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.
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