Player jump only on the Y axis and ignore other axis velocity

Hello, i have some problem with a script im writing, when i press the jump button, even if im going forward, backward etc.... the player will lose all velocity and jump only on the y axis, i would like him to keep other axis velocity so you can run then jump and the player will continue his initial motion while jumping. i use a rigidbody. here is my code: using UnityEngine; using System.Collections;

 public class PlayerMovement : MonoBehaviour
 {
     public float speed = 6.0f;
     public float jumpSpeed = 5.0f;
     public float SlopeAngle = 45;
 
     private Vector3 moveDirection = Vector3.zero;
     private Vector3 desiredDirection;
 
     private bool isGrounded = false;
     private bool isJumping = false;
     private bool Sliding = false;
 
     private Rigidbody rigidBody;
     private CapsuleCollider capsule;
 
 
     void Start()
     {
         rigidBody = GetComponent<Rigidbody>();
         capsule = GetComponent<CapsuleCollider>();
     }
 
     void FixedUpdate()
     {
         Debug.Log(isGrounded);
         Grounding();
         Movement();
 
     }
 
 
     void Movement()
     {
 
         if (isGrounded)
         {
             moveDirection = new Vector3(Input.GetAxis("Horizontal") * 0.7f, 0, Input.GetAxis("Vertical"));
             if (moveDirection.z <= 0f) { moveDirection.z = Input.GetAxis("Vertical") * 0.7f; }
             moveDirection *= speed;
             desiredDirection = transform.localRotation * moveDirection;
             rigidBody.AddForce(desiredDirection, ForceMode.Impulse);
         }
 
 
         if (Input.GetButton("Jump") && isGrounded == true)
         {
             rigidBody.AddForce(Vector3.up * jumpSpeed, ForceMode.VelocityChange);
             isGrounded = false;
             isJumping = true;
         }
 
     }
 
 
     void Grounding()
     {
         RaycastHit hit;
         if (Physics.Raycast(transform.position, Vector3.down, out hit, 0.1f ))
         {
             float angle = Vector3.Angle(Vector3.up, hit.normal);
             if(angle > SlopeAngle)
             {
                 isGrounded = true;
                 Sliding = true;
             }
             else
             {
                 isGrounded = true;
                 Sliding = false;
             }
 
         }
     }
 
 }