Effects of tendon elastic energy and electromyographic activity pattern on jumping height and pre-stretch augmentation during jumps with different pre-stretch intensity

The present study aimed to investigate the effects of tendon elastic energy and electromyographic activity patterns (ratio of pre-landing to concentric: mEMG PLA/CON; ratio of eccentric to concentric; mEMG ECC/CON) on jump performance. Twenty-nine males performed five kinds of unilateral jumps using only ankle joint (no-countermovement jump: noCMJ; countermovement jump: CMJ; drop jumps at 10, 20 and 30 cm drop height: DJ10, DJ20 and DJ30). Jumping height, pre-stretch augmentation and electromyographic activity of the plantar flexor muscles were measured. The elastic energy of the Achilles tendon was measured during isometric contractions. Relative tendon elastic energy (to body mass) was highly correlated with jumping heights of CMJ, DJ10 and DJ20 but not with noCMJ and DJ30, whereas that was significantly correlated with pre-stretch augmentation in CMJ, but not with three DJs. The mEMG PLA/CON was significantly correlated with the pre-stretch augmentation of DJ20 and DJ30, but not with DJ10, whereas the mEMG ECC/CON was significantly correlated with the pre-stretch augmentation of DJ20 and DJ30, but not with CMJ and DJ10. These results suggested that jumping exercises with low pre-stretch intensity benefited from tendon elastic energy, but those with high pre-stretch intensity benefited from electromyographic activity patterns.

Effect of angular velocity on active muscle stiffness in vivo

We previously reported that active muscle stiffness could be evaluated in vivo. However, we were not able to investigate active muscle stiffness as more than 250 deg·s^-1 due to the limitation of the torque motor of dynamometer. The aim of the present study was to investigate the effect of angular velocities (including higher angular velocities of more than 250 deg·s^-1) on active muscle stiffness. Eighteen males volunteered for this study. Active muscle stiffness of the medial gastrocnemius muscle was calculated according to changes in the estimated muscle force and fascicle length during fast lengthening at five different angular velocities (100, 200, 300, 500, and 600 deg·s^-1). Electromyographic activities of the lateral gastrocnemius muscle (LG) and soleus muscle (SOL) were evaluated over two different phases: before the stretch (mEMGa) and after the stretch (mMEGb). Active muscle stiffness was higher at 300 than at 100 deg·s^-1, but decreased as the angular velocity increased from 300 to 600 deg·s^-1. There were no differences in mEMGa and mEMGb values among the five angular velocities, whereas mEMGb values were higher than mEMGa for all angular velocities. In conclusion, active muscle stiffness was highest at 300 deg·s^-1 and decreased at both slower and faster angular velocities.

The differences in the structure of the motor nucleus of the medial gastrocnemius muscle in male and female rats

There are many reports describing sexual dissimilarities in the CNS, particularly in the brain and cortical regions. However, knowledge regarding sexual dissimilarities in the spinal cord and in particular in the hindlimb muscle-motoneuron connectivity controlling locomotion is limited. In order to recognise sex differences in the architecture of the medial gastrocnemius (MG) motor nucleus in rats of the same age, retrograde-labelled motoneurons were identified following a bath of the proximal stump of the transected MG nerve in horseradish peroxidase. The rostrocaudal distribution of motoneurons along the spinal cord and on transverse sections as well as the size and density of motoneurons in the motor nucleus were determined from serial microscopic images. It was shown that the MG pool extended throughout the L4-L6 segments, with a length that was 32% greater in males. The position of the moto pool within the ventral horn of the spinal cord transversal sections was also different in both sexes: the pool was located more dorsally and laterally in males. Three size categories of motoneurons with different soma diameters were distinguished: α₁ (27.5-40.0μm), α₂ (>40.0μm) and γ (<27.5μm). The density of α (α₁ and α₂) motoneurons as well as γ motoneurons was higher in females, by about 13% and 23%, respectively. The number of α₁ motoneurons was 8% higher in females, whereas there were 46% more α₂ motoneurons in males. The most significant differences in the distribution concerned α₂ motoneurons, which revealed a lower density in the rostral parts of the MG motor nucleus in females. It was therefore concluded that the length of the MG motor nucleus was shorter, whereas the mean density of α and γ motoneurons was higher in females.