Résumé : This study investigated the effect of protein supplementation enriched in leucine on muscle hypertrophy andstrength gain in response to a 6-month strength training programme of the plantar flexormuscles in older adults(aged over 60 yr). To this end, changes in neuromuscular parameters were compared between two groups(matched for age, sex and initial maximal strength) that trained twice a week for 24 weeks, with one group receivinga protein supplementation enriched with leucine (supplementation group, n = 12), and another groupreceiving a placebo (control group, n=13) that provided subjects with a similar energy intake that the proteinsupplementation. Muscle thickness and pennation angle of gastrocnemius medialiswere used as indices of musclehypertrophy.Maximal strength and associated electromyographic activity of leg muscles, and voluntary activation(assessed by the interpolated twitch technique) were recorded. Peak torque in response to 3-pulse traindelivered to the tibial nerve was also recorded. After training, muscle thickness (+8.4%) and pennation angle(+13.4%) were increased, regardless of training groups (p b 0.05). Maximal plantar flexion strength (+28.2%)and voluntary activation (+29.5%) increased to the same extent for both training groups. Peak torque evokedby 3-pulse train increased similarly between training groups (+13.4%, p b 0.001). Regardless of protein supplementation,results obtained in twelve subjects indicate that muscle hypertrophy mainly occurred in the last12 weeks of the programme. Our results suggest that protein supplementation enriched in leucine neither potentiatedthe neuromuscular adaptations to strength training, nor influenced the time course of these adaptations inolder adults.