Author(s): MACMILLAN, C., SEWRY, N., Institution: UNIVERSITY OF PRETORIA, Country: SOUTH AFRICA, Abstract-ID: 2166

INTRODUCTION:
The Dynamic Strength Index (DSI) is widely used to guide strength–power training prescription by expressing maximal ballistic force relative to maximal isometric force. Although the ballistic peak force is commonly derived from the countermovement jump (CMJ), the drop jump (DJ) may better reflect reactive stretch–shortening cycle demands. The aim of this study was to determine if the choice of ballistic task (CMJ vs DJ) influences DSI values and athlete classification, and to examine relationships between DSI and jump performance.
METHODS:
Twenty-nine elite female netballers completed a maximum effort isometric squat push (ISP), CMJ, and DJ using force-plate systems (VALD Performance). Jump height was determined using the impulse–momentum method. DSI derived from CMJ (DSI_CMJ) was obtained directly from the software, while DSI derived from DJ (DSI_DJ) was calculated as DJ peak concentric force divided by ISP peak force. Athletes were categorised using established DSI thresholds (<0.60 ballistic emphasis, 0.60–0.80 concurrent, >0.80 maximal force). Descriptive statistics were calculated, normality assessed, and Pearson or Spearman correlations performed. Agreement between DSI methods was examined using Bland–Altman analysis. One-way ANOVA was used for group comparisons (p<0.05).
RESULTS:
Median CMJ and DJ heights were 28.1 cm (IQR 4.9) and 25.6 cm (IQR 5.4), respectively. Mean DJ peak concentric force (3169 ± 886 N) exceeded ISP force (2091 ± 370 N). Using DSI_CMJ, 31% of athletes were classified <0.60, 59% between 0.60–0.80, and 10% >0.80. In contrast, DSI_DJ classified 100% of athletes >0.80. Mean CMJ heights across DSI_CMJ categories were 27.0 cm (<0.60), 29.9 cm (0.60–0.80), and 26.7 cm (>0.80), while DJ heights were 26.1 cm, 29.6 cm, and 23.1 cm, respectively. No significant (p>0.05) differences in CMJ or DJ height were observed between categories. No significant correlations were found between DSI_CMJ and CMJ height (r=0.21; 95%CI: -0.15-0.55), DJ height (r=0.12; 95%CI:-0.42-0.31), or DJ:CMJ height ratio (r=–0.29; 95%CI:-0.66-0.09). DSI_DJ also showed no meaningful associations with jump height. Agreement between DSI_CMJ and DSI_DJ was poor (r=-0.23; 95%CI:-0.67-0.09).
CONCLUSION:
DSI values and athlete classification were highly sensitive to the ballistic task used, with DJ-derived DSI uniformly categorising athletes as requiring maximal force training. Importantly, neither DSI approach was associated with jump performance during stretch–shortening cycle–challenging tasks. Practitioners should therefore exercise caution when using DSI for exercise prescription, as DSI may not reflect functional explosive performance. A more task-specific, multi-metric approach incorporating jump kinetics and performance outcomes may provide superior insight for individualized strength–power profiling and training design. These findings question the interchangeability and practical validity of DSI-based decision-making in applied settings.