<qa>  How many groups of volunteers will participate in a training program?Four groups of volunteers 65 years old and over will participate in a training program directed at elbow-flexor muscles. One group will be trained with an intensity near the level of maximal voluntary contraction MVC group; a second group will be trained with high mental effort, low muscle intensity contractions HME group; a third group will be trained with low mental effort, low muscle intensity elbow-flexion contractions LME group; and the fourth control group will not be trained but will participate in the strength tests. Training will be performed every weekday for 12 weeks. Participants will be evaluated by functional MRI fMRI, EEG-derived motor activity-related cortical potential MRCP, surface EMG signals, and the MRI T2 relaxation time. 