ABSTRACT The series connection of sarcomeres in a muscle raises the possibility of instabilities due to some sarcomeres being capable of greater isometric tension than others, due to non-uniform activation, cross-sectional area or sarcomere length. Such instabilities are heavily damped by the force-velocity curve and stabilized by passive tension. The variation of active tension with sarcomere length may act to stabilize or destabilize the situation. Such instabilities can affect tension in a complex manner, causing both increases and decreases in different situations. Some examples of situations where sarcomere non-uniformities have been observed or inferred are given, and their mode of action explained.