Aging leads to decreases in muscle volume, strength, and functional ability; resistance training (RT) is commonly used to mitigate these negative effects. RT leads to hypertrophy and neuromuscular adaptation; however, relative changes in various measures of strength following upper extremity RT in older adults remain unclear. It is also unknown which upper limb muscles contribute most to performance of important functional tasks; these muscles may be critical targets for RT interventions. The goals of this dissertation were to determine the effects of RT in healthy older adults on 1RM, maximum isometric joint moment, functional strength, and muscle volume; evaluate maximum isometric joint moment and muscle volume as predictors of functional reaching and pulling strength; and determine individual muscle contributions to these reaching and pulling tasks. Sixteen healthy older adults (8M, 8F) were randomized (blocked on sex) to a six week upper extremity RT program or control group. Muscle volume (via magnetic resonance imaging), maximum isometric joint moment, and functional strength were assessed at baseline and follow-up. RT participants performed 1RM testing at baseline, 3, and 6 weeks. An upper extremity musculoskeletal model was developed for use in dynamic simulation, a simulation approach was validated for upper limb movement, and this approach was used to determine muscle contributions to horizontal and vertical hand acceleration during forward reaching and pulling tasks. The RT group increased 1RM significantly (p<0.01 for all exercises) and these increases were linear for all exercises and time points (p<0.001). Randomization to RT was associated with greater increases in functional reaching strength (p=0.0034), maximum isometric shoulder adduction moment (p=0.041), elbow flexor volume (p=0.018), and shoulder adductor volume (p=0.009). Shoulder abductor and adductor muscle volumes (R2>0.40, p<0.008 for both) and maximum isometric joint moments at the shoulder (R2>0.59, p<0.11) were significant predictors of functional strength. Eight of the 12 muscles identified using simulation as having major contributions to hand acceleration during reaching and pulling movements actuate the shoulder. Therefore, we conclude that shoulder strength appears to be a critical factor in functional reaching and pulling tasks and may be a key target for RT interventions in older adults.