Pointe Shoe Stiffness Alters the Vertical Ground Reaction Force During Single-Leg Drop-Landing in Female Ballet Dancers

Introduction: It is well known that the pointe shoes are essential for ballet's unique postural control and repetitive wear makes the pointe shoes vary in stiffness. However, the effects of different shoe stiffness conditions on the vertical ground reaction force (vGRF) in ballet dancers are unknown. The purposes of this study were to quantify the stiffness of the sole of pointe shoes and to investigate its effect on vGRF during single-legged landings by ballet dancers. Methods: Eleven female ballet dancers participated, with 22 pairs of pointe shoes categorized based on sole stiffness measurements into hard or soft conditions. The relationship between sole stiffness and usage time was examined based on the Spearman's rank correlation coefficient (P < .05). Participants performed 10 single-leg drop-landings on each leg under both hard and soft shoe conditions. A principal component analysis (PCA) of the 0% to 100% time-normalized vGRF signals during landing was conducted for each leg to determine the time periods that best differentiated the two shoe conditions. Results: There was a significant weak negative correlation between sole stiffness and usage time (r = -.353, P = .0188). Most shoes with short usage had greater stiffness values than shoes with long usage, however, for two out of the 22 pairs of shoes, this trend was reversed. The loading vector magnitude of the selected principal components increased 25% time period (corresponding to the ground contact time of the ball of the foot) and 100% time period (corresponding to heel contact), indicating that vGRF during these time periods was influenced by sole stiffness. Conclusion: Prolonged use of pointe shoes does not consistently result in lower sole stiffness compared to shoes used for shorter durations and different sole stiffness levels can affect foot landing impact, suggesting these findings may help dancers, instructors, and manufacturers recognize the importance of shoe stiffness in reducing the risk of injury.