Dynamic Strength Index + Assessment Excel Sheet
Christopher Cervantes, Assistant Director for Athletic Performance, University of Tulsa
Dynamic Strength Index, also referenced as the dynamic strength deficit or explosive strength deficit, is an assessment conducted to evaluate the relationship between an athlete’s ballistic peak force and their maximal isometric peak force. More pragmatically it can be thought of as explosive power capabilities in comparison to their maximal force capacities (how well an athlete can express their strength potential). This assessment evaluates what training stimulus emphasis an athlete could benefit from based on the ratio of these two aspects of their force velocity profile. Based on the value calculated, DSI gives insight whether the athlete would benefit from more maximal strength training, concurrent training or more ballistic strength training.
Without some level of relative strength, an athlete cannot express high bouts of force quickly. Put simply, if they cannot first express high bouts of force initially, then the rate is irrelevant. The ability to generate high amounts of muscular force is well understood in strength and conditioning. Maximal muscular force generation is defined as a summation of contractile morphology and motor unit recruitment and synchronization. The combination of these factors result in an athlete’s maximal muscular force production. An increase in an athlete’s maximal muscular force production (athlete’s maximal strength) allows for an increase their muscular force potential. An athlete cannot possess high peak power output without first having the corresponding relative strength levels.
The Force Velocity Relationship
There is a caveat to voluntary muscular force production and peak ballistic force output. Power is defined by a velocity component and as such velocity becomes a rate limiting factor. This rate limiting component comes in the form of cross-bridge cycling (the binding of contractile units in a muscle filament) has a fixed rate. As the velocity of voluntary concentric muscle contraction increases (increase in contraction speed), less and less cross-bridges are able to attach and cycle as the muscle shortens. Maximal muscular force is directly impacted by number of attached cross-bridges. Therefore, this limitation creates an inverse relationship between force and velocity, known as the force-velocity relationship.
Peak Ballistic Force
The force-velocity relationship becomes relevant when looking at dynamic muscle action (nomenclature for sporting action) where the window of time for peak force generation is limited. Research shows “in moderately trained athletes maximal force generation generally occurs from 0.4-0.7 seconds. The majority of dynamic actions, such as cutting, jumping, throwing, etc. occur in roughly 0.2 seconds or less”. Research also shows “as the level of competition increases so does the demand for proficiency in expressing RFD through power. More elite level competitors have shown significantly higher peak power outputs than their more junior counterparts”. The concept of generating high force in nearly instantaneous periods of time is thus vital to the previously mentioned sporting actions. This is often practically observed and measured in the form of jump testing for athletes.
DSI take the peak force value of a ballistic movement, often either a CMJ (Countermovement Jump) or SJ (Squat Jump) and compares what percent of peak maximal isometric force, most commonly in the form of IMTP (Isometric Mid-Thigh Pull) that movement falls under. It is an examination at the force expression potential in applying total force production. While both the CMJ or the SJ can be used as this expression, and despite the wide variety of research and resources examining the CMJ, based on the research conducted but JB Morin and Pierre Samozino it is suggested that best practice is the SJ and its absences of SSC and SR and associated elastic recoil (unlike its counter the CMJ). This allows for a more detailed looked at voluntary concentric ballistic force production as well as accounting for more valid measurement in accounting for push-off distance. For eccentric application of elastic recoil the EUR is suggested. In order to ensure reliably and validity of equations some anthropometrics will be required. The proctor will need to measure from the athlete’s iliac crest to the tip of their toes for an Lower Limb Length. The proctor will then need to establish an initial height or Hi (knees at 90˚) to determine the Push-off Distance or Hpo. This comes from Morin an Samozino’s equations for calculating Jump force rather than jump power, as is common practice for most vertical jump calculations without force plates. It is imperative when testing the SJ, that the same Hi be used for every repetition and every testing session thereafter. It is suggested that a tangible marker (i.e. a string or band) be used to allow the athlete to locate and identify where this position is.
Links to Squat Jump
Isometric Mid-Thigh Pull is the gold standard for maximal isometric force generation. It involves placing an apparatus, often a bar, at the athletes mid-thigh or more aptly at the accentuated peak contraction location. The athlete then pulls with maximal intent for a given duration of time, usually about 3 seconds. The peak force value is recorded and considered to be their maximal isometric peak force, or their highest force potential (maximal strength).
DSI takes the ratio of the peak forces in both the ballistic and maximal effort isometric. In the absence of force plates, the gFlight and gStrength apparatuses can be very useful in allowing the collection of the data needed for these peak forces.
DSI = Peak Ballistic F / IMTP Peak F
Peak IMTP Force
Using the gStrength peak force can be recorded and converted to N. To ensure the application of force due to gravity is as valid as possible, however, it is imperative that the pulling anchor be located as close to directly under an athlete as possible, as to not skew the vector.
Now that the data has been gathered, how does a coach interpret the results.
If an athlete has a DSI score less than 0.6 or 60%, it is recommended that the athlete requires more tendon strength work. This can be thought of as a strong tire (the muscle) but poor shocks (the tendon). The athlete is able to produce lots of force but is not able to apply it quickly and thus dissipates the energy through heat and sound. Introducing and emphasizing plyometric work (where ground reaction forces and ground contact are utilizing) may help both the stiffening of the tendon as well as the reorganization of collagen alignment to better utilize the kinetic force generated in movements. Forms of complex or contrast training may also help the synchronization and recruitment of motor units. It is important, however, when introducing ballistic work, that appropriate accumulation towards volume is considered. It does not serve to phase in high volume of plyometric work or rapid increase in plyometric intensity if the tissue is intolerant to start with. Progression is key.
If an athlete has a DSI score between 0.6-0.8 or 60-80%, it is recommended that the athlete requires a combination of plyometric and strength work (concurrent training). There is some application of elastic recoil and RFD, but it could be better. Such training methodologies as contrast, complex or accommodating resistance could help increase this athlete’s application of peak force potential.
If an athlete has a DSI score of higher than 0.8 or 80%, it is recommended that the athlete is effectively utilizing their peak force potential, and this requires more peak force. This athlete would require more emphasis on peak force or maximal strength work to allow for a higher ceiling of force generation.
How often should I test?
It is important to test, but to understand when to test as well and what the implications are. It is generally recommended that the athletes be tested at the start of an offseason cycle or between training cycles to ensure that the desired training cycle adaptation has been reached. After testing, the coach can make an evaluation on where the athlete is baselined or with which training stimulus the athlete would most benefit from in the next training cycle.
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Dynamic Strength Index - Francisco Tavares