Add force to my back ??,,

how can i add addforce to my Back??. i have a script, but addforce not go to my back, it always go to z axis, when i rotate my character, it always add force to z axis. I need to addforce to my back, when rorate my character anywhere, always give addforce to myback not just on z axis, this my Script :

public class AddForceMyBack : MonoBehaviour
{
	Rigidbody myRb;
	public float forceSpeed;

	void Start () 
	{
		myRb = GetComponent<Rigidbody> ();
	}
	
	void Update () 
	{
		myRb.AddForce (Vector3.back * forceSpeed, ForceMode.Impulse);
        // i want addforce to myback, not just on z axis
	}
}

Replace line 13 with this

myRb.AddForce(-transform.forward * forceSpeed, ForceMode.Impulse);

transform.forward indicates the forward direction of the gameobjet’s transform. Negating it points to backward direction.