A more direct assessment of muscle damage can be obtained by applying electrical stimulation at different frequencies to a muscle before and after intense exercise and assessing the reduction in force output in response to low-frequency and high-frequency stimulation. After intense exercise, the force output at low frequencies of stimulation is often reduced, while the force output at high frequencies is maintained; a phenomenon termed "low frequency fatigue". When muscle is stimulated to contract (either voluntarily by the nervous system or involuntarily through electrical stimulation) calcium is released inside muscle. This calcium release leads to muscle contraction. When muscle undergoes intense exercise, there is damage to muscle membranes, including membranes inside muscle that are responsible for calcium release. This causes a lower amount of calcium to be released with each muscle contraction. Normally, if high frequencies of electrical stimulation are applied to muscle, a very large amount of calcium is released inside muscle - an amount which is "more than enough" to cause a high amount of muscle contraction and high force output. If muscle fibre membranes responsible for release of calcium are damaged, a lower amount of calcium is released, but because "more than enough" calcium is usually released with high frequency stimulation, the lower amount of calcium released with muscle damage is still enough to cause high force of muscle contraction. The force response to low frequencies of stimulation; however, is dramatically reduced when muscle is damaged - usually only a small amount of calcium is released when low frequencies of stimulation are delivered to muscle. Following muscle damage, the smaller amount of calcium released causes lower force production at low stimulation frequency. Low force production at low stimulation frequencies, with a relatively maintained force production at high stimulation frequencies therefore indicates that muscle damage has occurred. This lower muscle force capability at low frequencies of stimulation has dramatic effects on endurance performance because typical endurance performance relies on repeated low-force muscle contractions, as opposed to the few high-force contractions that might be required in other sports (i.e. short sprinting events or field events such as shot put).