The Dynamic Strength Index, often referred to as the dynamic strength deficit, measures the difference between an athlete’s maximal and explosive strength capacity. However, the term “index” is preferred over “deficit”, as it’s an index of the athlete’s current performance ability. The dynamic strength index can be used to identify whether the athlete may require maximal strength training, ballistic strength training, or concurrent training (i.e. a combination) as a stimulus in their programme. It can also be used to reliably measure the performance capabilities in both the lower- and upper-body and in recreational, university, and elite athletes.
Keywords: strength deficit, strength diagnosis, CMJ, IMTP, peak force.
What is the Dynamic Strength Index?
The Dynamic Strength Index (DSI), otherwise known as the Dynamic Strength Deficit (1) or the Explosive Strength Deficit (2-4), is simply a ratio between an athlete’s ballistic peak force and their dynamic or isometric peak force (5). In another sense, it may be viewed as a “strength potential” test. An example of ballistic peak force would be an athlete’s maximal force production during a countermovement jump (CMJ) – as this is a ballistic movement. On the other hand, dynamic peak force may be measured using a 1-repetition maximum (1RM) back squat, whilst an isometric mid-thigh pull (IMTP) may be used to measure their peak force production during an isometric test. It is very common for practitioners to use a CMJ or squat jump (ballistic tests) and an IMTP (isometric test) to calculate an athlete’s DSI (3, 5, 6). Put simply, the DSI measures the difference between an athlete’s ability to produce force during a dynamic or isometric test, versus their ability to produce force during a ballistic exercise. This allows the strength and conditioning coach to identify the athlete’s “strength potential” and how much of that potential they can use during a high-speed ballistic movement.
Calculating the DSI allows the strength and conditioning coach to do two things:
- Determine the maximal amount of force an athlete can produce (e.g. using the IMTP test).
- How much of that total force can they produce in a high-speed movement with a short timeframe – i.e. 400-ms is the time it takes to achieve peak force during the CMJ (7).
Just to clarify, the IMTP or 1RM dynamic back squat is used to measure an athlete’s maximal force capabilities (i.e. maximal strength), whilst the CMJ, which is a ballistic movement, is used to determine how much of their total force capability can they produce in a very short timeframe.
Why is the Dynamic Strength Index important?
The DSI provides the strength and conditioning coach with valuable information regarding how forceful (i.e. strong) the athlete is, and how much of that strength they can use during fast movements. This information allows the coach to design a more specific training programme focussed on developing an athlete’s strength and/or power capacities (6).
The Importance of Maximal Strength
High muscular strength is consider as a vital element of athletic performance. Greater muscular strength has been shown to enhance the ability to perform general sport skills such as: jumping, sprinting, and change of direction tasks, improve overall performance, allow athletes to potentiate earlier and to a greater extent, and even reduce the risk of injury (8). To add to this, maximal strength is strongly related to power output (r = 0.77-0.94 ) for both the lower- (10-13) and upper-body (10, 14-17). Therefore an athlete’s ability to produce power is largely dependent on their maximal force capacity (i.e. strength) (18-20). As a result, the “first box to be checked” so to speak when performing a strength diagnosis, is perhaps an athlete’s maximal strength.
The importance of being able to produce high-force in a short timeframe
Kawamori et al., (7) observed that it took approximately 260-ms to achieve peak force during the IMTP and 400-ms during the CMJ. Other research has shown time to peak force during the CMJ to take approximately 240-ms (21). So whilst the time to achieve peak force during the IMTP (260-ms) can be somewhat similar to the CMJ (240-400-ms), it is in fact how ‘much’ force is developed which separates the two exercises. Table 1 provides a clear example of how the peak force differs between the IMTP and the CMJ. Thus, the important factor is to determine how much force an athlete can produce under no time constraints (e.g. IMTP) versus how much force they can produce under short time constraints (e.g. CMJ).
Below are some examples of common ground contact times during particular sporting movements:
- Basketball lay-up shot = 218-ms (22)
- Sprinting = 80-90-ms (23)
- Long Jump = 140-170-ms (24)
As many sporting movements, such as those above, happen in a very short timeframe and are all ballistic in nature, it is vital to analyse the athlete’s ballistic force production capabilities under short time constraints.
How to calculate Dynamic Strength Index
The equation below is used to calculate an athlete’s DSI. And although it is referred to as a “ratio”, it is not displayed as a one, and is instead a simple division between ballistic and dynamic or isometric peak forces.
Dynamic Strength Index (DSI) = Ballistic Peak Force / Dynamic or Isometric Peak Force
Using the data from Table 1.
Dynamic Strength Index (DSI) = CMJ peak force (N) / IMTP peak force (N)
DSI = 1450 / 3178
DSI = 0.46
Validity and Reliability
The DSI has been proven to be both a valid and reliable measure of maximal and explosive strength capacity in recreational (6), university (5), and elite athletes (25). Furthermore, the following combination of exercises have been proven to be reliable when measuring DSI:
- CMJ and IMTP (3)
- Static SJ and IMTP (5, 6)
- Ballistic Bench Throw and Isometric Bench Press (26)
It has also been verified as a sensitive and useful tool to evaluate and monitor performance changes over the course of a training programme (26).
Once the athlete’s DSI has been calculated, the strength and conditioning coach needs to know what that score tells them, and therefore how to design the training programme based on that score. Table 2 provides some simple examples of various DSI scores.
The DSI reflects the percentage of maximal strength “potential” which is not being used within a given motor task (e.g. jump) (27). In other words, it demonstrates the athlete’s ability to use their full “force potential” during a ballistic exercise such as a CMJ. So theoretically speaking, if an athlete can express a DSI score of 1 (i.e. Athlete C in Table 2), they are capable of using their full “force potential’. This means the higher the athletes DSI, the more capable they are at utilising their “force potential” during a ballistic exercise.
In contrast, the lower the athlete’s DSI, the less capable they are at utilising their “force potential” during a ballistic exercise. A higher DSI means more time should be spent on developing maximal strength (i.e. force production). A smaller DSI means more time should be spent developing RFD using ballistic strength training methods (Table 3) (6).
This article was co-authored by Francisco Tavares. Francisco is a PhD candidate at the Waikato
University. He is also the Head of S&C at the Portuguese Rugby Union, a S&C Coach at the Chiefs Super Rugby in New Zealand and a guest lecturer for various universities in Portugal and Waikato University.
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