Muscle Slack and High Velocity Training: An Integrative Approach

Velocity Deficient


The idea of measuring and training for velocity deficiencies has become popular since the recent studies of JB Morin and colleagues. In one of their studies, they examined several different subjects and based on their profiling methods, determined whether or not the individuals had a force-velocity profile that was either velocity deficient or force deficient. Once the deficiency was determined, the subjects were trained using specific methods emphasizing the velocity component of the movement (slow velocity for max force and fast velocity for speed of movement). After the study’s training cycle, J.B Morin and colleagues were able to show that the specific training methods, either slow or fast, improved vertical jump performance and overall balance of the subjects’ force velocity profiles.

This was one of the first large scale studies to truly profile, test and train according to the individual needs of their force-velocity profile. Most coaches already understand that if someone needs to get stronger they have to increase their max strength, but the idea of training at high velocities had really yet to be established as widely effective. However, after the methods in the study were shown to be effective, there was now scientific evidence to support the usage of high speed and over-seed exercises to induce performance improving force-velocity profile changes.


The only issue with jumping to conclusions about whether or not high for fast velocity methods should be used is the fact that velocity is an output. Increases in velocity do not just occur from moving fast, but there are underlying unknown physiological adaptations at play. There have been many assumptions as to what is occurring, but nothing has been definitively proven. This is not to say high velocity training is bad, it just to say that high velocity training does guarantee high velocity adaptations. In a sense, by just doing high velocity movements to fix a velocity problem, the coach is essentially working backwards. They are taking the byproduct, and hoping that it fixes the core issues. Granted, going about it this way is better than not going about it at all. However, recent published papers by Frans Bosch may help coaches get a better understanding of what adaptations are causing the increases in velocity and how they can best train for these adaptations, even if the movements needed for training are not as high of a velocity as we thought they need to be.



Muscle slack


Frans Bosch has popularized the concept of muscle slack. It is hinges on the early stage rate of force development and the speed at which the muscle and tendon can go from “slack” to “tense”. When a muscle is not activated, it is relaxed and there is slack in both the muscle and tendon. Bosch uses the analogy of a rope to help describe how muscle slack works. You are holding one end of the rope and the other end is tied to a car. Before you can pull the car with the rope, the rope first has to become tense. This is the point where the rope goes from lying slack on the ground, to now in a straight line from your hands to the car. This is synonymous with the process of the muscle fibers aligning. The second part of the slack is that the rope now needs to become tense enough so that force can be applied to the truck. At this point, the rope goes from being in a straight line from your hand to the car, to now taut, from you producing a force on the rope. This is synonymous with the muscles co-contracting to produce enough force on the tendon so the muscle can become tense. Bosch says that some athletes struggle to produce this tension and removal of slack fast enough (early phase rate of force development) and it can hinder their performance. Because movements occur extremely fast in sport, in order to generate the most force possible in a short period of time, the athlete needs to remove slack out of the muscle as fast as possible.


Influences On Training


Bosch argues that any movement that “unnaturally” aids in co-contractions and therefore reduction in muscle slack might be detrimental for improving pre-tensing mechanisms in the body, because it essentially allows the athlete to cheat in training. For example, when performing a back squat, the external load is applied during the eccentric phase, which means the muscles are extremely active during this phase and muscle slack is already taken up by the time the athlete moves into the concentric phase. However, in sport there is rarely an external load during the eccentric phase. Instead, this phase is extremely rapid and does not activate co-contractions and muscle slack reducing mechanisms in the same way that an external load does. Bosch argues, that such external loads may be beneficial during early training stages of an athlete, because the positive physiological changes out weigh the negative ones, but as the athlete progresses, heavy training might be harmful for performance because it reduces the ability to rapidly take up muscle slack.


Brining It All Together


J.B Morin and colleagues have shown that high velocity work can be an effective training tool to improve velocity deficits in an athlete’s profile. Bosch stated that movements that unnaturally increase co-contractions and aid in taking up muscle slack might hinder the athlete’s ability to pretense in sport, by reducing development of early stage RFD. It can probably be assumed that increases in velocity from the J.B Morin’s study were associated with and increase in early stage RFD, because the faster force can be developed, the faster it can be displayed and the faster your velocity will be. Both cases focus on early stage RFD, and when combining their ideas, it is possible to obtain more clarity as to how they are related. Remember, J.B Morin trained an output (velocity) and Bosch advocates the training of a mechanism (reduction in muscle slack).


It is possible that the lack of velocity in someone’s velocity profile is actually just an inability to reduce muscle slack rapidly (early stage RFD). If true, this means doing movements that emphasize early RFD might be most beneficial. Doing movements from static starts or with small amplitudes, despite their lower velocities outputs to their dynamic, larger amplitude counterparts, might actually be a better way of increasing velocity. Theoretically, speed of movement should not be over emphasized, instead the qualities that influence speed of movement should be targeted. When looking at a countermovement jump, despite having a higher velocity than its static counterpart, the squat jump, the higher movement velocity from the countermovement jump is byproduct of longer movement time and “unnaturally” aided pretensioning and muscle slack reduction (unnatural because in sport there isn’t enough time for larger countermovement’s), which means the actual underlying mechanisms of velocity (early stage RFD and reduction in muscle slack) are not being trained.


The goal of training should be to focus on the mechanisms of the movement that increase the output. There are times where high velocity movements will innately have aspects of muscle slack reducing qualities, however this does not guarantee that all high velocity movements will train muscle slack reduction, as seen by the countermovement jump and squat jump comparison. It is critical that coaches understand that outputs are merely products of many complex interactions. In order to obtain better outputs, these mechanisms need to be understood and trained.



I want to thank J.B Morin, Frans Bosch, and their respective teams of researchers. Their positive discoveries have helped propel the world of human performance into a better place. I have the utmost respect for both individuals and am very excited to learn more from both as they continue to push further with their research


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For Bosch

Van Hooren B & Bosch F (2016). Influence of muscle slack on high-intensitiy sport performance: a review. Strength and Conditioning Journal, 38 (5), 75-87.

Frans Bosch (2010) Strength Training and Coordination: An Integrative Approach

For Morin

Jimenez-reyes, P. (2016). Effectiveness of an optimized training using Force-velocity profile analysis, (October).

Samozino, P., Rejc, E., Di Prampero, P. E., Belli, A., & Morin, J. B. (2012). Optimal force-velocity profile in ballistic movements-Altius: Citius or Fortius? Medicine and Science in Sports and Exercise, 44(2), 313–322.

Samozino, P., Morin, J., & Samozino, P. (2015). Interpreting Power-Force-Velocity Profiles for Individualized and Specific Training Interpreting Power-Force-Velocity Profiles for Individualized and Specific Training, (March 2016).

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