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Objective: To improve the comprehensive performance of starter press machine. Methods: Based on the finite element analysis method for die box component modal and harmonic response analysis, and based on the finite element model by using variable density method was carried out on the die box component topology optimization, and the topology structure was also improved, which will improved the quality of the die box components and 1 order natural frequency as the optimization goal. The multi-objective optimization design was carried out based on non-dominated sorting genetic algorithm. Results: In the fourth mode, large torsional deformation appeared on the overall edge of the die box components, which did not meet the requirements of stiffness performance. The highest speed of the servo motor was close to the first-order critical speed, and the probability of low-frequency coupling resonance was very large. At the same time, the region where the response peak occurs was basically the same as the region where the deformation occurs in the modal analysis, which verified the accuracy of the dynamic characteristic results. After optimization, the mass of the die box component decreased by 15.13%, and the first four natural frequencies increasd by 14.09%, 14.14%, 3.54% and 10.37%, respectively. Conclusion: The dynamic characteristics of the starter press machine mainly affect its production efficiency and starter press quality. Under the optimized die box component, the starter press machine has good dynamic performance and achieves the goal of lightweight design.

Previous studies examining the flexion–relaxation phenomenon (FRP) in back muscles through trunk forward flexion tests have yielded inconsistent findings, primarily due to variations in leg posture control. This study aimed to explore the influence of leg posture control and individual flexibility on FRP in back and low limb muscles. Thirty-two male participants, evenly distributed into high- and low-flexibility groups, were recruited. Activities of the erector spinae, biceps femoris, and gastrocnemius muscles, alongside the lumbosacral angle (LSA), were recorded as participants executed trunk flexion from 0° to 90° in 15° increments, enabling an analysis of FRP and its correlation with the investigated variables. The findings highlighted significant effects of all examined factors on the measured responses. At a trunk flexion angle of 60°, the influence of leg posture and flexibility on erector spinae activities was particularly pronounced. Participants with limited flexibility exhibited the most prominent FRP under constrained leg posture, while those with greater flexibility and unconstrained leg posture displayed the least FRP, indicated by their relatively larger LSAs. Under constrained leg posture conditions, participants experienced an approximate 1/3 to 1/2 increase in gastrocnemius activity throughout trunk flexion from 30° to 90°, while biceps femoris activity remained relatively constant. Using an inappropriate leg posture during back muscle FRP assessments can overestimate FRP. These findings offer guidance for designing future FRP research protocols. [ABSTRACT FROM AUTHOR]