Comparisons between different methods of calculating dynamic strength index: Effect on training recommendations

Area of Science:

• Sports Science
• Biomechanics
• Human Movement Analysis

Background:

• The impulse-based dynamic strength index (iDSI) considers time-dependent force expression, potentially offering greater insight for training recommendations than the traditional peak force-based DSI (fDSI).
• A limitation of iDSI is its sensitivity to changes in countermovement jump (CMJ) propulsion phase duration, which can complicate longitudinal comparisons.
• A fixed-duration iDSI (e.g., 250 ms) may overcome the longitudinal comparison limitations associated with variable propulsion durations.

Purpose of the Study:

• To investigate the effect of different Dynamic Strength Index (DSI) calculation methods (fDSI, iDSI matched, iDSI fixed) on DSI values.
• To determine how these calculation methods influence resultant training prioritization for athletes.
• To compare the consistency of training recommendations derived from each DSI calculation method.

Main Methods:

• Thirty-seven team sport athletes performed maximal effort countermovement jumps (CMJ) and isometric mid-thigh pulls (IMTP) on force plates.
• fDSI was calculated based on peak force.
• Fixed iDSI was calculated using the initial 250 ms of the IMTP.
• Matched iDSI was calculated using a duration matched to the CMJ propulsion phase.
• Training recommendations were categorized based on DSI thresholds (>0.80, 0.60-0.80, <0.60).

Main Results:

• Fixed iDSI values (0.88 ± 0.11) were significantly and meaningfully higher than both fDSI (0.82 ± 0.12) and matched iDSI (0.82 ± 0.11).
• There were no significant differences between fDSI and matched iDSI.
• Training recommendation consistency was higher between fixed iDSI and matched iDSI (84.2%) compared to fDSI and matched iDSI (44.7%) or fDSI and fixed iDSI (55.3%).

Conclusions:

• Meaningful differences exist between various iDSI calculation methods, impacting training recommendations.
• A fixed-duration iDSI provides more consistent training recommendations and may be preferable for tracking physical performance longitudinally.
• The choice of DSI calculation method significantly influences athlete assessment and training prescription.