diff --git a/src/jaxsim/parsers/rod/utils.py b/src/jaxsim/parsers/rod/utils.py
index 5950a0fa8..ca69e12e0 100644
--- a/src/jaxsim/parsers/rod/utils.py
+++ b/src/jaxsim/parsers/rod/utils.py
@@ -117,22 +117,19 @@ def create_box_collision(
 
     center = np.array([x / 2, y / 2, z / 2])
 
-    box_corners = (
-        np.vstack(
-            [
-                np.array([0, 0, 0]),
-                np.array([x, 0, 0]),
-                np.array([x, y, 0]),
-                np.array([0, y, 0]),
-                np.array([0, 0, z]),
-                np.array([x, 0, z]),
-                np.array([x, y, z]),
-                np.array([0, y, z]),
-            ]
-        )
-        - center
+    # Define the bottom corners.
+    bottom_corners = np.array([[0, 0, 0], [x, 0, 0], [x, y, 0], [0, y, 0]])
+
+    # Conditionally add the top corners based on the environment variable.
+    top_corners = (
+        np.array([[0, 0, z], [x, 0, z], [x, y, z], [0, y, z]])
+        if not os.environ.get("JAXSIM_COLLISION_USE_BOTTOM_ONLY", "0")
+        else []
     )
 
+    # Combine and shift by the center
+    box_corners = np.vstack([bottom_corners, *top_corners]) - center
+
     H = collision.pose.transform() if collision.pose is not None else np.eye(4)
 
     center_wrt_link = (H @ np.hstack([center, 1.0]))[0:-1]
@@ -170,23 +167,31 @@ def create_sphere_collision(
 
     # From https://stackoverflow.com/a/26127012
     def fibonacci_sphere(samples: int) -> npt.NDArray:
-        points = []
-        phi = np.pi * (3.0 - np.sqrt(5.0))  # golden angle in radians
-
-        for i in range(samples):
-            y = 1 - (i / float(samples - 1)) * 2  # y goes from 1 to -1
-            radius = np.sqrt(1 - y * y)  # radius at y
-
-            theta = phi * i  # golden angle increment
-
-            x = np.cos(theta) * radius
-            z = np.sin(theta) * radius
+        # Get the golden ratio in radians.
+        phi = np.pi * (3.0 - np.sqrt(5.0))
+
+        # Generate the points.
+        points = [
+            np.array(
+                [
+                    np.cos(phi * i)
+                    * np.sqrt(1 - (y := 1 - 2 * i / (samples - 1)) ** 2),
+                    y,
+                    np.sin(phi * i) * np.sqrt(1 - y**2),
+                ]
+            )
+            for i in range(samples)
+        ]
 
-            points.append(np.array([x, y, z]))
+        # Filter to keep only the bottom half if required.
+        if os.environ.get("JAXSIM_COLLISION_USE_BOTTOM_ONLY", "0"):
+            # Keep only the points with y <= 0.
+            points = [point for point in points if point[2] <= 0]
 
         return np.vstack(points)
 
     r = collision.geometry.sphere.radius
+
     sphere_points = r * fibonacci_sphere(
         samples=int(os.getenv(key="JAXSIM_COLLISION_SPHERE_POINTS", default="50"))
     )