Contractile behavior of the gastrocnemius medialis muscle during running in simulated hypogravity

Vigorous exercise countermeasures in microgravity can largely attenuate muscular degeneration, albeit the extent of applied loading is key for the extent of muscle wasting. Running on the International Space Station is usually performed with maximum loads of 70% body weight (0.7 g). However, it has not been investigated how the reduced musculoskeletal loading affects muscle and series elastic element dynamics, and thereby force and power generation. Therefore, this study examined the effects of running on the vertical treadmill facility, a ground-based analog, at simulated 0.7 g on gastrocnemius medialis contractile behavior. The results reveal that fascicle−series elastic element behavior differs between simulated hypogravity and 1 g running. Whilst shorter peak series elastic element lengths at simulated 0.7 g appear to be the result of lower muscular and gravitational forces acting on it, increased fascicle lengths and decreased velocities could not be anticipated, but may inform the development of optimized running training in hypogravity. However, whether the alterations in contractile behavior precipitate musculoskeletal degeneration warrants further study.

Gastrocnemius Medialis Contractile Behavior Is Preserved During 30% Body Weight Supported Gait Training

Rehabilitative body weight supported gait training aims at restoring walking function as a key element in activities of daily living. Studies demonstrated reductions in muscle and joint forces, while kinematic gait patterns appear to be preserved with up to 30% weight support. However, the influence of body weight support on muscle architecture, with respect to fascicle and series elastic element behavior is unknown, despite this having potential clinical implications for gait retraining. Eight males (31.9 ± 4.7 years) walked at 75% of the speed at which they typically transition to running, with 0% and 30% body weight support on a lower-body positive pressure treadmill. Gastrocnemius medialis fascicle lengths and pennation angles were measured via ultrasonography. Additionally, joint kinematics were analyzed to determine gastrocnemius medialis muscle–tendon unit lengths, consisting of the muscle's contractile and series elastic elements. Series elastic element length was assessed using a muscle–tendon unit model. Depending on whether data were normally distributed, a paired t-test or Wilcoxon signed rank test was performed to determine if body weight supported walking had any effects on joint kinematics and fascicle–series elastic element behavior. Walking with 30% body weight support had no statistically significant effect on joint kinematics and peak series elastic element length. Furthermore, at the time when peak series elastic element length was achieved, and on average across the entire stance phase, muscle–tendon unit length, fascicle length, pennation angle, and fascicle velocity were unchanged with respect to body weight support. In accordance with unchanged gait kinematics, preservation of fascicle–series elastic element behavior was observed during walking with 30% body weight support, which suggests transferability of gait patterns to subsequent unsupported walking.

Design and Modeling of a Series Elastic Element for Snake Robots

In this work, we detail the design, fabrication, and initial modeling of a compact, high-strength series elastic element designed for use in snake robots. The spring achieves its elasticity by torsionally shearing a rubber elastomer that is bonded to two rigid plates, and it is able to achieve mechanical compliance and energy storage that is an order of magnitude greater than traditional springs. Its novel design features a tapered conical cross-section that creates uniform shear stress in the rubber, improving the ultimate strength. Tests show that the torque-displacement profile of these springs is approximately linear, and initial results are reported on creating more accurate models that account for the element’s hysteresis and viscoelastic properties. Low-bandwidth force control is demonstrated by measuring the element’s torsional deflection to estimate the torque output of one of our snake robot modules.

Contribution of Muscle Series Elasticity to Maximum Performance in Drop Jumping

The contribution of muscle in-series compliance on maximum performance of the muscle tendon complex was investigated using a forward dynamic computer simulation. The model of the human body contains 8 Hill-type muscles of the lower extremities. Muscle activation is optimized as a function of time, so that maximum drop jump height is achieved by the model. It is shown that the muscle series elastic energy stored in the downward phase provides a considerable contribution (32%) to the total muscle energy in the push-off phase. Furthermore, by the return of stored elastic energy all muscle contractile elements can reduce their shortening velocity up to 63% during push-off to develop a higher force due to their force velocity properties. The additional stretch taken up by the muscle series elastic element allows only m. rectus femoris to work closer to its optimal length, due to its force length properties. Therefore the contribution of the series elastic element to muscle performance in maximum height drop jumping is to store and return energy, and at the same time to increase the force producing ability of the contractile elements during push-off.

Effect of series elasticity on delay in development of tension relative to stiffness during muscle activation

Experimental data have indicated that during activation, the attachment of myosin to actin, measured by mechanical stiffness, precedes tension generation by 10-30 ms. Using computer simulation, we have investigated the effect of a series elastic element on the lag between stiffness and tension development during muscle activation. Two versions of the two-state cross-bridge model originally proposed by Huxley and a three-state model were considered. After simulated activation, stiffness and tension increased with rates that were strongly dependent on the series elastic strain. In the absence of a series elastic element, the rise in stiffness preceded, lagged, or was coincident with the increase in tension, depending on the model. For large elastic strains, tension lagged stiffness for all models. Lags of 10-30 ms could be obtained with elastic strains of 0.3-1% of the muscle length. This is a realistic value in experiments without sarcomere length servocontrol, suggesting that series elasticity may be an important contributor to the experimentally observed lag between tension and stiffness.