Denervation (DEN) results in a rapid loss of excitability in skeletal muscles. We here examined the effects of stretching (STR) on muscle hypoexcitability during the early stage of denervation. Wistar rats were divided into the DEN and DEN+STR groups. DEN was induced unilaterally by cutting the sciatic nerve and was developed for 2 days. Denervated plantar flexor (PF) muscles were maximally stretched with nonelastic tape to keep an ankle joints at 60° dorsiflexion for 0.5 or 12 hours per day. There was a remarkable increase in in situ chronaxie in PF muscles from the DEN group. This was accompanied by decreased endplate area and mRNA levels of acetylcholinesterase (AChE) and peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) in denervated gastrocnemius muscles. Moreover, depolarization-induced force production, but not maximal Ca²⁺-activated force, was markedly reduced in mechanically skinned fibres from denervated gastrocnemius muscles. Importantly, DEN-induced prolonged chronaxie and decreased endplate area, but not depolarization-induced force depression in skinned fibres and reduced mRNA levels of AChE and PGC-1α, were alleviated by daily STR treatment. Our results suggest that mechanical load induced by STR inhibits post-synaptic dysfunction and hence an activation failure in early denervated skeletal muscle. Thus, STR can be used to preserve muscle contractile function during early stage of denervation.

To consider the characteristics that influence falls in older adults, we assessed their ability to recognize obstacle height and compared it with younger adults. The LEG condition was used to reproduce the height of an object by raising the lower limb after gazing at it, and the STICK condition was used when the subject stopped the bar raised by the examiner from the floor with verbal instructions, and these were compared between the elderly and the young. There was a significant difference between the elderly and the young in the LEG condition, whereas there was no significant difference in the STICK condition. It was suggested that in older adults, the ability to reproduce recognized heights with the body is reduced.

Mechanical stress, especially loading, is essential for the histological and functional maintenance of articular cartilage. However, in clinical practice, treatments for several diseases are accompanied by bed rest, and there are opportunities to reduce the loading on the lower limbs. Immobility and inactivity have been reported to cause disuse histological changes in both the skeletal muscle and the bone. Similarly, many researchers in basic and clinical studies reported that the unloading environment induces the disuse histological changes in the articular cartilage, and in 2019, “disuse atrophy in articular cartilage” was proposed. The histological changes were mainly due to the thinning of the cartilage and the decrease of the matrix staining intensity.
Also, in 2019, our research group reported that the unloading environment for 4 weeks induced these histological changes of the cartilage in the medial tibiofemoral joint using the hindlimb suspension rat model. In this review, the findings on articular cartilage, loading, and osteoarthritis that our research group has been working on were reviewed, and also results of the ongoing research were introduced.

Hip osteoarthritis is a representative disease with hip pain and decreases the range of joint motion, muscle strength of lower limbs, ability of daily activities, and quality of life. However, no evidences have been established to delay the progression of hip osteoarthritis. We focused on the hip joint moment impulse as a hip joint load during a movement and examined the hip moment impulse in the frontal plane during walking and hip moment impulse in the sagittal plane during the sit-to-stand movement. Regarding walking, we noted that the use of a contralateral cane decreased the hip moment impulse in the frontal plane, and a decrease in the walking speed increased the hip moment impulse in the frontal plane. Furthermore, we observed that a decrease in the sit-to-stand movement speed increased the hip moment impulse in the sagittal plane during the sit-to-stand movement. Our results help to understand a gait pattern with a low (or high) hip moment impulse in the frontal plane and a sit-to-stand movement pattern with a low (or high) hip moment impulse in the sagittal plane. Furthermore, our results may be useful to delay the progression of hip osteoarthritis.

Joint angle specificity in strength training, which means that muscle strength improves only around the joint angle where training is performed has conventionally been reported. However, this concept has only been verified in high-load training. We examined whether the joint angle specificity is also valid under low-load training. As a result, we found that muscle strength gains did not follow the joint angle specificity in ankle isometric training, ankle isotonic training, and hip isometric training. On the other hand, the muscle fascicle length measured by the ultrasound was shown to be consistent between the training conditions and the conditions in which muscle strength was improved, suggesting that muscle strength gains were specific to the fascicle length. This hypothesis is a new concept that has never been proposed before, and we reported it as the specificity of fascicle length in strength training.

Spinal reciprocal inhibition (RI) is an important mechanism to carry out smooth joint movements and optimize gait. This mechanism includes disynaptic RI and presynaptic inhibitions. In disynaptic RI, direct synaptic coupling from primary afferent (Ia) fibers in the main operating muscle inhibits the spinal anterior horn cells of the antagonist muscle via Ia inhibitory interneurons. In presynaptic inhibitions, afferent Ia fibers in the main operating muscle bind to the terminals of afferent Ia fibers in the antagonist muscle via depolarization of primary afferent interneurons. Through these inhibitory pathways, RI works to inhibit excessive muscle contraction in the antagonist muscle, thereby enabling coordinated movement. Upper motor neuron disorders and aging cause functional decline of the RI. In recent years, the RI enhancement method has attracted attention because the decrease in RI function causes excessive co-contraction. Brain stimulation and peripheral stimulation are effective interventions for RI enhancement. Therefore, we focused on stimulating the supplementary motor area in the brain and on repetitive passive movement in our experiments. In each stimulation method, RI potentiation was observed at a shorter intervention time than in previous studies, and the after-effect was sustained. We believe that these interventions are simple and inexpensive, can decrease excessive co-contraction, have a high spillover effect, and can be applied in rehabilitation.

When we perform two tasks simultaneously, we often cannot perform each task completely, and performance in either one or both tasks is inadequate. This decrement is defined as dual-task interference. Dual-task interference causes various accidents, such as falls. Thus, dual-task interference is one problem that physiotherapists need to solve. To repeat dual tasks (i.e., dual-task training) was a famous method to reduce dual-task interference. However, the effect of dual-task training might be limited to repeated dual tasks. That is, it has become clear that the transfer effect of dual-task training might be poor. In this review, we outlined the methods and limitations of dual-task training. Subsequently, as new methods to reduce dual-task interference, we presented potentials for transcranial direct current stimulation and cognitive tasks. Through this review, we hope to contribute to the reduction of dual-task interference in clinical fields.

Medial tibial stress syndrome (MTSS) is considered to be a microinjury at the junction of the tibial periosteum and fascia. The most common site affected by MTSS is the attachment of flexor digitorum muscle to the posteromedial border of tibia. The development of MTSS could be related to the mechanical properties of the toe flexor muscles, namely strength and muscle stiffness. In addition, a history of MTSS is a risk factor for its relapse, suggestive of some physical factors associated with MTSS development in runners with a history of MTSS. Therefore, we investigated these physical factors of the posterior lower leg muscle such as strength and stiffness in runners with a history of MTSS, who did not have pain in the lower limbs on the day of measurement, using a newly developed toe flexor strength measuring device and shear wave elastography. The results showed that the runners with a history of MTSS had higher stiffness in flexor digitorum longus and tibialis posterior muscles and increased plantar flexor strength of the first metatarsophalangeal joint, suggesting an association between MTSS and the mechanical properties of the toe flexor muscles.