In vivo investigation of muscle behaviour during voluntary and electrically induced muscle contraction using B-Mode ultrasound imaging

Musculoskeletal Ultrasound Imaging (USI) is a growing field in literature. It has been proven to be a useful tool for investigating the properties of the muscle. There is growing interest in ultrasound imaging techniques for the description of skeletal muscle function, and different algorithms have been developed for this purpose. The majority of studies limit their focus on a particular area of the muscle, such as the aponeuroses, or on architectural parameters such as fiber length and pennation angle. The investigation of the entire muscle visualised on the ultrasound image may help elucidate the muscle function under normal conditions or when external factors compromise or alter the muscle function. Functional electrical stimulation (FES) is a technique based on the use of electrical current to activate skeletal muscles and facilitate their contraction. It is commonly used for strength training or in rehabilitation to accelerate or enhance the recovery of skeletal muscle's function. The ability of this technique to improve muscle performance in both healthy and diseased muscles has been demonstrated in research and in clinical practice. However the artificial nature of the muscle activation during FES leads to some important differences from the voluntary muscle contraction. Ultrasound Imaging (USI) is a potential tool that could provide objective measurements of the muscle's response during electrical stimulation, thus helping to describe and understand these differences. The aim of this study is to develop techniques based on USI that helps to elucidate the muscle function during electrical stimulation and allow comparison with voluntary contractions. Ultrasound videos were collected from healthy participants during experimental procedures involving voluntary and electrically induced muscle contractions. The videos were analysed using software algorithms for the tracking of features in US images. The resulting parameters were used as the basis for characterisation methods to describe the muscle contraction, both globally and locally. The effectiveness of the USI analysis techniques was tested and methods for extraction of physiological information from the video analysis were implemented. The regional distribution of muscle displacement during the tasks was analysed. Larger displacements were observed at deeper portions of the muscle in both the voluntary and the electrically induced contractions. Differential displacements across muscle depths were observed to differ during voluntary and FES contractions. The electric currents applied induce a uniform muscle contraction across different depths, most likely influenced by the way the electric field recruits muscle fibers. Muscle displacement was correlated to the force exerted by the muscle. Areas close to the deep aponeurosis have higher correlation with torque exerted and a second order polynomial can be used to define the relationship between displacement and torque. The relationship between the whole muscle displacement at different depths and the torque exerted was described using a polynomial surface fitting. Mechanical strain was used to map the muscle activation. Middle areas of the muscle undergo higher positive vertical strain (i.e. the muscle thickens) while deeper portions of the muscle are the most affected by shortening horizontal strain (i.e. the muscle shortens) in both voluntary and FES contractions. The muscle contractility was analysed through strain rate. A time-frequency analysis of the strain rate was performed. More frequency components and higher bandwidths were observed in FES induced contractions when compared to the voluntary. The frequency components might reflect the motor unit activation, suggesting that during FES all the motor units, firing at different rates, are recruited. In this project, USI was used as a tool to characterise the muscle behaviour locally. Regional muscle displacement and strain distribution have been used to elucidate the muscle function and quantify how different muscle areas are mechanically involved in the contraction. Strain rate was correlated with the muscle contractility and hypotheses regarding the correlation with motor units firing rate have been proposed. In conclusion a number of techniques were developed with the purpose to investigate the muscle function in normal conditions and when external factors, such as electrical stimulation, alter the natural muscle behaviour.