The relationship between skeletal muscle length and tension
Tension in the muscle can very broadly be defined as a tight stretch in certain structures of the muscle or it can be defined as the amount of force the muscle can produce in that position, this is to say that a muscular length which experiences the most tension is the position where the most amount of force can be generated in the muscle, do not get confused though, as this is usually not the position where you are strongest, that depends on the exercise at hand and its biomechanics. For instance, the biceps in a dumbbell curl, at the very stretched position, a large amount of tension is experienced, also the moment arm between the dumbbell and the joint in which the dumbbell is working around is small, which is the elbow, this is why once you can no longer complete a rep of a dumbbell curl through its full range of motion you can still squeeze out a good amount of reps just at the very stretched position, during a bicep curl the biceps typically experience the second most amount of tension at a mid muscular length, this is when the dumbbell and elbow are around parallel, however, like any long time gym goer has experienced you are certainly not strong in this position because the leverage is so long. The topic of leverages goes beyond the scope of this short article but you can find more information on the topic in my article “how to optimise leverages for powerlifting”.
There are two types of tension, active tension and passive tension. Active tension is the tension experienced in the contractile proteins of a muscle, in other words, the sarcomere. Active tension is caused by the myosin filaments binding to the actin filaments creating an actomyosin bond, the amount of actomyosin bonds is greater at a mid muscular length because when the muscle begins to shorten the actin filaments begin to overlap and the myosin filaments can no longer bind to as many actin filaments forming actomyosin bonds, in fact, there will come a point where the actin filaments are packed so densely that they can no longer be shortened anymore and the muscle can no longer generate any active tension. On the other hand, if a muscle is lengthened the actin bonds begin to travel away from the myosin and the myosin filaments begin to not be able to bind to as many actin filaments, eventually the actin filaments can be pulled so far from the myosin filaments that they no longer overlap at all and no actomyosin bonds can be formed and thus no active tension can be generated.
Passive tension is the tension generated by passive structures of the muscle, and it is generated rapidly when the muscle is in the stretched position, in fact so much so that in the very stretched position the muscle is at its strongest despite there being minimal active tension. Passive tension is generated mostly in titin and partly in other connective tissues such as the endomysium. When and to what extent people experience passive tension seems to vary greatly among individuals. The fact that passive tension is so strong may make you question why at times in the stretched position of certain exercises it is often the hardest, such as at the bottom of the squat, well this is because this is the position where the leverages are the longest and thus the difficulty of the lift is at its highest.
Tension is almost certainly the most important variable when it comes to hypertrophy, and more studies on the subject are showing that tension is experienced to a far greater degree in the stretched position of an exercise compared to the middle or shortened position. But why? After all there is still a significant amount of tension experienced in the muscle at a mid muscular length, this is because when it comes to hypertrophy we are typically talking about mechanical tension, this can be sensed by mechanosensors located all throughout the muscle and signal various different growth pathways to signal for muscle protein synthesis, particularly in the stretched position and during the eccentric portion of an exercise, I hypothesise that the stretched position signals for more muscle growth than other muscular positions because this is where the titin of the sarcomeres is stretched the most which works as a powerful mechanosensor signalling for an increase in muscle protein synthesis.
Disclaimer: use the information provided in this article at your own risk, as I will not be liable for any harm that may be caused by it.
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