The length-tension curve of muscle is one of the important descriptors of mechanical performance, and also a direct reflection of the underlying structure, particularly the number of sarcomeres connected in series in muscle fibres. This number is one of the most plastic properties of muscle, changing within days after changes in activity patterns. We propose that this adaptation is to prevent eccentric contractions from occurring beyond the optimum length for tension generation, since this is the region of sarcomere instability and muscle damage. Evidence for this is presented for muscles from rats trained on a treadmill, and from motor units of the gastrocnemius muscle of the cat.