<PAGE>


                                                                     Exhibit 4.3
                                                                     -----------

                         CONCENTRA OPERATING CORPORATION

                                   as Obligor

                                       AND

                       NATIONAL HEALTHCARE RESOURCES, INC.

                                  as Guarantor

                                       AND

                              THE BANK OF NEW YORK

                                   as Trustee

                         -------------------------------

                             SUPPLEMENTAL INDENTURE

                          Dated as of November 20, 2001

                                       to

                                    Indenture

                           Dated as of August 17, 1999

                         -------------------------------

                 13% Series A Senior Subordinated Notes due 2009

                 13% Series B Senior Subordinated Notes due 2009

<PAGE>

      SUPPLEMENTAL INDENTURE dated as of November 20, 2001, by and among
Concentra Operating Corporation, a Nevada corporation (the "Company"), National
Healthcare Resources, Inc., a Delaware corporation (the "New Subsidiary
Guarantor"), and The Bank of New York, as successor to United States Trust
Company of New York, as trustee (the "Trustee").

      WHEREAS, the Company and certain subsidiary guarantors have heretofore
executed and delivered to the Trustee an Indenture dated as of August 17, 1999,
as it may be amended or supplemented (the "Indenture"), providing for the
issuance of 13% Series A Senior Subordinated Notes due 2009 and 13% Series B
Senior Subordinated Notes due 2009 (the "Notes");

      WHEREAS, pursuant to that certain Agreement and Plan of Merger dated
November 2, 2001, by and among Concentra Inc., a Delaware corporation and owner
of all the capital stock of the Company (the "Parent Company"), the New
Subsidiary Guarantor and NHR Acquisition Company, Inc., a Delaware corporation
and wholly-owned, direct, subsidiary of the Company and wholly-owned, indirect,
subsidiary of the Parent Company ("Mergeco"), among other things, Mergeco merged
with and into the New Subsidiary Guarantor (the "Merger") and the New Subsidiary
Guarantor became a wholly-owned subsidiary of the Company;

      WHEREAS, the Company and the New Subsidiary Guarantor desire by this
Supplemental Indenture pursuant to and as contemplated by the provisions of the
Indenture relating to the addition of guarantors, including, without limitation,
Section 1018, to add the New Subsidiary Guarantor as a guarantor pursuant to the
terms of the Indenture;

      WHEREAS, the execution and delivery of this Supplemental Indenture has
been authorized by resolutions of the Boards of Directors of the Company and the
New Subsidiary Guarantor; and

      WHEREAS, all conditions and requirements necessary to make this
Supplemental Indenture a valid, legal, binding and enforceable instrument in
accordance with its terms have been performed and fulfilled by the parties
hereto and the execution and delivery thereof have been in all respects duly
authorized by the parties hereto.

      NOW, THEREFORE, in consideration of the above premises, each party hereby
agrees, for the benefit of the others and for the equal and ratable benefit of
the holders of the Notes, as follows:

                                   ARTICLE I

                     ASSUMPTION OF OBLIGATIONS AS GUARANTOR

      Section 1.1 Assumption. The New Subsidiary Guarantor hereby expressly and
      -----------------------
unconditionally assumes each and every covenant, agreement and undertaking of a
Guarantor in the Indenture as of the date of this Supplemental Indenture, and
also hereby expressly and unconditionally assumes each and every covenant,
agreement and undertaking of a Guarantor in each Note outstanding on the date of
this Supplemental Indenture.

<PAGE>

                                   ARTICLE II

                            MISCELLANEOUS PROVISIONS

      Section 2.1 Defined Terms. For all purposes of this Supplemental
      --------------------------
Indenture, except as otherwise defined or unless the context otherwise requires,
terms used in capitalized form in this Supplemental Indenture and defined in the
Indenture have the meanings specified in the Indenture.

      Section 2.2 Indenture. Except as amended hereby, the Indenture and the
      ----------------------
Notes are in all respects ratified and confirmed and all the terms shall remain
in full force and effect.

      Section 2.3 Governing Law. THIS SUPPLEMENTAL INDENTURE SHALL BE GOVERNED
      --------------------------
BY AND CONSTRUED IN ACCORDANCE WITH THE LAWS OF THE STATE OF NEW YORK WITHOUT
REGARD TO PRINCIPLES OF CONFLICT OF LAWS.

      Section 2.4 Successors. All agreements of the Company and the New
      -----------------------
Subsidiary Guarantor in this Supplemental Indenture and the Notes shall bind
their successors. All agreements of the Trustee in this Supplemental Indenture
shall bind its successors.

      Section 2.5 Duplicate Originals. All parties may sign any number of copies
      --------------------------------
of this Supplemental Indenture. Each signed copy shall be an original, but all
of them together shall represent the same agreement.

      Section 2.6 Severability. In case any one or more of the provisions in
      -------------------------
this Supplemental Indenture or in the Notes shall be held invalid, illegal or
unenforceable, in any respect for any reason, the validity, legality and
enforceability of any such provision in every other respect and of the remaining
provisions shall not in any way be affected or impaired thereby, it being
intended that all of the provisions hereof shall be enforceable to the full
extent permitted by law.

      Section 2.7 Trustee Disclaimer. The Trustee accepts the amendment of the
      -------------------------------
Indenture effected by this Supplemental Indenture and agrees to execute the
trust created by the Indenture as hereby amended, but on the terms and
conditions set forth in the Indenture, including the terms and provisions
defining and limiting the liabilities and responsibilities of the Trustee, which
terms and provisions shall in like manner define and limit its liabilities and
responsibilities in the performance of the trust created by the Indenture as
hereby amended, and without limiting the generality of the foregoing, the
Trustee shall not be responsible in any manner whatsoever for or with respect to
any of the recitals or statements contained herein, all of which recitals or
statements are made solely by the Company and the New Subsidiary Guarantor, or
for or with respect to (i) the validity or sufficiency of this Supplemental
Indenture or any of the terms or provisions hereof, (ii) the proper
authorization hereof by the Company and the New Subsidiary Guarantor by
corporate action or otherwise, (iii) the due execution hereof by the Company and
the New Subsidiary Guarantor or (iv) the consequences (direct or indirect and
whether deliberate or inadvertent) of any amendment herein provided for, and the
Trustee makes no representation with respect to any such matters.

<PAGE>

      Section 2.8 Effectiveness. This Supplemental Indenture shall become
      --------------------------
effective, once executed, upon receipt by the Trustee of a certificate of the
appropriate officers of the Company and an Opinion of Counsel (as defined in the
Indenture), each of which shall be dated no earlier than the date hereof.

            [THE REMAINDER OF THIS PAGE IS INTENTIONALLY LEFT BLANK]

<PAGE>

      IN WITNESS WHEREOF, the parties hereto have caused this Supplemental
Indenture to be duly executed as of the day and year written above.

Attest:                                CONCENTRA OPERATING CORPORATION,
                                       as Obligor

      /s/ Richard A. Parr II           By:          /s/ Daniel T. O'Brien
---------------------------------         --------------------------------------
Richard A. Parr II, Secretary          Name:   Daniel T. O'Brien
                                       Title:  Vice President


Attest:
                                       NATIONAL HEALTHCARE RESOURCES, INC.,
                                       as Guarantor

      /s/ Richard A. Parr II           By:          /s/ Daniel T. O'Brien
---------------------------------         --------------------------------------
Richard A. Parr II, Secretary          Name:   Daniel T. O'Brien
                                       Title:  Vice President


                                       THE BANK OF NEW YORK,
                                       as Trustee

                                       By:         /s/ Margaret Ciesmelewski
                                          --------------------------------------
                                       Name:   Margaret Ciesmelewski
                                       Title:  Authorized Signer