Isometric Strength in Human Skeletal Muscle

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Isometric Strength in Human Skeletal Muscle

by Ross Forsyth

An isometric contraction is when a muscle is activated when held at a constant length as a pose to being lengthened or shorted. Concentric strength is roughly 80% of total isometric strength.

One factor that is thought to influence isometric strength is muscle mass. However a study by Beliaeff et al. (2008) stated that “muscle mass does not play a crucial role in the variations of isometric muscle strength in well-functioning elderly.”

Furthermore muscle volume compared to muscle cross-sectional area is thought to be a more prominent factor when talking about isometric strength. Akagi R et al. (2009)

In addition anatomical and physiological architecture must be taken into account. Muscle fibre cross-sectional area (CSA) plays is more important in strength compared to anatomical muscle CSA “increase in fibre pennation angle allowed physiological CSA, and thereby maximal force-generating capacity, to increase significantly more than anatomical CSA “ Aagaard Pet al. (2001)

Tetanic tension with isometric strength in human skeletal muscle
Figure 1 - Plot demonstrating maximal tetanic force prior to and immediately following an exercise bout. While passive stretch causes negligible force decrement, isometric causes a moderate loss and eccentric causes a significant loss of force.

The graph in Figure 1 shows that muscle tetanic tension is reduced post exercise in isometric contractions. Therefore skeletal muscle damage will influence the amount of force that can be applied during muscle contraction.

Doherty T. J et al. (1993) found that “motor unit losses, even in healthy active individuals, are a primary factor in the age-associated reductions in contractile strength.” Therefore the amount of motor neurons in each muscle fibre is important when speaking about contractile strength.


Doherty T. J., Vandervoort A. A., Taylor A. W. and Brown W. F.(1993) Effects of motor unit losses on strength in older men and women. Journal of Applied Physiology, Vol 74, Issue 2 868-874

Akagi R. Takai Y., Ohta M. Kanehisa H. Kawakami Y. Fukunaga T. Muscle volume compared to cross-sectional area is more appropriate for evaluating muscle strength in young and elderly individuals. (2009)

Fridén, J., Kjorell, U., and L-E. Thornell. (1984). Delayed muscle soreness and cytoskeletal alterations. An immunocytological study in man. Int. J. Sports Med. 5:15-18.

Beliaeff S, Bouchard DR, Hautier C, Brochu M, Dionne IJ. J Aging Phys Act 2008 University Institute of Geriatrics, Universityof Sherbrooke, Quebec, Canada.

Aagaard P, Andersen J L, Dyhre-Poulsen P, Leffers A, Wagner A, Magnusson S P, Halkjær-K J and Simonsen EB (2001) A mechanism for increased contractile strength of human pennate muscle in response to strength training: changes in muscle architecture , The Journal of Physiology, 534, 613-623.