Introduction to Sports Biomechanics: Analysing Human Movement ...

stimulation rate – involves increasing the rapidity of stimulation to produce wave
summation. The second – increasing motor unit recruitment – involves recruitment of
increasingly more motor units to produce multiple motor unit summation.
Wave summation and tetanus
The duration of an action potential is only a few milliseconds, which is very short
compared with the following twitch. It is therefore possible for a series of action
potentials, known as an action potential train, to be initiated before the muscle has
completely relaxed. As the muscle is still partially contracted, the tension developed as
a result of the second stimulus produces greater shortening than the first, as seen in
Figures 6.11(b) and (c). The contractions are additive and the phenomenon is called
wave summation. Increasing the stimulation rate will result in greater tension development
as the relaxation time decreases until it eventually disappears. When this occurs, a
smooth, sustained contraction results (Figure 6.11(d)) called tetanus; this is the normal
form of muscle contraction in the body. It should be noted that prolonged tetanus leads
to an eventual inability to maintain the contraction and a decline in the tension to zero.
This condition is termed muscle fatigue.
Multiple motor unit summation
THE ANATOMY OF HUMAN MOVEMENT
The wide gradation of contractions within muscles is achieved mainly by the differing
activities in their various motor units – in stimulation rate and in the number of units
recruited. The repeated, asynchronous twitching of all the recruited motor units leads
Figure 6.11 Muscle responses: (a) muscle twitch; (b) wave summation; (c) incomplete and (d) complete tetanus (adapted from
Marieb, 2003; see Further Reading, page 280).
249