This　paper　proposes　a　new　criterion　for　redundancy　resolution　of　human　arm　in　reaching　tasks.　First,　an　assumption　of　minimum　Total　Potential　Energy　(TPE)　principle　is　proposed　to　explain　how　human　exploit　the　kinematic　redundancy　of　their　arms　to　choose　the　natural　arm　postures　in　reaching　tasks.　This　is　the　basis　as　well　as　the　physical　meaning　of　the　new　criterion.　The　TPE　of　human　arm　includes　Gravitational　Potential　Energy　(GPE)　and　Elastic　Potential　Energy　(EPE)　stored　in　the　muscles.　Then,　a　creative　simple　equivalent　model,　a　virtual　torsion　spring　model　with　variable　stiffness,　is　proposed　to　represent　the　EPE.　The　stiffness　of　the　torsion　spring　is　related　to　the　target　point.　Last,　to　obtain　the　stiffness　of　the　torsion　spring　in　different　target　points,　many　experiments　are　performed,　and　multiple　linear　regression　analysis　is　introduced　to　determine　the　relation　between　the　stiffness　and　target　point.　©　2013　IEEE.