Frans Bosch has popularized the concept of muscle slack (Van Hooren has publications on it). It is hinges on early stage rate of force development and the speed at which the muscle, tendon, and series elastic element can go from “slack” to “tense”. When a muscle is not activated, it is relaxed and there is slack in the muscle, tendon, and series elastic element as it hangs from its origin and insertion. 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, you are the origin and the car is the insertion. 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 from the origin and insertion. 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 muscle co-contracting to produce enough force on the tendon so the muscle can become tense. Muscle slack uptake occurs during start of where the contractile element receives the chemical signal to align all the way to the point where both the musculotendon unit and the series elastic element are tense.
Why Does Muscle Slack Matter?
A muscle cannot produce force until the muscle slack is taken up. The speed of taking up slack can theoretically vary between athletes. This process might be dependent on co-contractions and the rate at which the primary acting muscle and its co-contracting synergist muscles can develop force. In other words, the muscle has to have all slack removed before it can begin to produce force. The faster someone can develop tension, the better their muscle slack uptake might be and visa versa
Muscle Slack In Sport
Sporting movements do not allow for enough time for an athlete to produce a large amplitude, muscle pre-loading a movement. When a sprinter fires off the blocks, they do not have time for a large countermovement. In basketball, a player under the hoop does not have time to make a large countermovement before jumping in the air to block shot. Without a large countermovement, the body doesn’t have much time to develop force during the lowering phase. Instead, the muscle has to find other ways to reduce slack quickly, which means the performance of these kinds of movements might be highly dependent on the rate at which the athlete can tense the muscle. Once slack has been taken up, the athlete can begin to a propulsive force.
Importance Of Amplitude
As noted earlier, sport does not always allow time for a large countermovement, which might lead one to believe that training with a large countermovement might not be specific. Bosch points out that the utilization of the stretch shortening cycle and elastic energy might not actually be as beneficial as once believed. Instead, he states that early rate of force development and muscle slack uptake might be of greater importance. Assuming that muscle slack and early RFD are key physiological aspects of sporting development, it is possible that training with a large countermovement might be detrimental. Not only do these movements not occur in sport, but the innate neurological and physiological characteristics of a large amplitude movement, in regards to muscle slack, might be quite different to that of a small amplitude movement. For example, a large amplitude movement will increase the amount of time the athlete has to reduce muscle slack and in essence, teaches the body to rely on a countermovement for pretensioning purposes. Instead, training with a smaller amplitude movement might be more beneficial for muscle slack reduction, because the time required to perform a movement is much lower and possibly encourages faster neural firing rates.
Another aspect of muscle slack that Bosch touches upon is the idea of loaded eccentric movements. In sport, rarely will an athlete encounter a slow, externally loaded eccentric movement. Instead, the athlete will encounter a higher velocity, possibly passively lowered, eccentric movement. For example, a basketball player getting ready to block a shot will not slowly lower their body before jumping up. Instead, the player may passively lower their body (actively bend, but let gravity do the work) over a small rage of motion before exploding upward. Due to these stark differences between eccentric movements encountered in sport and the weight room, it is possible that eccentrically loading an exercise with an external resistance might not be beneficial. It may increase co-contraction during the eccentric phase and by the time the concentric phase rolls around, the muscle have already reduced their slack, which is very unlike that of sport. Similar to the issues with a large amplitude, this short of eccentric external resistance might hinder the body’s ability to rapidly reduce slack. Again, this could be teaching the body to rely on an external load to produce co-contractions, which may actually have a negative influence on sport.
Trained Versus Untrained
Dealing with trained athletes and untrained athlete is very different. Bosch states that traditional resistance training might be more beneficial than specific muscle pretensioning methods, because the positive physiological adaptations that come from traditional resistance training may outweigh some of the negative aspects of increased muscle slack. However, as the athlete becomes more trained and physiological changes are no longer as pronounced from traditional training, it might be more beneficial to move towards muscle slack reducing methods. The reason for this is that these methods might be more specific than that of traditional weight training, in the sense that they will help more with the pretensioning required in sport.
Reducing Muscle Slack (cocontractions)
According to Bosch’s papers, the best way to reduce muscle slack is through cocontractions. This method requires a light load, minimal to no countermovement, and lots of practice (skill acquisition). Specific methods such as unstable surfaces, small amplitude, and concentric only movements might serve as the best means for increasing cocontractions. However, because this is an under studied aspect of strength and conditioning, it is hard to label specific methods as most effective. Despite the lack of clarity, from the above suggestions, one might be best served to avoid large amplitudes and external eccentric loads.
My Take On Exercises For Muscle Slack Reduction
High knee angle jumps from box (pretensioning) . . It is theorized that top athletes have the best muscle slack reducing qualities. Theoretically, the faster someone can tense the muscle via cocontractions the quicker they can develop force. . . Most sporting movements (jumping and sprinting) have minimal time for tension to be built. Unlike what is typically seen in training, large counter movements rarely occur during sport. Athletes don't typically have time to "load up", when their sport is reactive. By training small ranges of motion, concentric only movements, muscle slack capabilities might possible be improved via increase in neural firing rates and tendon stiffness. . . Even tho this quality may be important, it should be noted that the theorizer behind all of this, Frans Bosch, has mentioned that traditional strength training in untrained athletes may still be more beneficial due to its positive physiological adaptations. However, it may reach a point where large amplitude strength movements may hinder more than they help when it comes to rapid nervous system activation. #trackandfield #sportsperformance #exercisescience #training #train #science #strengthandconditioning #strength #strengthtraining #strengthtrain #train #lift #weights #conjugate #powerlifting #weightlifting #cscs #exercisephysiology
Many thanks to Bosch and Van Hooren for all of their time, work, and effort towards propelling the world of human performance in a positive direction
Here are the links to Bosch and Van Hooren’s publications
Van Hooren, B, Bosch, Meijer. Can resistance training enhance the rapid force development in unloaded dynamic isoinertial multi-joint movements? A systematic review. Strength Cond J, 2017
Muscle Slack Table
Squat Muscle Slack