The objective was to determine if strength training the free limb during a 3-week period of unilateral immobilization attenuates strength loss in the immobilized limb, through cross- education. Thirty right-handed participants were assigned to three groups. One group (n=10) wore a cast and trained the free arm (CAST-TRAIN). A second group (n=10) wore a cast and did not train (CAST). A third group (n=10) received no treatment (CONTROL). Casts were applied to the non-dominant (left) wrist and hand by a physician. Strength training was maximal isometric ulnar deviation (right hand) 5 days/week. Peak torque (dynamometer), electromyography (EMG), and muscle thickness (ultrasound) were assessed in both arms before and after the intervention. CAST-TRAIN improved right arm strength (14.3[5.0] to 17.7[4.8] Nm;p<0.05) with no significant muscle hypertrophy (3.73[0.43] to 3.84[0.52] cm;p=0.09). The immobilized arm of CAST-TRAIN did not change in strength (13.9[4.3] to 14.2[4.6] Nm) or muscle thickness (3.61[0.51] to 3.57[0.43] cm). The immobilized arm of CAST decreased in strength (12.2[3.8] to 10.4[2.5] Nm;p<0.05) and muscle thickness (3.47[0.59] to 3.32[0.55] cm;p<0.05). CONTROL showed no changes in the right (strength:15.3[6.1] to 14.3[5.8] Nm; muscle thickness:3.57[0.68] to 3.52[0.75] cm) or left arm (strength:14.5[5.3] to 13.7[6.1] Nm; muscle thickness:3.55[0.77] to 3.51[0.70] cm). Agonist muscle activation remained unchanged after the intervention for both arms (Right: 302[188] to 314[176] µV; Left: 261[139] to 288[151] µV) with no group differences. Strength training of the free limb attenuated strength loss in the immobilized limb during unilateral immobilization. Strength training may have prevented muscle atrophy in the immobilized limb.