The Dynamic Strength Index (DSI) is a simple ratio that compares an athlete's ballistic peak force to their isometric peak force. It has also been referred to as the Explosive Strength Deficit or Dynamic Strength Deficit.
An athlete's maximal force production during a dynamic movement, such as a countermovement jump (CMJ), represents their ballistic peak force. On the other hand, an isometric mid-thigh pull (IMTP) can be utilised to measure peak force production during an isometric test. For more on the IMTP, check out my YouTube video.
The DSI represents the (dis)parity between an athlete's ability to generate force in an isometric action and their ability to produce force during a high-speed ballistic movement. Crucially, this information can be used to help guide training prescription for our athletes.
Calculating the Dynamic Strength Index
We are looking to compare the peak force generated in the CMJ and the IMTP. This can be calculated with force plate technology, such as VALD Performance's ForceDecks.
The DSI is calculated as follows:
Dynamic Strength Index (DSI) = Ballistic [CMJ] Peak Force / Isometric [IMTP] Peak Force
This is demonstrated in the table below with data for three hypothetical athletes.
Athlete | Ballistic (CMJ) Peak Force [N] | Isometric (IMTP) Peak Force [N] | DSI |
1 | 3100 | 5700 | 0.54 |
2 | 1900 | 2600 | 0.73 |
3 | 2600 | 2800 | 0.93 |
Programming with the Dynamic Strength Index
This method is said to ‘determine the extent to which the athlete is able to apply force dynamically, in relation to their total maximal force capabilities’ (Sheppard et al., 2011). By assessing these capabilities together, we can establish which training type may be best targeted to support further athletic gains.
Athletes with a DSI ≤ 0.60 may benefit from a ballistic training emphasis, as they can only produce 60% of their maximal isometric force during a jump. Whereas, athletes with DSI scores of ≥ 0.80 may benefit from gaining maximal strength, because as much as 80% of their maximal isometric force is being produced during a jump.
<0.6 Ballistic Training
0.6 – 0.8 Concurrent Training
>0.8 Maximal Strength Training
So, if we revisit the hypothetical data, we would get the following emphases for training prescription for each athlete.
Athlete | Ballistic (CMJ) Peak Force [N] | Isometric (IMTP) Peak Force [N] | DSI | Training Prescription |
1 | 3100 | 5700 | 0.54 | Ballistic |
2 | 1900 | 2600 | 0.73 | Concurrent |
3 | 2600 | 2800 | 0.93 | Max Strength |
Alternative exercises have also been investigated, such as the squat jump (SJ), but Comfort et al. (2013) found CMJ peak force to be a more reliable measure than SJ peak force. There appears to be growing interest in using an isometric belt-squat as opposed to a IMTP, but more research is needed to understand the validity of this assessment and what it would mean for the DSI calculation.
For upper body, the isometric bench press and ballistic bench throw have been shown to calculate a sensitive DSI (Young et al., 2014).
Practical Application of the Dynamic Strength Index
An athlete’s power production relies heavily on both their maximal force capacity and their ballistic qualities. Therefore, identifying relative strengths and weaknesses that can help guide program emphasis is appealing for sports science and strength and conditioning practitioners.
Of course, this is just one result and many underpinning factors in physical performance need to be considered when building an individual’s training programme. In addition, concerns have been raised regarding mathematical appropriateness of ratios widely used in sports science, as discussed in this recent editorial by Chris Bishop, Ian Shrier, and Matt Jordan. So the DSI should be used alongside other assessments of physical capacities, including subjective observations.
We should seek to minimise the data collection burden on our athletes, as I continue to discuss on this blog. The DSI offers an efficient way to gain further value from our testing data. The CMJ is a widely conducted test. While, the IMTP offers a safe and non-fatiguing assessment of maximal strength. Combining these results for greater information into our athletes can be of great value.
From my anecdotal experience, it also ‘makes sense’ to both coaches and athletes. So it can be a means to demonstrate both value and application from testing. I like to present it visually on a spectrum to athletes, as shown below.
Below are a number of practical considerations for utilising the DSI in your own environment:
Conduct the CMJ and IMTP on the same day, in the same testing session.
An IMTP can potentiate athletes so be consistent with testing order; I prefer CMJ first.
Be consistent with testing protocols; including standardised warm-ups, cueing, and verbal encouragement. With the IMTP, be consistent with the use (or not) of chalk and/or straps.
Conduct multiple trials, ideally at least three, and be consistent with using the maximum or average of trials for analysis.
Remember ratios can add noise by adding the error for two different measures. Focus on the original force measures to investigate a worthwhile change in physical qualities.
Key Takeaway
The DSI combines an assessment of an athlete's force capacity in both maximal (via an IMTP) and explosive strength (in a CMJ) capabilities. By understanding an athlete's force potential and their ability to apply force during high-speed movements, practitioners can design training programs to target individual physical development, either via maximal strength, concurrent training, or ballistic emphasis.