Contractile abilities of normal and "mini" triceps surae muscles from mice (Mus domesticus) selectively bred for high voluntary wheel running. J Appl Physiol 99: 1308 -1316, 2005. First published June 9, 2005; doi:10.1152/japplphysiol.00369.2005.-As reported previously, artificial selection of house mice caused a 2.7-fold increase in voluntary wheel running of four replicate selected lines compared with four random-bred control lines. Two of the selected lines developed a high incidence of a small-muscle phenotype ("mini muscles") in the plantar flexor group of the hindlimb, which apparently results from a simple Mendelian recessive allele. At generations 36 -38, we measured wheel running and key contractile characteristics of soleus and medial gastrocnemius muscles from normal and mini muscles in mice from these selected lines. Mice with mini muscles ran faster and a greater distance per day than normal individuals but not longer. As expected, in minimuscle mice the medial and lateral gastrocnemius muscles were ϳ54 and 45% the mass of normal muscles, respectively, but the plantaris muscles were not different in mass and soleus muscles were actually 30% larger. In spite of the increased mass, contractile characteristics of the soleus were unchanged in any notable way between mini and normal mice. However, medial gastrocnemius muscles in mini mice were changed markedly toward a slower phenotype, having slower twitches; demonstrated a more curved force-velocity relationship; produced about half the massspecific isotonic power, 20 -50% of the mass-specific cyclic work and power (only 10 -25% the absolute power if the loss in mass is considered); and fatigued at about half the rate of normal muscles. These changes would promote increased, aerobically supported running activity but may compromise activities that require high power, such as sprinting. experimental evolution; fatigue; muscle mechanics; power; selective breeding; work SWALLOW ET AL. (34) describe an artificial selection experiment using a base population of outbred, Hsd:ICR house mice (Mus domesticus) in which four replicate lines were subject to selective breeding for high levels of voluntary wheel running whereas another four lines were random bred as controls. Compared with mice from the control lines, mice from the selected lines ran ϳ70% more revolutions/day after 10 generations of selection (34), 100% more after 14 generations (17), and 170% more revolutions and about double the average running speed after 23 generations (5, 11, 13). These mice have been the focus of anatomical, behavioral, physiological, and psychological investigations on the correlated effects of such selection (e.g., Refs. 7,11,[23][24][25]28,29,[35][36][37].More recently it has been noted that some individuals in both the selected and control lines express a small-muscle phenotype in which the plantar flexor muscle group (soleus, gastrocnemius, and plantaris) is 44 -50% lighter than normal for the body mass (5, 12, 17). Evidence suggests that this smallmuscle phenotype, coine...