diff --git a/pyproject.toml b/pyproject.toml
index 3ef1792..af1b1b2 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -63,7 +63,8 @@ isaac = [
     # pip install isaaclab isaaclab_assets isaaclab_tasks (from Isaac Lab source)
     "h5py>=3.0",
     "mujoco>=3.0.0",
-    "usd-core>=24.0",  # OpenUSD (pxr) for MJCF→USD asset conversion
+    "usd-core>=24.0; platform_machine != 'aarch64'",  # no aarch64 wheels
+    "warp-lang>=1.0.0",  # required by Isaac Lab
 ]
 newton = [
     "newton>=1.0.0",
diff --git a/strands_robots/newton/newton_backend.py b/strands_robots/newton/newton_backend.py
index 85de55f..533cb10 100644
--- a/strands_robots/newton/newton_backend.py
+++ b/strands_robots/newton/newton_backend.py
@@ -611,19 +611,20 @@ def create_world(
             gravity_mag = -abs(gy) if gy != 0 else -9.81
 
         try:
-            self._builder = newton.ModelBuilder(gravity=gravity_mag)
+            self._builder = newton.ModelBuilder(
+                up_axis=up_axis.upper(), gravity=gravity_mag
+            )
         except (TypeError, AttributeError):
             self._builder = newton.ModelBuilder()
-        # Set scalar gravity and up_vector
-        try:
-            self._builder.gravity = gravity_mag
-        except Exception:
-            pass
-        try:
-            self._builder.up_vector = up_vec
-            self._builder.up_axis = up_axis
-        except Exception:
-            pass
+            # Fallback: set gravity and up_vector as attributes
+            try:
+                self._builder.gravity = gravity_mag
+            except Exception:
+                pass
+            try:
+                self._builder.up_vector = up_vec
+            except Exception:
+                pass
 
         self._ground_plane_requested = ground_plane
         if ground_plane:
@@ -657,18 +658,21 @@ def _recreate_builder(self) -> None:
         newton = self._newton
         gravity_mag = getattr(self._builder, "gravity", -9.81)
         up_vec = getattr(self._builder, "up_vector", (0.0, 0.0, 1.0))
+        up_axis_str = "z" if up_vec[2] > up_vec[1] else "y"
         try:
-            self._builder = newton.ModelBuilder(gravity=gravity_mag)
+            self._builder = newton.ModelBuilder(
+                up_axis=up_axis_str.upper(), gravity=gravity_mag
+            )
         except (TypeError, AttributeError):
             self._builder = newton.ModelBuilder()
-        try:
-            self._builder.gravity = gravity_mag
-        except Exception:
-            pass
-        try:
-            self._builder.up_vector = up_vec
-        except Exception:
-            pass
+            try:
+                self._builder.gravity = gravity_mag
+            except Exception:
+                pass
+            try:
+                self._builder.up_vector = up_vec
+            except Exception:
+                pass
         if getattr(self, "_ground_plane_requested", True):
             try:
                 self._builder.add_ground_plane()
@@ -1111,7 +1115,14 @@ def replicate(self, num_envs: Optional[int] = None) -> Dict[str, Any]:
             self._dof_per_world = getattr(main_builder, "joint_count", 0)
 
             # Use Newton's replicate — instance method on a new builder
-            replicated_builder = newton.ModelBuilder()
+            # Propagate up_axis from main builder to replicated builder
+            _up_axis = getattr(main_builder, "up_axis", "Y")
+            if hasattr(_up_axis, "name"):
+                _up_axis = _up_axis.name  # Axis enum → string
+            try:
+                replicated_builder = newton.ModelBuilder(up_axis=str(_up_axis).upper())
+            except (TypeError, AttributeError):
+                replicated_builder = newton.ModelBuilder()
             replicated_builder.replicate(
                 main_builder,
                 world_count=num_envs,