The Length-Tension Relationship

Active vs passive tension

Active tension is created when actin and myosin filaments slide past each other inside a sarcomere. This requires the constant formation of cross-bridges (cross-bridge cycling) – a process that relies on adenosine triphosphate (ATP) to function. Passive tension refers to the force created when the connective tissue elements inside the muscle-tendon unit are stretched. Some studies have indicated that titin content was the most important contributor to passive tension, whereas the collagen and elastin of the endomysium contributed only minimally. Although active length-tension properties are consistent between sarcomeres, the passive length-tension properties of muscle fibers and muscles are less consistent and less known.

Interestingly, passive tension can reach significantly higher forces than active muscle tension. It is therefore considered the main limitor of movement in extreme ends of a joint’s range of motion (ROM). The increase in passive tension can occur either by passively stretching a muscle, or when an antagonist muscle contracts and causes the antagonist to lengthen. The increase in passive tension is especially evident in multi-joint exercises where muscles are stretched to their limits at all the joints they cross, preventing the full ROM of one or more joints (passive insufficiency). Conversely, if a multi-joint muscle shortens over two or more joints, the muscle creates too much slack and is unable to produce tension (active insufficiency).

Passive tension does not contribute to the muscle’s force production during movements in the middle range of joint motion. However, it plays an important part in low-force exercises and movements closer to the end of a joint’s range of motion. At maximal lengths, tension (resistance to further lengthening of the muscle) is virtually all passive.

Passive tension also allows for the ”pre-stretch” to occur in the stretch-shortening cycle. Thus, helping the muscle produce more force in the concentric phase of a movement.

Training implications of the length-tension relationship

Mechanical loading is considered to be the greatest physiological driver of hypertrophy (muscle growth). When muscle fibers produce contractile force and the passive structures are able to contribute to force production, the overall mechanical tension of the muscle fiber is increased. This added passive tension is due to presence of titin, which increases tension when the muscle is stretched to long lengths as well as when the muscle lengthens while activated (eccentric muscle actions).

During strength training, muscles experience higher tension at longer muscle lengths (larger ranges of motion) as well as during eccentric muscle actions. When the muscle lengthens while contracting, it allows titin to produce tension even at short muscle lengths. This happens because titin’s elastic segment cannot elongate when the muscle is active, forcing its stiff segments to stretch instead. Without muscle activation, titin’s stiff segment elongates only when the elastic segment has already stretched (occurs at long fiber lengths).

When muscle fibers contract at longer lengths, their passive structures (titin and collagen) are already elongated when the contraction takes place. This allows them to participate in force production, and thus, increase mechanical tension of the muscle fiber (up to 30% more than the equivalent concentric muscle action). The increased tension is also why training at long muscle lengths (traditional strength training or isometric training) often result in greater hypertrophy and increased fascicle length than training at short muscle lengths.