oms ▁below ▁are ▁most ▁common ▁in ▁the ▁severe ▁S MA ▁type ▁ 0 / I : ▁ ▁A ref lex ia , ▁particularly ▁in ▁extrem ities ▁ ▁Over all ▁mus cle ▁weak ness , ▁poor ▁mus cle ▁tone , ▁lim p ness ▁or ▁a ▁t endency ▁to ▁f lop ▁▁ ▁D iff icult y ▁ach ieving ▁development al ▁mil est ones , ▁difficulty ▁sitting / standing / walk ing ▁ ▁In ▁small ▁children : ▁adopt ing ▁of ▁a ▁f rog - leg ▁position ▁when ▁sitting ▁( hips ▁ab duct ed ▁and ▁kne es ▁flex ed ) ▁ ▁L oss ▁of ▁strength ▁of ▁the ▁resp ir atory ▁mus cles : ▁weak ▁c ough , ▁weak ▁cry ▁( inf ants ), ▁accum ulation ▁of ▁secret ions ▁in ▁the ▁l ungs ▁or ▁thro at , ▁resp ir atory ▁dist ress ▁ ▁Bell - sh aped ▁tor so ▁( ca used ▁by ▁using ▁only ▁ab dom inal ▁mus cles ▁for ▁resp iration ) ▁in ▁severe ▁S MA ▁type ▁▁ ▁F asc ic ulations ▁( tw itch ing ) ▁of ▁the ▁tongue ▁ ▁D iff icult y ▁su ck ing ▁or ▁sw allow ing , ▁poor ▁feed ing ▁ ▁C aus es ▁ ▁Sp inal ▁mus cular ▁at ro phy ▁is ▁linked ▁to ▁a ▁gen etic ▁mut ation ▁in ▁the ▁SM N 1 ▁gene . ▁ ▁Human ▁chrom os ome ▁ 5 ▁contains ▁two ▁nearly ▁identical ▁gen es ▁at ▁location ▁ 5 q 1 3 : ▁a ▁tel omer ic ▁copy ▁SM N 1 ▁and ▁a ▁cent rom