Neural Drive and Motor Unit Characteristics of the Serratus Anterior in Individuals With Scapular Dyskinesis.

OBJECTIVE: Scapular dyskinesis is one of the causes of shoulder disorders and involves muscle weakness in the serratus anterior. This study investigated whether motor unit (MU) recruitment and firing property, which are important for muscle exertion, have altered in serratus anterior of the individuals with scapular dyskinesis. METHODS: Asymptomatic adults with (SD) and without (control) scapular dyskinesis were analyzed. Surface electromyography (sEMG) waveforms were collected at submaximal voluntary contraction of the serratus anterior. The sEMG waveform was decomposed into MU action potential amplitude (MUAPAMP), mean firing rate (MFR), and recruitment threshold. MUs were divided into low, moderate, and high thresholds, and MU recruitment and firing properties of the groups were compared. RESULTS: High-threshold MUAPAMP was significantly smaller in the SD group than in the control group. The control group also exhibited recruitment properties that reflected the size principle, however, the SD group did not. Furthermore, the SD group had a lower MFR than the control group. CONCLUSIONS: Individuals with scapular dyskinesis exhibit altered MU recruitment properties and lower firing rates of the serratus anterior; this may be detrimental to muscle performance. Thus, it may be necessary to improve the neural drive of the serratus anterior when correcting scapular dyskinesis.

Sex differences in pelvis, thigh, and shank coordination during walking.

Differences in lower limb kinematics between males and females during functional activities may be attributed to sex differences in the incidence of patellofemoral pain, which is more common in females. To better comprehend the knee joint motion, it is necessary to understand both inter-segmental coordination patterns and angular amplitude. This exploratory study aimed to assess sex differences in pelvis-thigh and thigh-shank coordination patterns in the frontal and horizontal planes during walking. Data regarding the kinematic characteristics of the pelvis, thigh, and shank segments were collected from 26 males and 26 females performing walking at self-selected speeds using a 3D motion capture system. Furthermore, we compared the kinematics of the pelvis, thigh, and shank during walking as well as the pelvis-thigh and thigh-shank coordination patterns in the frontal and horizontal planes during the stance phase between males and females. Compared to males, females had greater thigh adduction (p < 0.001) and internal rotation (p < 0.001) throughout the stance phase; significantly greater frequency of the pelvis-thigh anti-phase pattern in the frontal plane in the early (p = 0.002) and mid-stance (p = 0.003); and significantly greater thigh-shank anti-phase pattern in the frontal plane in the early (p = 0.001) and mid-stance (p = 0.015). These results suggest the presence of sex differences in the inter-segmental coordination of the pelvis and lower limb during walking. However, as this study could not determine a causal relationship between female sex and knee joint injury, further longitudinal studies are needed to determine the effects of differences in coordination patterns on the pathophysiology of the injury and pain generation.

Effects of Gluteus Maximus Muscle Activity and Pelvic Width on Dynamic Frontal Plane Hip Joint Stiffness During Gait in Healthy Young Women.

CONTEXT: Excessive hip adduction and internal rotation are abnormal movements that may lead to the onset and progression of patellofemoral pain. Previous studies have reported that lower dynamic frontal plane hip joint stiffness in the gait of women is associated with the magnitude of hip adduction and internal rotation angles. However, the factors contributing to the lack of dynamic frontal plane hip joint stiffness in the gait of young women are unclear. This study aims to investigate the factors affecting dynamic frontal plane hip joint stiffness during the weight-acceptance phase of the gait of healthy young women. DESIGN: Cross-sectional study. METHODS: This study included 30 healthy women between the ages of 18 and 30 years. The pelvic width/femur length ratio was calculated by dividing the pelvic width by the femur length. Data on hip kinematics and kinetics and activation of the gluteus maximus and medius, tensor fasciae latae, and adductor longus muscles during gait were collected using a motion capture system, force plates, and surface electromyography. Stepwise multiple regression analysis was conducted to determine the extent to which each independent factor affected dynamic frontal plane hip joint stiffness. RESULTS: In healthy young women, decreased dynamic frontal plane hip joint stiffness was associated with decreased muscle activity of the gluteus maximus during the gait, as well as greater pelvic width/femur length ratio. CONCLUSIONS: Women with a relatively great pelvic width relative to femur length may have more difficulty in producing dynamic frontal plane hip joint stiffness. However, increasing the muscle activity of the gluteus maximus may contribute to increased dynamic frontal plane hip joint stiffness.

The effect of lumbopelvic region rotation relative flexibility on thorax-pelvis and pelvis-femur coordination during walking.

BACKGROUND: Lumbopelvic region rotation relative flexibility (LRRF), which is defined as lumbopelvic region that is relatively less stiffness than the hip region, is associated with low back pain (LBP) symptoms. However, how LRRF is influenced by lumbopelvic region motion during walking is unclear. RESEARCH QUESTION: What is the influence of LBP and LRRF on coordination patterns of the thorax, pelvis, and femur during walking? METHODS: The presence of LRRF was determined based on whether the lumbopelvic rotation occurred in the first 50% of knee flexion or hip external rotation movement. Participants with LBP and LRRF were classified into the LBP group. Participants with LRRF but without LBP were classified into the early pelvis rotation (ROT) group, and those without LBP and relative flexibility were classified as controls. The thorax-pelvis coordination and pelvis-femur coordination during the stance cycles were calculated from the segmental angles obtained by three-dimensional motion analysis using a modified vector coding technique. RESULTS: In the sagittal plane, the thorax-pelvis coordination of the LBP group showed more anti-phase patterns at both the early stance and midstance compared with controls and the ROT group. In the sagittal and horizontal planes, pelvis-femur coordination of the LBP and ROT groups showed more in-phase patterns during the early stance and midstance compared with controls. SIGNIFICANCE: Regardless of LBP, the presence of LRRF alters the intersegmental coordination during walking. In individuals with LRRF, stiffness of the hip may increase during walking. People who have LRRF without LBP may develop LBP in the future, and it is important for prevention to identify these differences in kinematics during walking.