Skeletal muscle is a vital tissue not only for maintaining posture and body movements but also for energy metabolism in human body. Skeletal muscle is highly plastic in response to various stimuli, resistance exercise or anabolic hormones can increase muscle mass, which is termed “muscle hypertrophy”. Contrary, immobility, aging and severe illness can reduce muscle mass, which is termed “muscle atrophy”. Loss of skeletal muscle mass is associated with loss of independent living, the morbidity of a variety of diseases and mortality throughout life. Therefore, understanding mechanisms that regulate skeletal muscle mass is essential for improving the quality of life. Recent studies reported microRNAs (miRNAs), which is a class of non-coding RNAs, play a crucial role in the regulation of muscle mass. This review provides a current understanding of the function of miRNAs in regulation of skeletal muscle mass.

Mitochondria play a central role in energy production but also are the main cellular source of reactive oxygen species (ROS). It is widely believed that aging is related to the accumulation of oxidative molecular damage due to ROS. Aging also induces a decrease in muscle function. Recent studies indicate that caloric restriction and physical exercise suppress the decrease in muscle function though the maintaining quality of mitochondria. This review provides the current understanding of the influence of caloric restriction and exercise on mitochondrial quality control in skeletal muscle.

Skeletal muscle is the dominant organ in locomotion and energy metabolism so that maintaining skeletal muscle function is a prerequisite for our health and independent living throughout the life. A loss or decrease in skeletal muscle function leads to increased morbidity and mortality through the development of secondary diseases such as metabolic syndrome, type 2 diabetes, obesity, cardiovascular and respiratory disease. Identifying mechanisms which influence the processes regulating skeletal muscle function is a key priority. The recent discovery of microRNAs (miRNAs) provides a new knowledge controlling skeletal muscle function. miRNAs suppress gene expression through either in inhibition of protein translation or in degradation of the mRNA transcripts through a process similar to RNA interference (RNAi). This review provides the current understanding in skeletal muscle miRNA biology and focuses on their role and regulation under physiological conditions with exercise.

The discovery of microRNAs (miRNAs) and following studies reporting that miRNAs are stably present in the circulation, paved the way for possible use of miRNAs as biomarkers. Identification of circulating miRNAs that are functional upon delivery to the recipient cells, suggests miRNAs play roles in cell-to-cell communication as well as hormones and cytokines. This review provides a basic understanding of miRNAs and the current understanding of circulating miRNA as biomarkers and focuses on their putative roles and regulation under stressed conditions including physical exercise.

The purpose of this study was to investigate the relation between alterations of salivary secretory immunoglobulin A (SIgA) and the occurrence of upper respiratory tract infections (URTI) in rugby football players.We examined the relationship between the onset of URTI and the daily alterations of SIgA levels in 32 male collegiate rugby football players (20.5±1.3 years) during summer training camp for 36 days.Total of 6 in 32 subjects had the appearance of URTI symptoms (18 %). SIgA secretion rate decreased significantly in the middle of training camp compared to the baseline (P＜0.05). Furthermore, SIgA secretion rate during the appearance of URTI (13.7±1.1μg/min) were significantly lower than that without symptoms (19.2±1.4 μg/min, P＜0.01). These results suggest that serial monitoring of SIgA may be useful to assess the risk status of URTI affection in athletes.

The purpose of this study was to investigate differences between underwater and land-based exercise in leg muscle activity. Nine healthy males (mean age : 21.7±0.5 years, mean height : 173.4±2.2 cm) had electrodes placed on their left leg muscles (Tibialis Anterior ; TA, Medial Gastrocnemius ; MG, Soleus ; SOL, Rectus Femoris ; RF, and Biceps Femoris ; BF), and their muscle activity was measured during various exercises. The subjects performed six types of exercise such as the forward walk, backward walk, squat, calf raise, leg range, and one leg wave, both in the water (waist level) and on land. These exercises were categorized as 3 types of leg movement according to direction ; horizontal, vertical, and mixed movement.
In the forward walk and backward walk, categorized as horizontal movements, the integrated electromyogram (IEMG) significantly increased during underwater exercise compared with on land. In the squat, as a vertical movement, the IEMG showed a similar change under both conditions. In the calf raise, as a vertical movement, and leg range and one leg wave, as mixed movements, the IEMG significantly decreased during underwater exercise compared with on land. These results suggest that leg muscle activity during underwater exercise is different based on the movement direction of the legs. In a word, it was apparent that movement in a horizontal direction underwater provides greater activity for leg muscles than on land; but movement in a vertical and/or mixed direction underwater provides less activity for leg muscles than on land. In prescribing an exercise program, it may be helpful to understand the differences between underwater and land-based exercise in leg muscle activity.