A muscle called upon by the nervous system to do work is known as an "active" muscle. Activated muscles create force by pulling on their attachments, which connect them to bones, thereby enabling them to move the joints. Muscles only pull - they never push - so they can only produce tensile (pulling) forces. Tensile force is also known as tension, and two types arise in muscles: active and passive tension.
Active tension is the force created by the actin and myosin filaments sliding past each other in the sarcomeres of working muscles. Producing active tension requires chemical energy input, which is stored in adenosine triphosphate (ATP).
Passive tension is the force created by elongating the connective tissue elements within the muscle-tendon unit. It occurs when an agonist contracts and causes its antagonist to lengthen, such as raising one's leg to the front by flexing the hip (agonist) and stretching the hamstring group (antagonist).
Passive tension also occurs when the antagonist elongates without the active input of the agonist, such as sliding into a relaxed split without contracting the shortening muscles. Passive tension is the familiar resistance to stretch that is felt when the muscle-tendon unit resists against elongation.
Passive tension can reach very high levels - more so than active tension - and some researchers have postulated that it might be responsible for muscle weakness following stretching (Simic et al., 2013). Passive tension does not contribute to muscle forces during normal movement in the middle range of joint motion, but it does become a significant factor during low-force movements, in neuromuscular disorders, and towards the end of range of motion (Siegler & Moskowitz, 1984; Muraoka et al., 2005; Lamontagne et al., 2000).
Passive tension of the muscle-tendon unit is the main limiter of movement at the extreme limits of joint range of motion. The increase in passive tension is most evident in multi-joint muscles lengthened to their fullest at all the joints they cross, thus preventing the full range of motion at one or more joints. In the biomedical literature, this phenomenon is called "passive insufficiency."
A classic example is the hamstring muscle group (which crosses the hip and knee) during a straight leg raise; maximally flexing the hip makes the full extension of the knee joint extremely difficult for most people. Insufficient hamstring flexibility could lead to poor performance or heightened risk of injury during movements requiring maximal hip flexion and knee extension, such as a front kick in martial arts.
The primary goal of flexibility training is to create change in passive tension by altering the stiffness of the muscle (its intrinsic resistance to stretch), increasing fascicle length, or improving one's ability to tolerate higher levels of passive tension (Nakamura et al., 2020; Freitas & Mil-Homens, 2015; Law et al., 2009). An active counterpart to passive insufficiency (imaginatively called active insufficiency) will be discussed in a future blog post.
Simic, L. et al. (2013) Does Pre-Exercise Static Stretching Inhibit Maximal Muscle Performance? A Meta-Analytic Review. Scandinavian Journal of Medicine & Science in Sports vol. 23, no. 2, pp. 131-148.
Siegler, S. & Moskowitz, G. D. (1984) Passive and Active Components of the Internal Moment Developed About the Ankle Joint During Human Ambulation. Journal of Biomechanics vol. 17, no. 9, pp. 647-652.
Muraoka, T. et al. (2005) Estimation of Passive Ankle Joint Moment During Standing and Walking. Journal of Applied Biomechanics vol. 21, no. 1, pp. 72-84.
Lamontagne, A. et al. (2000) Contribution of Passive Stiffness to Ankle Plantar Flexor Moment During Gait After Stroke. Archive of Physical Medicine and Rehabilitation vol. 81, no. 3, pp. 351-358.
Nakamura, M. et al. (2020) Effects of Static Stretching Programs Performed at Different Volume-Equated Weekly Frequencies on Passive Properties of Muscle-Tendon Unit. Journal of Biomechanics vol. 103, article 109670.
Freitas, S. R. & Mil-Homens, P. (2015) Effect of 8-Week High-Intensity Stretching Training on Biceps Femoris Architecture. Journal of Strength and Conditioning Research vol. 29, no. 6, pp. 1737-1740.
Law, R. Y. W. et al. (2009) Stretch Exercises Increase Tolerance to Stretch in Patients with Chronic Musculoskeletal Pain: A Randomised Controlled Trial. Physical Therapy vol. 89, no. 10, pp. 1016-1026.