This study proposes a mechanical model to investigate the relationship between gait speed and strength of the ankle plantarflexor muscles. The model calculates the muscular utilization ratio (MUR) of the plantarflexor muscles during gait by comparing the plantarflexion moment used while walking to the maximal moment of the plantarflexors estimated from dynamometric measurements. To verify the model, MURs of the plantarflexor muscles were calculated for five healthy subjects and one hemiparetic subject walking at different speeds (slow, self-selected, and fast). Generally, the results of the healthy subjects revealed that MURs increase with an increasing gait speed: average (+/-SD) peak values of MUR reached 58.8% (+/-18.5), 65.6% (+/-17.2) and 71.0% (+/-17.8) for the slow, self-selected, and fast speeds, respectively. The average peak value of MURs at the self-selected speed corresponds to values reported in electromyographic studies of the plantarflexor muscles. At self-selected gait speed, the hemiparetic subject presented a higher peak MUR (80.5%) of the plantarflexors and a lower gait velocity when compared to healthy subjects. For the hemiparetic subject, peak values of MUR of the plantarflexor muscles at maximal walking speed reached 100% suggesting that full activation of the plantarflexors had been reached preventing him from walking faster. From these preliminary results, it appears that MURs calculated by the proposed model are sensitive to the mechanical demands imposed on a group of muscles during a task (eg., increase in gait speed) and to change in the maximal plantarflexor's strength (eg., weakness). The proposed model seems to have the potential to demonstrate whether muscle weakness limits maximal gait speed in hemiparetic subjects. However, considering the complexity of gait speed regulation in hemiparetic patients, the model should be tested on a large number of hemiparetic subjects.