Author(s): QI, X., WANG, T., Institution: BEIJING INSTITUTION OF SPORTS SCIENCE , Country: CHINA, Abstract-ID: 1620

INTRODUCTION:
Agility is fundamental to volleyball, particularly in defensive and transitional actions requiring rapid braking, reorientation, and re‑acceleration. As the distal link of the lower limb kinetic chain, the plantar flexor muscles operate under tight time constraints and rely heavily on stretch‑shortening cycle (SSC) efficiency and rapid force production. However, direct comparisons between “static reserve” capacity (peak force, PF; force symmetry rate, FSR) and “SSC‑related” qualities (reactive strength index, RSI; rate of force development, RFD) in explaining agility performance remain scarce, and strength‑agility profiles is poorly understood. This study aimed to quantify the relative contributions of plantar flexor strength measures to pre‑planned (modified agility T‑test, MAT) and reactive (digging agility test, DAT) agility
METHODS:
19 male elite volleyball players from the Chinese Volleyball Super League (age 24.95 ± 3.54 yrs, height 195.3 ± 10.6 cm, mass 89.3 ± 8.7 kg) performed: (i) seated isometric plantar flexion testing to obtain bilateral PF, FSR, and RFD at 0–50/100/150/200/250 ms; (ii) 40 cm drop jumps to determine RSI, eccentric RFD, and surface EMG root mean square (RMS) of the soleus, gastrocnemius medialis and lateralis; (iii) MAT and DAT. Normality was assessed with Shapiro–Wilk tests. Pearson/Spearman correlations and multiple linear regression (covariates: height, mass, age, training experience) were used to examine relationships between strength measures and agility performance.
RESULTS:
MAT correlated moderately with RSI (r=–0.517, p=0.023), left‑leg RFD at 0–100 ms (r=–0.552, p=0.014), and right‑leg RFD at 0–200 ms (r=–0.550, p=0.015). RSI independently explained 37.2% of MAT variance (β=–0.651, p=0.006). Neither PF nor FSR showed significant independent prediction for MAT or DAT. DAT was not significantly associated with any strength or EMG variable.
CONCLUSION:
MAT is primarily determined by plantar flexor SSC efficiency (RSI) and early neural drive (0–100 ms RFD), with only minimal contribution from PF or FSR. The underlying reason lies in the time constraints of cutting tasks: ground contact time during directional changes is typically below 250 ms—far shorter than the time required to reach peak force—thereby limiting the translation of strength reserves into effective braking and re‑acceleration impulse. In contrast, DAT cannot be adequately explained by isolated strength measures; its performance appears more dependent on perceptual‑decision efficiency and motor coordination, suggesting that strength‑only training is insufficient to improve this task domain. From a practical standpoint, training emphasis should be placed on enhancing RSI and early RFD, with position‑specific interventions guided by individual strength profiles. PF and FSR are better reserved as monitoring tools for strength reserve and rehabilitation progress. Collectively，these findings establish a clear hierarchy of strength qualities for guiding agility training in volleyball.