diff --git a/bioptim/examples/muscle_driven_ocp/muscle_activations_tracker.py b/bioptim/examples/muscle_driven_ocp/muscle_activations_tracker.py
index f3f031958..f4f8d8306 100644
--- a/bioptim/examples/muscle_driven_ocp/muscle_activations_tracker.py
+++ b/bioptim/examples/muscle_driven_ocp/muscle_activations_tracker.py
@@ -153,6 +153,7 @@ def prepare_ocp(
     kin_data_to_track: str = "markers",
     use_residual_torque: bool = True,
     ode_solver: OdeSolver = OdeSolver.COLLOCATION(),
+    n_threads: int = 1,
 ) -> OptimalControlProgram:
     """
     Prepare the ocp to solve
@@ -177,6 +178,8 @@ def prepare_ocp(
         If residual torque are present or not in the dynamics
     ode_solver: OdeSolver
         The ode solver to use
+    n_threads: int
+        The number of threads
 
     Returns
     -------
@@ -240,6 +243,7 @@ def prepare_ocp(
         u_bounds,
         objective_functions,
         ode_solver=ode_solver,
+        n_threads=n_threads,
     )
 
 
diff --git a/bioptim/interfaces/interface_utils.py b/bioptim/interfaces/interface_utils.py
index 0739477c4..0f9b26351 100644
--- a/bioptim/interfaces/interface_utils.py
+++ b/bioptim/interfaces/interface_utils.py
@@ -251,7 +251,7 @@ def get_x_and_u_at_idx(_penalty, _idx):
         if penalty.multi_thread:
             if penalty.target is not None and len(penalty.target[0].shape) != 2:
                 raise NotImplementedError("multi_thread penalty with target shape != [n x m] is not implemented yet")
-            target = penalty.target if penalty.target is not None else []
+            target = penalty.target[0] if penalty.target is not None else []
 
             x = nlp.cx()
             u = nlp.cx()
diff --git a/tests/test_global_muscle_tracking.py b/tests/test_global_muscle_tracking.py
index 99852c0a5..e36f0b53a 100644
--- a/tests/test_global_muscle_tracking.py
+++ b/tests/test_global_muscle_tracking.py
@@ -12,7 +12,8 @@
 
 
 @pytest.mark.parametrize("ode_solver", [OdeSolver.RK4, OdeSolver.COLLOCATION, OdeSolver.IRK])
-def test_muscle_activations_and_states_tracking(ode_solver):
+@pytest.mark.parametrize("n_threads", [1, 2])
+def test_muscle_activations_and_states_tracking(ode_solver, n_threads):
     # Load muscle_activations_tracker
     from bioptim.examples.muscle_driven_ocp import muscle_activations_tracker as ocp_module
 
@@ -42,6 +43,7 @@ def test_muscle_activations_and_states_tracking(ode_solver):
         use_residual_torque=use_residual_torque,
         kin_data_to_track="q",
         ode_solver=ode_solver(),
+        n_threads=n_threads,
     )
     solver = Solver.IPOPT()
     # solver.set_maximum_iterations(10)