فهرست مطالب زانیار کریمی

Fatigue, caused by prolonged standing activities, affects postural stability through disrupting somatosensory system functions. The application of sub-threshold mechanical vibration can enhance the sensitivity of the mechanical receptors in the sole of the feet. Improved sensory function leads to better movement control and balance. Therefore, this study investigated the effectiveness of vibration-based active insoles in reducing the impacts of fatigue on postural stability among military forces.

Each of the 20 healthy male participants of the study completed three standing balance tests: 1) on both legs, 2) on one leg, and 3) on one leg with a cognitive secondary task for two states of the vibration system (on and off), before and after a fatigue protocol, on a force plate. The activity of leg muscles was also monitored by electromyography (EMG). The mean as a linear index (amount) and the sample entropy (SaEn) as a non-linear index (structure) related to the resultant distance of the body’s center of pressure (CoPRD) displacements, as well as the normalized percentage of the muscle activity level (linear envelope), were used for statistical analyses.

Significant changes in most of the postural and muscular parameters were recorded after the fatigue protocol for the off-mode of the vibration system. Meanwhile, running the vibration system of active insoles was accompanied with no significant differences in the amount and structure of the CoPRD displacements, as well as the activity level of the lateral gastrocnemius and soleus muscles after fatigue, compared to the measurements before fatigue.

The findings confirm the effectiveness of applying sub-threshold mechanical vibration to the sole of the foot in improving standing balance by reducing the negative effects of fatigue on postural stability parameters. Therefore, it is suggested that the use of active insoles can be an effective, feasible, and accessible ergonomic intervention to address fatigue among military forces.

Given the high prevalence of low back pain in manual handling activities, its known relationship with spinal loads, and the role of muscular fatigue and the body’s adaptive mechanisms to counteract fatigue, this study investigated the effect of repetitive lifting tasks on trunk muscular fatigue and the kinematics of the spine and load-in-hand.

Eighteen male volunteers lifted a box from the floor to their waist height at a pace of ten lifts per minute until they could no longer continue the task and reported the highest level of exhaustion. Kinematic data and muscle electromyographic activity were simultaneously recorded using a motion capture system and an electromyography device. In this study, average trunk flexion angle and trunk angular velocity were calculated as trunk kinematic variables, while average box vertical travel distance, average box horizontal displacement from L5-S1, and average box vertical displacement velocity were considered as box kinematic variables. The median frequency of electromyographic signals from selected muscles was quantified as a muscle fatigue indicator. Since subjects performed different lifting cycles, the total number of cycles was divided into five distinct blocks for data analysis.

The results showed significant effects of lifting trial blocks on trunk angle (p=0.004) and vertical box displacement (p<0.001). Median frequency was significantly affected by lifting blocks for right (p=0.016) and left erector spinae (p=0.014), right (p=0.021) and left multifidus (p<0.001), right latissimus dorsi (p=0.001), and left rectus abdominis (p=0.039).

Overall, the results highlight variations in most kinematic parameters and a reduction in the frequency content of EMG signal spectra. These changes serve as indices of the central nervous system’s control over lifting behavior under dynamic conditions. A better understanding of these central nervous system adaptations could have practical applications in interventions such as workstation design, exoskeleton development, and worker training to manage musculoskeletal disorders.