diff --git a/src/user/CMakeLists.txt b/src/user/CMakeLists.txt
index 55c8b407..035ef1fe 100644
--- a/src/user/CMakeLists.txt
+++ b/src/user/CMakeLists.txt
@@ -19,6 +19,7 @@ set(MUJOCO_USER_SRCS
     user_composite.h
     user_flexcomp.cc
     user_flexcomp.h
+    user_init.cc
     user_mesh.cc
     user_model.cc
     user_model.h
diff --git a/src/user/user_api.h b/src/user/user_api.h
index 8b30b485..9190ef19 100644
--- a/src/user/user_api.h
+++ b/src/user/user_api.h
@@ -155,9 +155,18 @@ MJAPI void mjm_setDefault(mjElement element, void* defspec);
 // Set frame.
 MJAPI void mjm_setFrame(mjElement dest, void* frame);
 
-// Compute quat and inertia from body->fullinertia..
+// Compute quat and inertia from body->fullinertia.
 MJAPI const char* mjm_setFullInertia(mjmBody* body, double quat[4], double inertia[3]);
 
+
+//---------------------------------- Initialization functions --------------------------------------
+
+// Default body attributes.
+MJAPI void mjm_defaultBody(mjmBody& body);
+
+// Default site attributes.
+MJAPI void mjm_defaultSite(mjmSite& site);
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/src/user/user_init.cc b/src/user/user_init.cc
new file mode 100644
index 00000000..22bc46e0
--- /dev/null
+++ b/src/user/user_init.cc
@@ -0,0 +1,54 @@
+// Copyright 2024 DeepMind Technologies Limited
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <cstring>
+#include <mujoco/mjmodel.h>
+#include <mujoco/mujoco.h>
+#include "user/user_api.h"
+#include "user/user_util.h"
+
+
+// Default body parameters
+void mjm_defaultBody(mjmBody& body) {
+  memset(&body, 0, sizeof(mjmBody));
+
+  // set non-zero defaults
+  body.fullinertia[0] = mjNAN;
+  body.explicitinertial = (mjtByte)false;
+  body.mocap = (mjtByte)false;
+  body.quat[0] = 1;
+  body.iquat[0] = 1;
+  body.pos[0] = mjNAN;
+  body.ipos[0] = mjNAN;
+  body.alt.axisangle[0] = body.alt.xyaxes[0] = body.alt.zaxis[0] = body.alt.euler[0] = mjNAN;
+  body.ialt.axisangle[0] = body.ialt.xyaxes[0] = body.ialt.zaxis[0] = body.ialt.euler[0] = mjNAN;
+  body.plugin.active = false;
+  body.plugin.instance = nullptr;
+}
+
+
+
+// Default site parameters
+void mjm_defaultSite(mjmSite& site) {
+  memset(&site, 0, sizeof(mjmSite));
+
+  // set non-zero defaults
+  site.type = mjGEOM_SPHERE;
+  site.quat[0] = 1;
+  site.size[0] = site.size[1] = site.size[2] = 0.005;
+  site.fromto[0] = mjNAN;
+  site.alt.axisangle[0] = site.alt.xyaxes[0] = site.alt.zaxis[0] = site.alt.euler[0] = mjNAN;
+  site.rgba[0] = site.rgba[1] = site.rgba[2] = 0.5f;
+  site.rgba[3] = 1.0f;
+}
diff --git a/src/user/user_objects.cc b/src/user/user_objects.cc
index ee8f9a4d..9f659a9c 100644
--- a/src/user/user_objects.cc
+++ b/src/user/user_objects.cc
@@ -552,33 +552,12 @@ mjCBody::mjCBody(mjCModel* _model) {
   // set model pointer
   model = _model;
 
-  spec.pos[0] = spec.ipos[0] = mjNAN;
-
-  // clear variables
-  spec.fullinertia[0] = mjNAN;
-  spec.explicitinertial = false;
-  spec.mocap = false;
-  mjuu_setvec(spec.quat, 1, 0, 0, 0);
-  mjuu_setvec(spec.iquat, 1, 0, 0, 0);
-  mjuu_zerovec(spec.pos+1, 2);
-  mjuu_zerovec(spec.ipos+1, 2);
-  spec.mass = 0;
-  mjuu_setvec(spec.inertia, 0, 0, 0);
+  mjm_defaultBody(spec);
   parentid = -1;
   weldid = -1;
   dofnum = 0;
   lastdof = -1;
   subtreedofs = 0;
-  spec.gravcomp = 0;
-  spec_userdata_.clear();
-  spec.alt.axisangle[0] = spec.alt.xyaxes[0] = spec.alt.zaxis[0] =
-      spec.alt.euler[0] = mjNAN;
-  spec.ialt.axisangle[0] = spec.ialt.xyaxes[0] = spec.ialt.zaxis[0] =
-      spec.ialt.euler[0] = mjNAN;
-
-  spec.plugin.active = false;
-  spec.plugin.instance = nullptr;
-
   contype = 0;
   conaffinity = 0;
   margin = 0;
@@ -593,6 +572,7 @@ mjCBody::mjCBody(mjCModel* _model) {
   sites.clear();
   cameras.clear();
   lights.clear();
+  spec_userdata_.clear();
 
   // point to local
   spec.element = (mjElement)this;
@@ -1982,23 +1962,13 @@ void mjCGeom::Compile(void) {
 
 // initialize default site
 mjCSite::mjCSite(mjCModel* _model, mjCDef* _def) {
-  // set defaults
-  spec.type = mjGEOM_SPHERE;
-  mjuu_setvec(spec.size, 0.005, 0.005, 0.005);
-  spec.group = 0;
-  mjuu_setvec(spec.quat, 1, 0, 0, 0);
-  mjuu_setvec(spec.pos, 0, 0, 0);
-  spec_material_.clear();
-  spec.rgba[0] = spec.rgba[1] = spec.rgba[2] = 0.5f;
-  spec.rgba[3] = 1.0f;
-  spec.fromto[0] = mjNAN;
-  spec_userdata_.clear();
-  spec.alt.axisangle[0] = spec.alt.xyaxes[0] = spec.alt.zaxis[0] =
-      spec.alt.euler[0] = mjNAN;
+  mjm_defaultSite(spec);
 
   // clear internal variables
   body = 0;
   matid = -1;
+  spec_material_.clear();
+  spec_userdata_.clear();
 
   // reset to default if given
   if (_def) {