The purpose of this study was to clarify the effects of lower limb muscle mass growth on sprinting ability in children aged 3 to 8 years. The subjects were 514 unimpaired children (266 boys and 248 girls). We measured their lower limb muscle thickness (anterior thigh: MTa, posterior thigh: MTp, and calf: MTC) and 25 meter sprinting time. Muscle thickness was measured using a B-mode ultrasound diagnostic imaging unit. From the 25 meter sprint, we measured the following characteristics in relation to sprinting ability: results, maximum velocity, stride and pitch. The results revealed that sprinting ability significantly correlated with MTp and MTC in both boys and girls. This suggests that, in addition to morphological development, lower limb muscle mass growth contributes to an increase in stride (m/step) and affects sprinting ability during the period from infancy to early childhood. However, no relationship was seen between sprinting ability and anthropometric characteristics (body height and mass) or lower limb muscle thickness among 8-year-old boys. It is possible that lower limb muscle quality and power as well as improvement in elements such as sprinting movement have a stronger influence on sprinting ability than morphological elements such as physique and muscle mass in boys around that age. In contrast, a significant relationship was seen between lower limb muscle thickness and sprinting ability in girls of all age groups, suggesting that, unlike boys, innate lower limb muscle mass influences sprinting ability for girls.
Loss of muscle strength is not only associated with loss of muscle mass, but also affected by neural factors. It is well known that facilitatory and inhibitory responses of spinal motor neurons occur with cutaneous stimulation via spinal interneurons. The purpose of this study was to examine the neural adaptations associated with low load resistance training utilizing skin cooling (SC). 10 men trained both legs and each side was randomly assigned to SC training (SC-T) and non SC training (NSC-T). Subjects performed 30 isometric ankle dorsiflexion repetitions at 35% maximum voluntary contraction (MVC) 3 times weekly for 6 weeks. The skin cooling condition was defined as when skin temperature was 25°C while repetitive resistance training was being performed. Dorsiflexor MVC significantly increased in both SC-T (n = 9) and NSC-T (n = 9) by 12.8 and 3.8%, respectively. A significant increase in root mean square of EMG (rmsEMG) was observed for 30 isometric ankle dorsiflexion repetitions in SC-T both pre- and post-training. Lower leg girths did not significantly increase post-training. Therefore, the results of this study suggest that muscle strength might increase via changes in neural activation and that SC-T may lead to greater increases in muscle strength compared with NSC-T because of improved muscle activation during resistance training with SC. Therefore, we suggest that low load resistance training with SC is an effective method to increase muscle strength.
The purpose of the study was to examine the changes in soleus Hoffmann (H)-reflex and volitional (V)-wave after resistance training (RT) with and without neuromuscular electrical stimulation (NMES). Fourteen participants were randomly allocated to receive RT with NMES (RT+NMES, n = 7) or RT without NMES (RT, n = 7). Each participant trained for 10 sessions of right leg standing calf-raise for 2 weeks. For the RT+NMES group, NMES was applied to the tibial nerve using rectangular pulses (400 µsec duration, 75-Hz trains). Stimulation intensity was set at the maximal tolerable level. The H-reflex was elicited just above motor threshold during rest and during maximal voluntary isometric contraction (MVIC) conditions. The H-reflex and V-wave were normalized to the maximal motor response (Mmax). There was a significant interaction between time (pre/post) and group in the H-reflex during rest and during the MVIC condition (P = 0.05), but no significant difference in plantar torque, Mmax or V-wave. The H-reflex during rest decreased from 0.54 to 0.38 in the RT+NMES group (P = 0.01) but did not change in the RT group (from 0.48 to 0.47, P = 0.79). During MVIC, the H-reflex increased from 0.23 to 0.76 in the RT+NMES group (P = 0.02), but did not change in the RT group (from 0.44 to 0.58, P = 0.12). The results indicate that NMES-induced proprioceptive input during RT has an inhibitory effect in the resting muscle and an excitatory effect in the voluntarily activated muscle via spinal and/or supraspinal pathways.
The present study focused on the effect of the newly motor unit (MU) recruitment on mechanomyographic signal (MMG) by the analysis on motor unit mechanical signal (MUMS) during prolonged isometric constant contraction (PICC) at low torque levels of the knee extension. The mechanical and myoelectric signals (MES) from m. vastus medialis or lateralis were recorded by condenser microphone and disc electrode, respectively. In order to recruit the objective MU during the PICC, the target torque set at several levels below the recruitment threshold torque of the MU (≦ 7.4 %MVC). 1) iMMG and iMES sustained constant for initial several minutes and then increased during the PICC. 2) MUMS superimposed on MMG from back ground MUs activities and iMUMS increased significantly at the timing of MU recruitment. Subsequent iMUMS decreased according to the decrement of MUMS amplitude depend on the discharge trend of the MU. 3) Amplitude of MUMS (MS-Vpositive) showed different trend depended the recruitment timing during PICC. At the iMMG constant phase, MS-Vpositive sustained constant followed by the increment similar to iMMG trend. In contrast, at iMMG increment phase, MS-Vpositive showed increment trend without the constant phase. The present results suggested that the newly MU recruitment increase the iMMG during the PICC. IMMG increment at later period of the PICC could interpret from the MU recruitment and MS-Vpositive increment. It is necessary to investigate the factors to increase the MS-Vpositive from the muscle and muscle fibers conditions.
Past studies have suggested that the cortisol awakening response (CAR) is a robust index for assessing daily stress. The present study examined the stability of CAR in individuals and the relationship between CAR and moods states and the influence of gender on CAR. CAR was measured in two successive days. Participants were student athletes (men=36, women=12) in a university athletic club that participated in club practice over five days a week. They were instructed to collect saliva just after and 30 minutes following awakening. The Moods was measured using the short form of the Profile of Moods States (POMS-S). The results indicated high stability of CAR values for the two successive days. Furthermore, there was a relationship between CAR and negative moods. Moreover, women showed significantly lager increases of the early morning free cortisol levels after awakening compared to men.