Force history and fibre dynamics for human movements

It is well known and accepted for a long time that the steady state isometric force of a muscle after stretching is increased compared to the force of an isometric contraction at the corresponding muscle length. This effect is known as "steady state" or "residual" force enhancement. Similarly to the force enhancement, another effect can be observed when an isometric contracted muscle is shortened. In this case a depression of force can be observed compared to a purely isometric contraction at the corresponding muscle length. This effect is known as "force depression" after shortening. Both effects have been investigated on different muscle levels (single fibre to whole muscle during in vivo human experiments) and under different stretch or shortening conditions (magnitude, velocity, force, etc.) So far there hasn`t been a systematical observation of fibre lengths and angles of pennation during voluntary and electrically induced contractions of human muscles in vivo which induce force enhancement or force depression. The hypothesis is that fibre lengths and angles of pennation (and their changes prior to an isometric reference contraction) are different for purely isometric contractions and isometric contractions following active muscle shortening (force depression) or active muscle stretching (force enhancement).The goal of the study is to investigate force enhancement/depression while simultaneously measuring the fibre dynamics associated with different experimental protocols. To investigate the behaviour of the muscle fibres during force enhancement or depression, voluntary and electrically induced contractions of M. tibialis anterior will be executed. Classical experimental approaches measure force, angle, angle velocity and EMG during force enhancement/force depression. Additional to these approaches and therefore novel we will use ultrasonic measurements to receive information of the fibre dynamics. As the history dependence for isometric muscle force has been observed also for sub-maximal contractions and can therefore be expected in everyday movements, it is important to understand the behaviour of the muscles fibres during such contractions. We are convinced to get new insights into the mechanisms underlying history dependence which are still not fully known.