Does Muscle Morphology Affect The Weight Transfer Phase During Backward, Forward, and Lateral Stepping?

Background & Aim: The choice reaction step test (CST) is a widely-used test to assess the capacity of a person to rapidly respond to a stimuli and perform a voluntary step. The weight transfer phase (WTP), which represents the shift of weight preceding the step, is part of CST. A faster WTP results in a faster step, which is associated with better balance recovery. Moreover, muscle morphology, such as muscle size and quality, is critical for human movement. Larger muscles with better quality, usually are associated with better performance. Yet, previous research showed no significant correlations between WTP and hip muscle size. However, the authors investigated a limited number of muscles (gluteus medius [GM], tensor fascia latae [TFL], and adductor magnus) and only one step direction (lateral stepping). Thus, this study aimed to investigate the impact of muscle size and quality from eight different lower limb muscles on WTP and its impact across the different step directions. We hypothesize that muscle quality will have a significant correlation with WTP, but there will be no significant correlation for muscle size. Methods: Fourteen adults (5 males; 42±25y; 1.65±0.08m; 63.9±10.2kg; X±SD) visited the laboratory for a single session. Upon arrival, participants rested for 10 minutes before undergoing ultrasound measurements. B-mode ultrasound images were taken from the vastus lateralis, bicep femoris, TFL, GM, tibialis anterior, and medial gastrocnemius. From the images of vastus lateralis and medial gastrocnemius, information was also extracted from the vastus intermedius and soleus muscles, respectively. Using ImageJ, muscle size (measured as muscle thickness) and echo intensity (index of muscle quality) were extracted. After ultrasound measurement, participants performed 18 CST across three directions- forward, backward, and lateral- performing six repetitions in each direction, with three repetitions per side (right vs. left). From the CST, we extract the weight transfer onset and weight transfer duration. For this preliminary study, only data from the dominant side of the participant was used. To understand the correlation of those measured times with muscle thickness and quality, after checking normality of data, Pearson bivariate correlation was used, using SPSS software, with the significance threshold set at P≤0.05. Results: There was no statistically significant correlation found between the size or quality of the eight muscles and weight transfer onset (r≤ -0.417; P≥0.096). Furthermore, no significant correlation was observed between muscle morphology and weight transfer duration (r≤ -0.417; P≥0.072). Thus, the size and quality of the investigated muscles did not influence the WTP in any step direction. Conclusion: This preliminary study found that muscle morphology, including size and quality, was not significantly correlated with WTP. Given the lack of association between muscle morphology and WTP, it is likely that other physiological factors, such as neuromuscular activation and/or muscle contractile properties, may play a role in influencing WTP. Future studies should explore this possibility, as a better understanding of the factors affecting stepping performance could inform strategies to prevent falls. The University of Maryland Claude D. Pepper Older Americans Independence Center This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.