Background/Objectives: Maintaining an upright posture is a complex, adaptive neurophysiological process that relies on a set of stereotypical muscle synergies. Despite extensive research on postural control, the interaction between muscle dynamics and ground reaction forces during progressive loading remains poorly understood. Methods: Twenty-five healthy men, aged 21–25, performed a 30 s quiet standing task on a tensometric platform. The protocol involved seven loading levels—from no load to higher loads using progressive weights in a backpack. The three components, Rx, Ry, and Rz, of the ground reaction force (GRF) and surface electromyographic activity from six muscles of the lower limb were recorded, namely medial (GAM) and lateral heads (GAL) of m. gastrocnemius, rectus femoris (RF), and vastus lateralis (VL) from m. quadriceps femoris, m. biceps femoris (BF), and m. semitendinosus (ST). The recently developed intercriteria analysis (ICrA) was utilized to enhance understanding of the central nervous system’s adaptive mechanisms under changing load conditions. Results: The ICrA results show that the RF–VL muscle pair is stable and does not depend on load, while the BF–ST pair is semi-stable across different weights. Also, ICrA indicates a significant increase in synchronization between specific muscle groups at higher load levels, suggesting the recruitment of additional muscle coupling. Regarding the influence of increasing weight (W) on GRF, the analysis reveals a positive consonance relation between W and Rz in 100% of subjects. Conclusions: The study shows that during progressive external loading, the central nervous system maintains vertical stability by reorganizing muscle coupling. Some of these are specific to individuals. Coactivation of the antagonistic muscle pairs at higher loads suggests a transition from a basic to a more complex adaptive strategy.
Angelova et al. (2026) studied this question.