diff --git a/sim/scripts/drop_urdf.py b/sim/scripts/drop_urdf.py
index aa05e87a..20eb8c04 100644
--- a/sim/scripts/drop_urdf.py
+++ b/sim/scripts/drop_urdf.py
@@ -14,6 +14,7 @@
 
 from sim.env import stompy_urdf_path
 from sim.logging import configure_logging
+from sim.stompy.joints import Stompy
 
 logger = logging.getLogger(__name__)
 
@@ -138,6 +139,9 @@ def load_gym() -> GymParams:
 
 
 def run_gym(gym: GymParams) -> None:
+    flag = False
+    joints = Stompy.legs.all_joints()
+
     while not gym.gym.query_viewer_has_closed(gym.viewer):
         gym.gym.simulate(gym.sim)
         gym.gym.fetch_results(gym.sim, True)
@@ -145,6 +149,12 @@ def run_gym(gym: GymParams) -> None:
         gym.gym.draw_viewer(gym.viewer, gym.sim, True)
         gym.gym.sync_frame_time(gym.sim)
 
+        # Every second, set the target effort for each joint to the reverse.
+        effort = 1.0 if flag else -1.0
+        flag = not flag
+        for joint in joints:
+            gym.gym.set_dof_force_target(gym.sim, joint, effort)
+
     gym.gym.destroy_viewer(gym.viewer)
     gym.gym.destroy_sim(gym.sim)
 
@@ -156,5 +166,5 @@ def main() -> None:
 
 
 if __name__ == "__main__":
-    # python -m sim.stompy
+    # python -m sim.scripts.drop_urdf
     main()
diff --git a/sim/stompy/joints.py b/sim/stompy/joints.py
index 6795a262..c5b6edc5 100644
--- a/sim/stompy/joints.py
+++ b/sim/stompy/joints.py
@@ -19,6 +19,22 @@ def children(cls) -> List["Union[Node, str]"]:
             if isinstance(attr, (Node, str))
         ]
 
+    @classmethod
+    def joints(cls) -> List[str]:
+        return [
+            attr
+            for attr in (getattr(cls, attr) for attr in dir(cls) if not attr.startswith("__"))
+            if isinstance(attr, str)
+        ]
+
+    @classmethod
+    def all_joints(cls) -> List[str]:
+        leaves = cls.joints()
+        for child in cls.children():
+            if isinstance(child, Node):
+                leaves.extend(child.all_joints())
+        return leaves
+
     def __str__(self) -> str:
         parts = [str(child) for child in self.children()]
         parts_str = textwrap.indent("\n" + "\n".join(parts), "  ")