Previous research has indicated increased performance for movements with a prior stretch (stretch-shortening cycle or ssc) than for those without prior stretching. These comparisons have nearly always been made for movements with unequal muscle torque levels at the initiation of the concentric phase (pretension). Also, there has been little examination of the influence of pretension on these movements. This study compared concentric phase performance for ssc and prior isometric muscle action (IC) movements at equivalent pretensions and also determined the influence of pretension on these movements. In addition, the influence of the rate of torque development at the initiation of the concentric phase was considered for equivalent pretension movements with no prior stretch. Several factors were examined using both experimental and computer modeling techniques for the ankle plantarflexor muscles. A standard piece of variable weight exercise equipment was used for the experimental phase, while an eighth order muscle model was used to gain insight on internal variables. Each subject lowered and/or raised a weight which represented a mean value of 35.6% of maximum isometric plantarflexor strength. At any pretension level, ssc displayed improved performance over IC movements. The contribution from the series elastic component was greater and occurred earlier in the concentric phase for ssc movements, while force enhancement and the stretch reflex provided minor contributions. Higher pretension levels resulted in improved performance for both movements, although ssc performance was enhanced to a greater extent than IC as pretension increased. The power derived from the series elastic component was of increasing importance at higher pretension levels for both movements. Nearly all performance differences across movement conditions and/or pretension levels were exhibited during approximately the first 100 ms of the concentric phase. For high rates of torque development, concentric phase performance improved only slightly. Overall, experimental and model trends were very similar when comparing movement conditions and/or pretension levels. It was concluded that the interaction among the mass being lifted, the contractile component, and the series elastic component was important in determining the relative contributions of these two muscle components to overall muscle output.