During muscle contractions, the muscle fascicles may shorten at a rate different from the muscletendon unit, and the ratio of these velocities is its gearing. Appropriate gearing allows fascicles to reduce their shortening velocities and allows them to operate at effective shortening velocities across a range of movements. Gearing of the muscle fascicles within the muscle belly is the result of rotations of the fascicles and bulging of the belly. Variable gearing can also occur as a result of tendon length changes that can be caused by changes in the relative timing of muscle activity for different mechanical tasks. Recruitment patterns of slow and fast fibres are crucial for achieving optimal muscle performance, and coordination between muscles is related to whole limb performance. Poor coordination leads to inefficiencies and loss of power, and optimal coordination is required for high power outputs and high mechanical efficiencies from the limb. This paper summarizes key studies in these areas of neuromuscular mechanics and results from studies where we have tested these phenomena on a cycle ergometer are presented to highlight novel insights. The studies show how muscle structure and neural activation interact to generate smooth and effective motion of the body.Keywords: power; efficiency; gearing; pennation PROLOGUEMuscles have a surprising variety of functions during locomotion and can serve as motors, brakes, springs and struts [1]. These muscle functions depend on the arrangement of the muscle fibres within the muscle belly, the length and velocity of the fibres and also on the timing of their activation relative to those length changes. When the muscle fibres undergo substantial length change the muscle can act as a motor if it is active during shortening, or a brake if it is active during lengthening; when the length change is minimal then the muscle can act like a strut if it actively develops force to resist stretch [1]. Muscle fibre velocity may differ from the velocity of the muscletendon unit (MTU) in a process known as gearing. Thus, the extent to which the muscle fibres change length during a limb motion depends on how they are geared relative to the MTU. The timing and the level of activation are under neural control. This paper is a synthesis of previous literature on both gearing and activation, with a particular focus on some of the more recent findings in man. We introduce additional findings about structural mechanics in man and propose that some of the features of muscle gearing actually result from the interactions between the structural mechanics and the activation patterns of the muscles. MUSCLE STRUCTURE AND FASCICLE GEARINGMTUs are comprised of both tendinous structures containing a large proportion of collagen fibres, and skeletal muscle fibres containing the actin-and myosin-rich contractile machinery. The skeletal muscle fibres typically lie parallel to each other and are bundled to form fascicles that combine to form the muscle belly. The mechanics of fibres and fascicles can be ...

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Brain network activity associated with altered motor control in individuals with chronic pain is not well understood. Chronic Prostatitis/Chronic Pelvic Pain Syndrome (CP/CPPS) is a debilitating condition in which previous studies have revealed altered resting pelvic floor muscle activity in men with CP/CPPS compared to healthy controls. We hypothesized that the brain networks controlling pelvic floor muscles would also show altered resting state function in men with CP/CPPS. Here we describe the results of the first test of this hypothesis focusing on the motor cortical regions, termed pelvic-motor, that can directly activate pelvic floor muscles. A group of men with CP/CPPS (N = 28), as well as group of age-matched healthy male controls (N = 27), had resting state functional magnetic resonance imaging scans as part of the Multidisciplinary Approach to the Study of Chronic Pelvic Pain (MAPP) Research Network study. Brain maps of the functional connectivity of pelvic-motor were compared between groups. A significant group difference was observed in the functional connectivity between pelvic-motor and the right posterior insula. The effect size of this group difference was among the largest effect sizes in functional connectivity between all pairs of 165 anatomically-defined subregions of the brain. Interestingly, many of the atlas region pairs with large effect sizes also involved other subregions of the insular cortices. We conclude that functional connectivity between motor cortex and the posterior insula may be among the most important markers of altered brain function in men with CP/CPPS, and may represent changes in the integration of viscerosensory and motor processing.

Human pelvic floor muscles have been shown to operate synergistically with a wide variety of muscles, which has been suggested to be an important contributor to continence and pelvic stability during functional tasks. However, the neural mechanism of pelvic floor muscle synergies remains unknown. Here, we test the hypothesis that activation in motor cortical regions associated with pelvic floor activation are part of the neural substrate for such synergies. We first use electromyographic recordings to extend previous findings and demonstrate that pelvic floor muscles activate synergistically during voluntary activation of gluteal muscles, but not during voluntary activation of finger muscles. We then show, using functional magnetic resonance imaging (fMRI), that a region of the medial wall of the precentral gyrus consistently activates during both voluntary pelvic floor muscle activation and voluntary gluteal activation, but not during voluntary finger activation. We finally confirm, using transcranial magnetic stimulation, that the fMRI-identified medial wall region is likely to generate pelvic floor muscle activation. Thus, muscle synergies of the human male pelvic floor appear to involve activation of motor cortical areas associated with pelvic floor control.