Length-tension relationship

The length-tension relationship, also called the force-length relationship, describes how the force a musculotendinous unit produces depends partly on muscle length.^{1Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 67.} ISSA describes an optimal sarcomere length within a muscle fiber for maximum force production.^{2International Sports Sciences Association, Strength and Conditioning, p. 240.}

The active, contractile part of the curve forms an inverted U: too much filament overlap in a short sarcomere reduces force, an optimal range of overlap produces maximum force, and further lengthening reduces overlap and force again.^{1Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 67.} The NSCA’s strength and conditioning text explains this through cross-bridge sites: a muscle generates the most force at resting length and less when stretched well beyond it or shortened well below it.^{3Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 46.} Connective tissue, tendons and titin add resistive tension when the muscle stretches past resting length.^{1Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 67.}

In practice

• The gastrocnemius crosses the knee and ankle, so it can produce substantially more force in a standing calf raise than a seated one; bending the knee shortens it and shifts more demand to the soleus.^{1Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 67.}
• At the extreme, a fully flexed knee with a plantarflexed ankle leaves the gastrocnemius unable to produce active force, a position Zatsiorsky and colleagues describe for selectively training the soleus.^{4Zatsiorsky, Kraemer and Fry, Science and Practice of Strength Training, 3rd ed., p. 53.}
• Joint torque varies through the range of motion largely because of this relationship and changes in muscle and tendon leverage, or moment arms.^{3Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 46.}
• Treat the curves as approximate: they’re usually recorded during isometric muscle actions at set lengths, while tension is greater during stretching and smaller during shortening.^{4Zatsiorsky, Kraemer and Fry, Science and Practice of Strength Training, 3rd ed., p. 53.}