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
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
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)
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
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.
Fridén, J., Kjorell, U., and L-E. Thornell. (1984). Delayed
muscle soreness and cytoskeletal alterations. An immunocytological
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Beliaeff S, Bouchard DR, Hautier C, Brochu M, Dionne IJ. J Aging
Phys Act 2008 University Institute of Geriatrics, Universityof Sherbrooke,
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.