Longitudinal muscle growth (increased serial sarcomere number (SSN) or fascicle length (FL)) has been observed following downhill running training in rats. The influence of longitudinal muscle growth on static contractile function is well-studied in the literature, however, the influence on dynamic contractile function is unclear. Additionally, previous body-weight downhill running programs did not induce longitudinal muscle growth in the rat soleus. The purpose of this thesis was to assess soleus SSN, FL, and work loop performance following 4 weeks of weighted downhill running training in rats. I observed increases in soleus SSN (+8%) and FL (+13%) in trained compared to control rats. Work loop performance improved in only the slowest work loops, at length changes of 1 and 3 mm (+78-209%), however, net work output was more related to maximum specific force (R2=0.17-0.42, P<0.05) than SSN (R2<0.01, P>0.05). Therefore, training-induced sarcomerogenesis likely contributed little to the improved work loop performance.