to the motoneuron pool must be adjusted in response to an increased or decreased force generation capacity of the muscle fibers, and the firing rates of all motor units respond consequently with a decreased or increased firing rate. The simulation of prolonged contractions showed that the increase in force variability may be attributed to the gradual recruitment of higherrecruitment threshold larger-amplitude force twitch motor units. The level of cross-correlation between firing rates appeared to influence force variability, whereas the variability in the firing rates had no clear effect on force variability.Il controllo della forza muscolare si basa principalmente su due fenomeni: il reclutamento di unità motorie e la regolazione della loro frequenza di scarica. Molti aspetti riguardanti i meccanismi coinvolti nel controllo delle unità motorie e nella generazione di forza muscolare restano ancora da investigare. Parte del lavoro di questa tesi ha riguardato lo studio del comportamento della frequenza di scarica delle unità motorie e dei parametri alla base dell’incremento delle fluttuazioni dell’output di forza durante l’esecuzione di contrazioni muscolari sostenute fino all’affaticamento. Inoltre, è stato analizzato il comportamento della frequenza di scarica delle unità motorie durante lo svolgimento di contrazioni muscolari a livelli di forza crescente fino alla massima forza di contrazione volontaria (a diverse velocità di incremento della forza); ed è stata messa a punto una equazione in grado di modellare il comportamento della frequenza di scarica in funzione dell’eccitazione ricevuta dal pool di unità motorie. I risultati di questa prima analisi sono serviti per creare un modello di produzione della forza muscolare basato su dati fisiologici verificabili. Il modello include il concetto di “common drive”, ovvero di un input oscillatorio comune ricevuto da tutte le unità motorie del pool; la dipendenza temporale dei “twitch” di forza delle unità motorie; ed un “feedback loop” per simulare la generazione di forza in contrazioni in “target-force tracking mode”. Si è dimostrato come il modello sviluppato sia in grado di simulare il pattern di forza e il comportamento delle unità motorie sperimentalmente osservati durante l’esecuzione di contrazioni prolungate e sostenute fino all’affaticamento. In particolare, si è potuto osservare come l’eccitazione ricevuta dal pool di unità motorie si modifichi in seguito ad un aumento o ad una diminuzione della capacità di produrre forza delle fibre muscolari e come la variazione dell’eccitazione comporti di conseguenza una diminuzione o un aumento della frequenza di scarica delle unità motorie e del numero di unità motorie attive. La simulazione di contrazioni muscolari prolungate ha anche evidenziato come la crescente variabilità della forza muscolare sia da attribuire al reclutamento di unità motorie caratterizzate da “twitch” di ampiezza maggiore e da un maggiore grado di cross-correlazione tra la frequenza di scarica delle unità motorie attive, mentre la variabilità della frequenza di scarica non sembra influire sull’output di forza

Is the notion of central fatigue based on a solid foundation?

Exercise-induced muscle fatigue has been shown to be the consequence of peripheral factors that impair muscle fiber contractile mechanisms. Central factors arising within the central nervous system have also been hypothesized to induce muscle fatigue, but no direct empirical evidence that is causally associated to reduction of muscle force-generating capability has yet been reported. We developed a simulation model to investigate whether peripheral factors of muscle fatigue are sufficient to explain the muscle force behavior observed during empirical studies of fatiguing voluntary contractions, which is commonly attributed to central factors. Peripheral factors of muscle fatigue were included in the model as a time-dependent decrease in the amplitude of the motor unit force twitches. Our simulation study indicated that the force behavior commonly attributed to central fatigue could be explained solely by peripheral factors during simulated fatiguing submaximal voluntary contractions. It also revealed important flaws regarding the use of the interpolated twitch response from electrical stimulation of the muscle as a means for assessing central fatigue. Our analysis does not directly refute the concept of central fatigue. However, it raises important concerns about the manner in which it is measured and about the interpretation of the commonly accepted causes of central fatigue and questions the very need for the existence of central fatigue

Atypical erythema nodosum in atypical tuberculosis presentation

Erythema nodosum (EN) is an inflammatory disease of the skin and subcutaneous tissue that may be found in association with many systemic diseases such as infectious diseases, sarcoidosis, Behçet disease, inflammatory bowel diseases and tumours, in particular lymphoma. EN may be also induced by some drugs, including mainly estroprogestinics, salicylic acid, minocycline and sulfamidic acid. Due to the numerous possible causes, sometimes it may be very difficult to achieve a correct diagnostic interpretation, especially when an isolated EN represents the revealing feature, as in the following case. We describe the case of a patient, of young age and good clinical condition, who developed EN during the course of abdominal tuberculosis. The diagnosis was obtained by histologic examination of the abdominal formation since positron emission tomography and total body axial tomography were not useful in discriminating EN from malignancies