Pre-symptomatic muscle weakness in Smn-/-; SMN2 and Smn2B/-mice. (A) P2 Smn-/-;SMN2 force measurements revealed a 67% decrease in maximal tetanic force production compared with controls. Force data were normalized to the muscle cross-sectional area. (B) The average peak tetanic force was reduced by 61% in P9 pre-symptomatic Smn2B/- TA muscles compared with control littermates. (C) Mean fiber area of P2 TA muscles from Smn-/-;SMN2 and control mice. (D) Average fiber cross-sectional area for P9 Smn2B/- and control TA muscle. N = 3 for all experiments. *, P < 0.05.