Bugfix! First leg couldn't pull back

manduca/Manducas.py

@@ -253,7 +253,7 @@ class Manduca(object):
                     M_forwards = spring_forwards = damp_forwards = 0
 
                 # Compute the backward forces
-                if prev_idx > 0: # all but the first leg
+                if prev_idx >= 0: # all but the first leg
                     M_backwards = current_muscles[prev_idx]
                     spring_backwards = k*(pos[curr_idx] - pos[prev_idx] - L0)
                     damp_backwards = c*(vel[curr_idx] - vel[prev_idx])