Training-induced changes in shoulder muscle mechanical properties in volleyball players

Study aim This pilot study aimed to quantify acute and short-term pre- to post-session changes in the mechanical properties of shoulder girdle muscles in volleyball players during an in-season preparatory mesocycle. Material and methods Eight male volleyball athletes competing at the national academic level underwent assessments once per week across a 3-week in-season preparatory mesocycle immediately preceding the Polish Academic Volleyball Championships. At each session, MyotonPRO was used to measure tone (Hz), stiffness (N/m), and elasticity (log. decrement, D) before and after standardized 120-min training. Upper-body explosive power was assessed via the seated medicine-ball throw. Week-specific pre–post comparisons were tested with paired t -tests (Cohen’s dz , 95% CI). Two-way repeated-measures ANOVAs examined Time (pre/post) × Week effects. Results Upper trapezius tone increased significantly post-training across all weeks ( dz = 0.90–1.01, p < 0.05). Lower trapezius elasticity improved consistently ( Δ = –0.13 to –0.17, dz ≈ –1.0, p < 0.05), while infraspinatus elasticity improved only in Week 3 ( dz = –1.13, p = 0.015). Exploratory repeated-measures ANOVA also identified a Time effect for upper trapezius tone ( F (1,7) = 12.1, p = 0.010, <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"> <m:msubsup> <m:mrow> <m:mi>η</m:mi> </m:mrow> <m:mrow> <m:mtext>p</m:mtext> </m:mrow> <m:mrow> <m:mn>2</m:mn> </m:mrow> </m:msubsup> </m:math> {η }ₚ² = 0.634), although ANOVA-based effect sizes should be interpreted cautiously given the pilot sample. Conclusions Preliminary data suggest that selected shoulder stabilizers exhibited short-term pre- to post-session changes in tone and elasticity during the monitored in-season preparatory mesocycle. Given the pilot sample, these localized changes should be interpreted cautiously.