HMB intake during Resistance Training (RT) is known to be an effective strategy to improve muscle mass & function and decrease FM. PURPOSE: To determine whether HMB administration during 10-wk RT would facilitate RT-induced improvement in neuromuscular functions, body composition, myofiber dimension and regenerative capacity in aged female rats. METHODS: After four animals were sacrificed for baseline muscle samples, twelve 19-month old female Sprague-Dawley rats (N=16) were randomly divided into two groups, HMB (0.46 g/kg/d, similar to 6 g/d in humans) and non-HMB. Both groups were trained every 3 day for 10 wks on ladder climbing exercise with weights attached to the tail. The MW, GS, inclined plane test were used to measure neuromuscular functions. Body composition was assessed by DXA before and after RT. DTI determined water diffusion in myofibers by calculating FA and eigenvalues. Muscle mRNA expressions of the regulators of myogenesis, mitogenesis, and cell cycle inhibition were analyzed using RT-PCR. The number of satellite cells and myonuclei associated with muscle regeneration capacity were analyzed using Pax7-stained stem-cell immunohistochemistry. RESULTS: There were significant (p<0.05) main effects for time for MW (pre vs. post +239%), and normalized MW (pre vs. post +262%). A significant time effect (p < 0.05) was found for normalized GS (pre vs. post +16.6%); a post hoc indicated significant increase (p < 0.05) from pre- to post-training in both groups (pre vs post, Non-HMB: +19.8 %; HMB: +13.5 %). A significant time effect was observed for LBM (p < 0.05, +20.35%), and post hoc analysis indicated a significant (p < 0.05) increase in LBM for both groups (pre vs post, Non-HMB: +22.9 %; HMB: +17.8 %). A significant time effect existed for FM (-19.25%, p < 0.05); post hoc analysis revealed a significant reduction only in the HMB group (-22.3%, p < 0.05). Eigenvalues indicating muscle fiber dimension significantly (p < 0.05) increased from baseline values in both gastrocnemius (ë2; +17% and ë3; +20%) and soleus (ë 1; +8.2%, ë 2; +18% and ë 3; +21%) muscles for both groups. FA significantly (p < 0.05) declined from baseline value in gastrocnemius (-12%) and soleus (-29%) muscles in both groups. Although the main group effect for IGF-I mRNA only approached significance (p=0.1), post hoc analysis revealed significant (p < 0.05) increases in IGF-I mRNA only for the HMB group soleus muscle (+33%). There was a significant (p < 0.05) effect for time for soleus MGF mRNA expression in both HMB (+32%) and non-HMB (+40%) with no significant between group differences (p > 0.05). Although a main group effect for soleus myogenin mRNA approached significance (p=0.07), post hoc analysis showed significant increases in both HMB (+32%) and non-HMB (+40%) (p < 0.05). Soleus SC significantly increased in both HMB (+108%) and non-HMB (+100%) (p < 0.05) with no significant between group differences. Soleus MN also increased in both HMB (+32.2%) and non-HMB (+31.8%) (p < 0.05) with no significant between group differences. RSC for soleus muscle significantly increased in both HMB (+51.5%) and non-HMB (+46.4%) (p < 0.05) with no significant between group differences. CONCLUSION: HMB did not to exert any additive improvements in all parameters during 10 wks of intensive RT in aged female rats. The excessive effects resulting from this intense RT protocol maybe far exceeded our expectations. Intriguingly, however, our findings indicated that HMB administration during RT can facilitate a reduction in fat mass. Thus, intense RT alone might serve as an effective strategy to promote body composition, functionality, and myofiber hypertrophy in aged skeletal muscle.