Corticospinal excitability remains unchanged in the presence of residual force enhancement and does not contribute to increased torque production

Background Following stretch of an active muscle, muscle force is enhanced, which is known as residual force enhancement (rFE). As earlier studies found apparent corticospinal excitability modulations in the presence of rFE, this study aimed to test whether corticospinal excitability modulations contribute to rFE. Methods Fourteen participants performed submaximal plantar flexion stretch-hold and fixed-end contractions at 30% of their maximal voluntary soleus muscle activity in a dynamometer. During the steady state of the contractions, participants either received subthreshold or suprathreshold transcranial magnetic stimulation (TMS) of their motor cortex, while triceps surae muscle responses to stimulation were obtained via electromyography (EMG), and net ankle joint torque was recorded. B-mode ultrasound imaging was used to confirm muscle fascicle stretch during stretch-hold contractions in a subset of participants. Results Following stretch of the plantar flexors, an average rFE of 7% and 11% was observed for contractions with subthreshold and suprathreshold TMS, respectively. 41–46 ms following subthreshold TMS, triceps surae muscle activity was suppressed by 19–25%, but suppression was not significantly different between stretch-hold and fixed-end contractions. Similarly, the reduction in plantar flexion torque following subthreshold TMS was not significantly different between contraction conditions. Motor evoked potentials, silent periods and superimposed twitches following suprathreshold TMS were also not significantly different between contraction conditions. Discussion As TMS of the motor cortex did not result in any differences between stretch-hold and fixed-end contractions, we conclude that rFE is not linked to changes in corticospinal excitability.

Study on an Analytical Solution to the Mechanical Response of Soil Cut by a Bionic Force Enhancement Grouser

With the depletion of land mineral resources, people have turned their attention to the sea. As an important part of deep-sea mining systems, the technical research and development of ore collectors has always been a difficult problem in many countries. According to the characteristic that buffalo hoof is suitable for walking on soft soil, a kind of bionic grouser for a deep-sea mining vehicle is designed in this paper. Through the optimization of Rankine’s passive earth pressure theory, the formula for calculating the tractive force of the grouser is obtained. The accuracy of the analytical solution is verified by finite element simulation, and the force enhancement mechanism of the bionic grouser is revealed. The results show that the design of the bionic grouser has a significant effect on the improvement of tractive force, and the tractive force of the No. 1 bionic grouser is 17.52% higher than that of the straight grouser. On this basis, the geometric parameters of the bionic grouser profile are optimized. The results show that when L is 0 mm and R is 183 mm, the force enhancement effect reaches the maximum of 27%, which provides a design basis for optimizing the grouser and improving the mining efficiency of the collector.

Effect of Active Lengthening and Shortening on Small-Angle X-ray Reflections in Skinned Skeletal Muscle Fibres

Our purpose was to use small-angle X-ray diffraction to investigate the structural changes within sarcomeres at steady-state isometric contraction following active lengthening and shortening, compared to purely isometric contractions performed at the same final lengths. We examined force, stiffness, and the 1,0 and 1,1 equatorial and M3 and M6 meridional reflections in skinned rabbit psoas bundles, at steady-state isometric contraction following active lengthening to a sarcomere length of 3.0 µm (15.4% initial bundle length at 7.7% bundle length/s), and active shortening to a sarcomere length of 2.6 µm (15.4% bundle length at 7.7% bundle length/s), and during purely isometric reference contractions at the corresponding sarcomere lengths. Compared to the reference contraction, the isometric contraction after active lengthening was associated with an increase in force (i.e., residual force enhancement) and M3 spacing, no change in stiffness and the intensity ratio I1,1/I1,0, and decreased lattice spacing and M3 intensity. Compared to the reference contraction, the isometric contraction after active shortening resulted in decreased force, stiffness, I1,1/I1,0, M3 and M6 spacings, and M3 intensity. This suggests that residual force enhancement is achieved without an increase in the proportion of attached cross-bridges, and that force depression is accompanied by a decrease in the proportion of attached cross-bridges. Furthermore, the steady-state isometric contraction following active lengthening and shortening is accompanied by an increase in cross-bridge dispersion and/or a change in the cross-bridge conformation compared to the reference contractions.

Spontaneous myogenic fasciculation associated with the lengthening of cardiac muscle in response to static preloading

Force enhancement is one kind of myogenic spontaneous fasciculation in lengthening preload striated muscles. In cardiac muscle, the role of this biomechanical event is not well established. The physiological passive property is an essential part for maintaining normal diastole in the heart. In excessive preload heart, force enhancement relative erratic passive properties may cause muscle decompensating, implicate in the development of diastolic dysfunction. In this study, the force enhancement occurrence in mouse cardiac papillary muscle was evaluated by a microstepping stretch method. The intracellular Ca2+ redistribution during occurrence of force enhancement was monitored in real-time by a Flou-3 (2 mM) indicator. The force enhancement amplitude, the enhancement of the prolongation time, and the tension–time integral were analyzed by myography. The results indicated that the force enhancement occurred immediately after active stretching and was rapidly enhanced during sustained static stretch. The presence of the force and the increase in the amplitude synchronized with the acquisition and immediate transfer of Ca2+ to adjacent fibres. In highly preloaded fibres, the enhancement exceeded the maximum passive tension (from 4.49 ± 0.43 N/mm2 to 6.20 ± 0.51 N/mm2). The occurrence of force enhancement were unstable in each static stretch. The increased enhancement amplitude combined with the reduced prolongation time to induce a reduction in the tension–time integral. We concluded that intracellular Ca2+-synchronized force enhancement is one kind of interruption event in excessive preload cardiac muscle. During the cardiac muscle in its passive relaxation period, the occurrence of this interruption affected the rhythmic stability of the cardiac relaxation cycle.

The static preload associated myogenic spontaneous fasciculation response in lengthening cardiac muscle

The passive tension force enhancement is one kind of myogenic spontaneous fasciculation in muscles. However, its physiological properties in cardiac fibres are not well known. In this study, mice cardiac papillary muscle spontaneous force enhancement was evaluated by micro stepping stretch method. The occurrence of spontaneous force and real time cardiac fibre Ca2+ redistribution was tranced by Flou-3 (2mM) indicator. Force enhancement amplitude, enhancement prolonging time, and tension–time integral were analysis by myograph analyser. The results indicated that the spontaneous force occurred immediately after the active stretch, rapidly enhanced during tolerating the sustained static stretch. The force occurrence and amplitude enhance synchronized with the Ca2+ recruitment and lightning transmitted to adjacent fibres. In high preload fibres, the enhancement was forceful to over its maximum passive tension (6.20 ± 0.51 N/mm2 to 4.49 ± 0.43 N/mm2). The force occurrences were unsteadiness in each stretch. The increased enhancement amplitude combining with the shortening prolonging time induced reduction of tension–time integral. We concluded that the intracellular Ca2+ synchronized force enhancement is one kind of interruption event in overloading cardiac fibres. This interruption occurred during the relaxation processing in cardiac muscle, therefore affect the rhythmic stability of cardiac relaxation-contraction cycle.