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1. Concepts Related to DSI and Issues Raised
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• Background: The rate of force development (RFD) in athletes is crucial for athletic performance and reducing the risk of injury. Strength and conditioning (S&C) professionals often ponder how to determine the focus of an athlete’s strength training (e.g., increasing strength or speed capabilities, enhancing peak strength or RFD, etc.). DSI has been proposed to help answer these questions.
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• Calculation Method of DSI: DSI is derived by comparing the peak strength of athletes under dynamic conditions (e.g., jumping) with that under isometric conditions (e.g., isometric mid-thigh pull, IMTP). For example, if an athlete’s peak strength during a jump is 1500N and their isometric task peak strength is 2500N, then DSI equals 0.60.
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• Research Purpose: To assess the effectiveness of DSI as a diagnostic tool, explore whether it can identify the focus of an athlete’s strength or speed training, and the benefits of increasing peak strength or RFD, while proposing alternative tools.
▲ Article Mind Map
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2. Analysis of Indicators Related to DSI
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• Relationship Between Jump Peak Strength and Rapid Dynamic Strength: Commonly used tests for DSI, such as squat jump (SJ) and countermovement jump (CMJ), cannot directly equate peak strength with rapid dynamic strength. For instance, in CMJ, peak strength and jump height may not be consistent due to displacement factors. SJ’s average and peak strength may be higher than CMJ, but the jump height is lower. Studies show that jump performance outcome indicators (e.g., jump height, take-off speed, or impulse) reflect rapid dynamic strength, while isolated peak strength does not. Moreover, the correlation between jump peak strength and DSI is weak, while the correlation between IMTP peak strength and DSI is strong, indicating that jump peak strength may be redundant, and IMTP peak strength has a greater influence on the DSI ratio.
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• Relationship Between DSI Ratio and Focus on Maximal or Rapid Dynamic Strength Training: Maximal strength affects the strength-speed curve, but when the target movement approaches the speed end, the influence of maximal strength training diminishes, and rapid dynamic strength training becomes more important. Although DSI aims to guide training strategies, the peak strength during CMJ is influenced by jump strategies, while SJ is affected by the athlete’s body structure, indicating flaws in the interpretation guidelines. IMTP can reveal the athlete’s maximal strength ceiling, helping to determine when to focus on rapid dynamic strength training, but the DSI ratio is not an effective indicator for assessing the athlete’s strength-speed orientation.
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• Relationship Between Jump Peak Strength and RFD: Research on the relationship between jump peak strength and RFD is limited and inconsistent, due to reasons including unstable RFD metrics and differences in calculation methods (e.g., differing time points and methods for calculating RFD). In CMJ, due to significant muscle pre-tension at the initial stage, peak strength may not accurately reflect RFD capability and could even be negative. In SJ, peak strength can somewhat reflect RFD capability, but due to the significant impact of maximal strength on the area under the force-time curve, the “impulse advantage” of RFD is reduced. Caution is needed when inferring RFD from SJ, as it may be influenced by changes in the athlete’s strength-speed orientation.
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• Relationship Between DSI Ratio and Focus on Peak Strength or RFD Training: From a contraction perspective, peak strength and RFD are related, but from a neural perspective, they can be separated, indicating different training focuses. The DSI ratio is not suitable for determining whether an athlete benefits more from RFD or maximal strength training, as its jump peak strength time point is not fixed, and the contraction phases of SJ and CMJ differ significantly from IMTP, making it unable to represent different neuromuscular/mechanical capabilities. However, IMTP peak strength can provide information on the athlete’s relative strength, aiding in determining training focus.
▲ Figure 1. Comparison of vertical ground reaction forces in countermovement jump (A) and squat jump (B).
3. Alternative Diagnostic Tools
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• Determining Focus on Maximal Strength or Rapid Dynamic Strength Training: Load-velocity curve analysis is a simpler and more effective assessment method, comparing jump outcome indicators with independently manipulated loads (rather than force values related to speed) to intuitively determine whether an athlete is lacking strength or speed. For example, a relatively strong athlete who cannot jump high may lack speed, while a relatively weak athlete who jumps high may lack strength. However, the load-velocity curve is task-specific, influenced by sport-specific factors, and dynamic strength relies on coordination, with neural adaptations potentially offering no benefits across different sports. There may also be measurement errors when using linear position sensors.
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• Determining Focus on Maximal Strength or RFD Training: RFD can be inferred by obtaining strength at specific time points after the contraction begins (e.g., 100ms) and comparing it with the final peak strength (relative RFD). It is recommended to use time points ≤100ms, as RFD significantly impacts net impulse at this time, and its influencing factors differ from maximal strength. Although reliability at 100ms remains challenging, errors can be reduced through multiple trials. Practitioners should establish their own relative RFD normative data to better assess athletes’ RFD conditions.
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• Logic-Oriented Approach: If load-velocity curve and relative RFD are unsuitable, a logic-oriented approach could be adopted. For example, comparing evaluations related to the movement action (e.g., CMJ positive impulse and IMTP) to judge an athlete’s strength and speed or ballistic capabilities, or observing the impact of maximal strength increases on ballistic performance through simple regression analysis. When the benefit transfer begins to diminish, ballistic training emphasis can be increased. Additionally, practitioners should fully understand the temporal and mechanical factors in the target task to make informed training decisions.
▲ Figure 2. Comparison of force-time and displacement-time curves in two countermovement jumps.
4. Conclusion: DSI has numerous issues as a diagnostic tool, with significant flaws in its jump peak strength indicator, and the interpretation guidelines for the DSI ratio are controversial and do not reflect the athlete’s strength-speed orientation. Although maximal strength, rapid dynamic strength, and RFD share common determinants, task conditions will affect their relative importance. It is recommended to establish training strategies based on the athlete’s maximal strength ceiling and further evaluate rapid dynamic strength and related qualities under specific conditions. At the same time, all strength assessments are task-specific, and practitioners should clarify the conditions under which athletes generate strength during their movements to choose appropriate assessment indicators and training methods.
▲ Figure 3. Example data showing comparisons of peak strength between athletes and strength at 100ms and 300ms time points. Note: “F100:PF” and “F300:PF” represent the percentage of strength at these time points relative to the peak strength value.
There are three tables in the text, as follows
Table 1. Operational Definitions of Common Terms in Literature
| Term | Operational Definition |
|---|---|
| Rapid Dynamic Strength | The ability to repeatedly apply force while maintaining high and/or continuously increasing movement speed. Practically manifested as superior ballistic performance (e.g., vertical jumps) or the ability to move at high speeds under low to moderate resistance (e.g., sprinters turning at maximum speed). |
| Rate of Force Development (RFD) | The ability to rapidly increase muscle force from a low-speed or stationary state (also referred to as RFD). Practically manifested as the ability to produce explosive contractions to overcome inertia and quickly accelerate external mass (e.g., head kicks in taekwondo). |
| Dynamic Strength Deficiency | DSI ratio <0.6 proposed by Shepperd et al., indicating a need to shift the focus of strength training towards ballistic training methods. |
| Rate of Force Development Deficiency (RFD Deficiency) | The inability to produce force within a limited time window after contraction onset, or to increase force from a low level within a limited time window relative to peak strength ceiling (also referred to as relative RFD). |
Table 2. Hypothetical DSI Calculation
| Test/Metric | Athlete A | Athlete B | Athlete C | Athlete D |
|---|---|---|---|---|
| Jump Peak Strength (N) | 1700 | 1500 | 950 | 2600 |
| IMTP Peak Strength (N) | 2750 | 1800 | 2200 | 3200 |
| DSI Ratio | 0.62 | 0.83 | 0.43 | 0.81 |
| Classification | Low | High | Low | High |
| Training Indication | Ballistic Training | Maximal Strength Training | *Ballistic Training | *Maximal Strength Training |
Table 3. Normative Data for Relative Rate of Force Development (RFD)
| Author | Subject Characteristics | Relative RFD (%) |
|---|---|---|
| West et al. | 39 rugby league players | 46% |
| Guppy et al. | 14 recreational weightlifters | 51%* |
| Guppy et al. | 17 strength and power athletes | 43/44%* |
| Beckham et al. | 12 weightlifters of varying levels | 48% |
| Lum et al. | 28 endurance runners | 55% |
Translation of Notes
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Note 1: Consideration of component values may impact assumptions drawn from the ratio data of athletes C and D. Specifically, athlete C has lower peak strength despite indications for ballistic training; athlete D has higher peak strength despite indications for maximal strength training. -
Note: *Force is obtained from the 90ms time point, rather than 100ms.
Table 4. Alternative Strength Diagnostic Methods for Guiding Strength Training Strategies
| Question/Specific Insight | Suggested Diagnostic Tool | Limitations |
|---|---|---|
| How to know if an athlete can improve power output by increasing strength or speed capabilities? | Load-velocity curve analysis: The relative ability to generate force from low speed (high load) to high speed (low load) obtained under independently manipulated loads. | Task specificity; measurement errors may occur when using linear position sensors; must consider that jump strategies may also change (unless performed on a Smith machine). |
| How to know if an athlete benefits most from increasing peak strength or rate of force development (RFD)? | Relative RFD assessment (isometric force @100ms/peak strength): The relationship between the ability to produce force within a limited time window (i.e., rate of force development) and the peak strength ceiling. | Structural validity – initial RFD may not relate to many movement actions. |
Translation of Notes
Note: RFD = Rate of Force Development; Isometric force @100ms/peak strength = The ratio of force at 100ms in isometric tasks (e.g., isometric mid-thigh pull) to peak strength.