When active muscles lengthen: properties and consequences of eccentric contractions.

exceeds that produced by the muscle, it will lengthen. Lengthening, or eccentric, muscle contractions have a surprisingly long history in physiological studies. (Note that this word was first introduced as “excentric” by Asmussen in 1953. This original spelling is more enlightening as it combines the prefix ex, “from or away,” with centric, “center,” hence a muscle contraction that is moving away from the muscle’s center.) In 1882, Fick observed that a muscle could exert greater force when stretched while contracting. Fifty years later, Hill reported another feature of eccentric contractions, namely that there is decreased energy liberation in a muscle that is stretched during a contraction. But the first revelation of the functional significance of these properties occurred by way of a clever demonstration devised by Bud Abbott, Brenda Bigland, and Murdoch Ritchie (1). They connected two stationary cycle ergometers back-to-back with a single chain, such that one cyclist pedaled forward and the other resisted this forward motion by braking the backward-moving pedals. Because the internal resistance of the device was low, the same force was being applied by both individuals, yet the task was much easier for the individual braking. This demonstration cleverly revealed that a tiny female resisting the movement of the pedals (in this case, Bigland) could easily exert more force than, and hence control the power output of, a large burly male pedaling forward (Ritchie). However intriguing, relatively little more was done to probe the properties of lengthening muscle contractions. Conventional wisdom continued to focus on work done by shortening muscles as essential during locomotion. Furthermore, most of the classic studies in muscle physiology, which have formed the foundation of our basic understanding of how muscle works, are founded on two important experimental approaches: isometric (constant length) and isotonic (shortening against a constant load) muscle contractions. As a consequence, much less is known of both the mechanics and the energetics of activated muscle during forced lengthening than during shortening or remaining at a fixed length. In fact, so little is known that the late muscle biomechanist Tom McMahon and his student Jason Harry characterized lengthening contractions as “the dark side of the force-velocity curve”; a reference to the relative lack of knowledge about this region of the classic model of Hill that describes the relationship between a muscle’s shortening velocity and its force production. Only recently are both the importance and prevalence of lengthening contractions in normal locomotion receiving increasing attention (see references in Ref. 5).