The ornamental design for a standinglumbar back distractor, as shown and described.
FIG. 1 is a perspective view of a standing lumbar back distractor showing my new design;
FIG. 2 is a left side elevational view thereof;
FIG. 3 is a right side elevational view thereof;
FIG. 4 is a rear elevational view thereof;
FIG. 5 is a front elevational view thereof;
FIG. 6 is top plan view thereof;
FIG. 7 is a bottom plan view thereof; and,
FIG. 8 is a right side elevational view thereof wherein environment is shown.
The broken lines shown in the views are included for the purpose of illustrating environment of the standing lumbar back distractor that form no part of the claimed design.