This paper considers the synthesis of the four-bar mechanism. It is treated here as an optimization problem, in which an objective function is defined. To solve this problem, a metaheuristic called the virus optimization algorithm is employed. Furthermore, a new path-repairing technique recently published by Sleesongsom and Bureerat is applied instead of the very common technique related to the application of a penalty function. This makes the search by means of the metaheuristic more efficient. Furthermore, the obtained results are very accurate.

(1) Background: This study aimed to assess whether older adults exhibit greater discrepancies between intended and actual motor unit recruitment, which could affect the quality of muscle activation and potentially increase the risk of falls. (2) Methods: Forty-eight physically active older women were assessed (65 ± 6 years, 164 ± 6 cm, and 76 ± 7 kg). The bioelectrical activity (EMG) of the vastus lateralis oblique (VLO) and vastus medialis oblique (VMO) muscles were assessed during isometric testing with the knee joint bent to 75 degrees. The participants were instructed to press against a stable bar for 5 s at a specific percentage of their perceived force level (at 15%, 30%, and 60% of MVC) when the EMG activity was recorded. Balance was assessed using a stabilometric platform in a standing position. (3) Results: In all three thresholds, the bioelectrical activity of the VLO and VMO muscles significantly deviated from what was expected under the assumption of a nearly linear relationship between muscle force and bioelectrical activity. In each of the three thresholds, it did not exceed 10% MVC and significantly differed only between the 15% and 60% MVC thresholds. No significant differences were found between the dominant and non-dominant sides. A significant relationship was observed between the sway area (Area 95%) and the activity of the non-dominant limb VLO muscle. (4) Conclusions: Our results suggest that older adults experience deficits in muscle activation perception, leading to discrepancies between intended and actual muscle engagement, which may affect functional task performance and potentially increase fall risk.

Background/Objectives. Despite growing interest in capacitive-resistive electric transfer TECAR) and Vibration therapy (VT), their comparative effectiveness in sports recovery remains unclear. This study aimed to evaluate and contrast the short-term effects of TECAR and VT on neuromuscular recovery following eccentric muscle fatigue, relative to passive rest, in active young adults. We hypothesized that both interventions would accelerate recovery and potentially reduce injury risk. Methods. Forty-one participants were randomized into two groups: TECAR therapy (Group 1) and VT (Group 2). Neuromuscular function was assessed at baseline, post-exercise, and post-intervention using tensiomyography (TMG) and electromyography (EMG). Results. Both groups showed a significant increase in EMG MDF intercept after exercise. Post-intervention, VT induced a further rise in this parameter, whereas TECAR stabilized values without significant change. In the contralateral resting limb, increases persisted after exercise and passive recovery. Between-limb differences were significant only in the TECAR group. TMG analysis revealed a non-significant but large-effect increase in contraction delay (Td) post-exercise, followed by significant reductions after both interventions. In the left limb, Td changes were not significant. For maximal displacement (Dm), both VMO and VLO muscles demonstrated a significant decrease post-exercise and a marked recovery after both therapies. Other TMG parameters (Ts, Tc, Tr) showed no significant changes. Conclusions. Both TECAR and VT effectively enhanced neuromuscular recovery after eccentric exercise. TECAR demonstrated a modest but consistent advantage, particularly in normalizing muscle recruitment and restoring mechanical properties, making it suitable in contexts requiring rapid recovery. VT, however, remains a more accessible and cost-effective modality. These findings support the application of both techniques in sports recovery, while highlighting the need for further research in professional athletes and diverse exercise settings to optimize regeneration strategies and reduce injury risk.

Objectives: Rupture of the anterior cruciate ligament (ACL) is a severe injury common in sports. It also has a high rate of re-injury. The aim of this work was to assess hamstring muscle fatigue in active football players after ACL reconstruction who were cleared to play and to determine symmetry between the lateral and medial hamstring muscles. Methods: In professional football players post ACL reconstruction (n = 25) and non-injured players (n = 26), the bioelectrical activity of the medial (biceps femoris—BF) and lateral (semimembranosus and semitendinosus—SEM) hamstring muscles was measured during 60 s of isometric contraction. The fatigue variables were calculated using the Continuous Wavelet Transform (CWT) tool. Results: The football players following ACL reconstruction demonstrated significant asymmetry in fatigue of the lateral and medial hamstring muscles, with greater fatigue in the SEM compared to the BF muscle. Moreover, in those after reconstruction, the changes are more pronounced, with higher muscle fatigue in both limbs (they have lower MDF than non-injured players) and more severe SEM muscle insufficiency (noted in both limbs but with greater intensity in the non-operated one). Conclusions: The higher SEM muscle fatigue observed in this study influenced the lateral-to-medial activation ratio within the hamstring muscle, which may be a probable cause of this muscle’s insufficiency in laterally stabilizing the knee in the frontal and transverse plane. Furthermore, the hamstring muscles after reconstruction were more fatigued in both limbs, which may be another risk factor for ACL graft rupture. Therefore, increased fatigue in specific hamstring muscles may indicate the direction in which knee stabilization is compromised due to ACL overload. A muscle that becomes fatigued and inefficient more quickly also becomes ineffective in performing its function sooner, which can lead to increased overloading forces acting on the ACL graft.

The paper proposes a two-dimensional model of human stability. The model allows for gaining data on forces and moments of forces being the effect of musculoskeletal interactions of body parts at human joints. The necessary input data are registered with the use of an optoelectronic motion capture system, two force plates, and a dynamometer. The latter measures the magnitude of the external force that is randomly applied to cause a backward fall. Therefore, the position of the participant’s body during the experiment, the external force used to cause the fall, and the ground reaction forces are known. The model proposed has the structure of a closed kinematic chain with one loop and two open subchains and uses the Euler–Newton approach to analyse the motion. In addition to external loads, linear and angular accelerations are also needed. To calculate those, we used a variant of approximation. This consists in carrying out a simultaneous approximation of a given function and its first and second derivatives in the subintervals that shift along the whole data range. The experiments conducted include two activities while maintaining a state of equilibrium and the fall itself. This gives the basis for examining whether the internal forces and moments at human joints differ significantly in both these states. The results and conclusions are discussed in the final part of the paper.

(1) Background: The aim of this study is to determine the intra-rater and test–retest reliability of an upper extremity sway test performed in the one-handed plank position on a force plate and to establish if there are any between-limb differences in assessment with either open or closed eyes. (2) Methods: Fifteen males participated in this study (23.1 ± 2.3 years; 71.2 ± 7.4 kg; 175 ± 4 cm). The sway test (ST) on the force plate was performed during two separate visits, with a one-week interval between tests performed with eyes open and closed, separately for the left and right upper limbs. (3) Results: Intra-rater reliability for measurements with eyes open and closed on both the left and right sides was good to excellent for almost all variables (ICC = 0.84–0.96). Moderate reliability was observed only for the center of pressure (COP) area. Test–retest reliability was somewhat lower. With eyes open and closed, good to excellent reliability was observed for COP length and velocity in the M-L direction and for the total COP length (ICC = 0.77–0.95). However, for COP length and velocity in the A-P direction, as well as for COP area, reliability was moderate (ICC = 0.58–0.70). No significant differences were found between the left and right sides in any variable. (4) Conclusions: The results of this study indicate that the upper limb sway test performed on a force plate in the one-handed plank position, both with eyes open and closed, is a reliable measurement option and has practical applications in clinical settings, including for baseline assessments, return-to-play protocols, and injury prevention strategies.