Document:

<PAGE>   1

                                                                 EXHIBIT 10.11

                          FIRST AMENDMENT AND CONSENT

         FIRST AMENDMENT AND CONSENT (this "Amendment"), dated as of September
1, 2000, among UNIVERSAL COMPRESSION HOLDINGS, INC. ("Holdings"), UNIVERSAL
COMPRESSION, INC. (the "Borrower"), various lenders party to the Credit
Agreement referred to below (the "Lenders") and BANKERS TRUST COMPANY, as
Administrative Agent (the "Administrative Agent"). All capitalized terms used
herein and not otherwise defined herein shall have the respective meanings
provided such terms in the Credit Agreement referred to below.

                                   WITNESSETH:

         WHEREAS, Holdings, the Borrower, Deutsche Bank Securities, Inc., as
Lead Arranger, the Lenders and the Administrative Agent are parties to a Credit
Agreement, dated as of May 30, 2000 (as amended, modified or supplemented to,
but not including, the date hereof, the "Credit Agreement"); and

         WHEREAS, Holdings and the Borrower have requested that the Lenders
provide the consents and amendment provided for herein and the undersigned
Lenders have agreed to provide such consents and amendment on the terms and
conditions set forth herein;

         NOW, THEREFORE, it is agreed:

         1. Notwithstanding anything to the contrary contained in sub-clause
(vi) of Section 9.02(viii) of the Credit Agreement, the Lenders hereby agree
that the Borrower may acquire (the "GCSI Acquisition") (i) all of the
outstanding capital stock of Gas Compression Services, Inc. ("GCSI") through the
merger of GCSI with and into the Borrower (with the Borrower being the surviving
corporation), (ii) all of the outstanding capital stock of Gas Compression
Finance Corporation ("GCFC"), and (iii) all of the outstanding membership
interests in each of the G.C.S. Distributing L.L.C. ("Distributing") and Gas
Compression Realty, L.L.C. ("Realty" and, together with GCFC and Distributing,
the "GCS Entities"), so long as the purchase price for the GCSI Acquisition
consists of approximately (I) $12,000,000 in cash, (II) $46,000,000 of newly
issued shares of common stock of Holdings, and (III) the refinancing and/or
assumption of $62,000,000 of existing Indebtedness and Operating Leases of GSCI
and the GCS Entities. The Lenders hereby further agree that the GCSI Acquisition
shall constitute a Permitted Section 9.02(viii) Acquisition so long as all of
the other terms and conditions of Section 9.02(viii) of the Credit Agreement, as
well as all of the terms and conditions of the other provisions of the Credit
Agreement (including, but not limited to, Sections 9.01, 9.04 and 9.15 thereof
(in the case of such Sections 9.01 and 9.04, after giving effect to the
designation of the GCSI Acquisition as a Permitted Section 9.02(viii)
Acquisition)), shall, except as provided below in this Section 1, be satisfied
in connection with the GCSI Acquisition. The Lenders hereby also agree that,
notwithstanding anything to the contrary contained in Sections 8.11 and 9.15(a)
of the Credit Agreement, the GCS Entities do not have to comply with the
provisions of such Sections 8.11 and 9.15(a) for a period of no more than 30
days following the consummation of the GCSI

<PAGE>   2

Acquisition so long as (i) such GCS Entities do not otherwise engage in any new
significant business activities and do not purchase any new significant assets
and (ii) if, by the 30th day following the consummation of the GCSI Acquisition,
any such GCS Entity shall not have merged with and into the Borrower pursuant to
Section 9.02(vi) of the Credit Agreement, the Borrower will cause each such GCS
Entity at such time to take all of the actions otherwise required to be taken by
a new Wholly-Owned Domestic Subsidiary pursuant to such Sections 8.11 and
9.15(a).

         2. Holdings, the Borrower and the Lenders hereby agree that (i) the
GCSI Acquisition shall not utilize any portion of the $75,000,000 basket or the
$10,000,000 basket referred to in sub-clause (vi) of Section 9.02(viii) of the
Credit Agreement for the current fiscal year of Holdings, and additional
Permitted Section 9.02(viii) Acquisitions may be consummated in accordance with
the terms of the Credit Agreement for the remainder of the current fiscal year
of Holdings subject to the provisions set forth below in this Section 2, and
(ii) for the fiscal years 2001 and 2002 of Holdings, such $75,000,000 basket and
such $10,000,000 basket referred to in sub-clause (vi) of Section 9.02(viii) of
the Credit Agreement shall be available from (x) in the case of fiscal year
2001, the period commencing on the Amendment Effective Date (as defined below)
through and including June 30, 2001 and (y) in the case of fiscal year 2002, the
period commencing on July 1, 2001 through and including March 31, 2002.

         3. Section 11.01 of the Credit Agreement is hereby amended by inserting
the following new text at the end of the definition of "Consolidated EBITDAR"
appearing therein:

     "; it being understood and agreed, however, that for purposes of
     calculating the Leverage Ratio and the Senior Secured Leverage Ratio under
     this Agreement, Consolidated EBITDAR shall be calculated on a pro forma
     basis to take into account any Permitted Section 9.02(viii) Acquisition
     consummated after the first day of the relevant Test Period (but only to
     the extent that the Person or assets so acquired had not been sold during
     such Test Period) as if each such Permitted Section 9.02(viii) Acquisition
     had been consummated on the first day of such Test Period (and assuming
     that any Indebtedness (including Capitalized Lease Obligations) and/or
     Operating Lease Obligations incurred, issued or assumed in connection with
     each such Permitted Section 9.02(viii) Acquisition had been incurred,
     issued or assumed on the first day of such Test Period, and had remained
     outstanding through the date of each such Permitted Section 9.02(viii)
     Acquisition (and with the methodology to give effect to such pro forma
     adjustments being satisfactory to the Administrative Agent).

         4. In order to induce the Lenders to enter into this Amendment,
Holdings and the Borrower hereby represent and warrant that (i) no Default or
Event of Default exists on the Amendment Effective Date, both before and after
giving effect to this Amendment, and (y) on the Amendment Effective Date, and
both before and after giving effect to this Amendment, all representations and
warranties contained in the Credit Agreement and in the other Credit Documents
are true and correct in all material respects as though such representations and
warranties were made on the Amendment Effective Date (it being understood and
agreed that any representation or warranty which by its terms is made as of a
specified date shall be required to be true and correct in all material respects
only as of such date).

                                      -2-
<PAGE>   3

         5. This Amendment is limited as specified and shall not constitute a
modification, acceptance or waiver of any other provision (or of any provision
beyond the specific waivers granted herein with respect to such provision) of
the Credit Agreement or any other Credit Document.

         6. This Amendment may be executed in any number of counterparts and by
the different parties hereto on separate counterparts, each of which
counterparts when executed and delivered shall be an original, but all of which
shall together constitute one and the same instrument. A complete set of
counterparts shall be delivered to the Borrower and the Administrative Agent.

         7. THIS AMENDMENT AND THE RIGHTS AND OBLIGATIONS OF THE PARTIES
HEREUNDER SHALL BE CONSTRUED IN ACCORDANCE WITH AND GOVERNED BY THE LAW OF THE
STATE OF NEW YORK.

         8. This Amendment shall become effective on the date (the "Amendment
Effective Date") when Holdings, the Borrower and the Required Lenders shall have
signed a counterpart hereof (whether the same or different counterparts) and
shall have delivered (including by way of facsimile transmission) the same to
the Administrative Agent at the Notice Office.

         9. From and after the Amendment Effective Date, all references in the
Credit Agreement and each of the other Credit Documents to the Credit Agreement
shall be deemed to be references to the Credit Agreement after giving effect to
this Amendment.

                                      * * *

                                      -3-
<PAGE>   4
                  IN WITNESS WHEREOF, each of the parties hereto has caused a
counterpart of this Amendment to be duly executed and delivered as of the date
first above written.

                                      UNIVERSAL COMPRESSION HOLDINGS, INC.

                                      By: /s/ RICHARD W. FITZGERALD
                                          --------------------------------------
                                          Name: Richard W. FitzGerald
                                          Title: Senior Vice President and Chief
                                                 Financial Officer

                                      UNIVERSAL COMPRESSION, INC.

                                      By: /s/ RICHARD W. FITZGERALD
                                          --------------------------------------
                                          Name: Richard W. FitzGerald
                                          Title: Senior Vice President and Chief
                                                 Financial Officer

                                      BANKERS TRUST COMPANY,
                                        Individually and as Administrative Agent

                                      By: /s/ MARCUS M. TARKINGTON
                                          --------------------------------------
                                          Name: Marcus M. Tarkington
                                          Title: Director

                                      FIRST UNION NATIONAL BANK

                                      By: /s/ ROBERT R. WETTEROFF
                                          --------------------------------------
                                          Name: Robert R.Wetteroff
                                          Title: Senior Vice President

                                      -4-
<PAGE>   5

                                      BANK ONE, N.A.

                                      By: /s/ J. CHARLES FREEL, JR.
                                          --------------------------------------
                                          Name: J. Charles Freel, Jr.
                                          Title: First Vice President

                                      THE BANK OF NOVA SCOTIA

                                      By: /s/ FCH ASHBY
                                          --------------------------------------
                                          Name: FCH Ashby
                                          Title: Senior Manager Loan Operations

                                      NATIONAL CITY BANK OF KENTUCKY

                                      By: /s/ SCOTT L. BREWER
                                          --------------------------------------
                                          Name: Scott L. Brewer
                                          Title: Assistant Vice President

                                      WELLS FARGO BANK (TEXAS), N.A.

                                      By: /s/ SPENCER SMITH
                                          --------------------------------------
                                          Name: Spencer Smith
                                          Title: Vice President

                                      WILMINGTON TRUST COMPANY

                                      By:
                                          --------------------------------------
                                          Name:
                                          Title:

                                      -5-<PAGE>   1
                                                                  EXHIBIT 10.1

                                VOTING AGREEMENT

               This Voting Agreement dated as of September 15, 2000 (this
"Agreement"), is made by and among Integrated Orthopaedics, Inc., a Texas
corporation ("IOI"), and each of the undersigned holders of shares of capital
stock (each, a "Stockholder" and collectively, the "Stockholders") of
PowerBrief, Inc., a Delaware corporation (the "Company").

                             PRELIMINARY STATEMENTS

               Concurrently with the execution of this Agreement, the Company
and IOI have entered into an Agreement and Plan of Merger (as the same may be
amended from time to time, the "Merger Agreement"), providing for the merger of
the Company with and into IOI, with IOI being the surviving corporation (the
"Merger"), which Merger is subject to the approval of the holders of shares of
capital stock of the Company as provided in the Merger Agreement, the Delaware
General Corporation Law, as amended, and the Company's Certificate of
Incorporation, as amended.

               The Stockholders own the shares of the Company common stock, par
value $0.01 per share (the "Common Stock"), set forth opposite their respective
names on Exhibit A hereto. As used herein, the term "Shares" includes all shares
of such Common Stock as to which each Stockholder (at any time prior to the
termination of this Agreement) is the beneficial or record owner or is otherwise
able to direct the voting thereof and all securities issued or exchanged with
respect to any such Shares upon any reclassification, recapitalization,
reorganization, merger, consolidation, spin-off, stock split, combination, stock
or other dividend or any other change in the Company's capital structure.

               To induce IOI to enter into the Merger Agreement, the Company has
agreed, upon the terms and subject to the conditions set forth herein, to cause
holders of not less than a majority of the voting power of the outstanding
shares of Common Stock and Series A Preferred Stock, par value $0.01 per share
(the "Series A Preferred Stock"), of the Company, which will vote together on
the Merger as a single class, to execute this Agreement.

               NOW, THEREFORE, for good and valuable consideration, the receipt,
sufficiency and adequacy of which are hereby acknowledged, the parties to this
Agreement agree as follows:

     1. Stockholders' Representations and Warranties. Each Stockholder, as to
itself only, represents and warrants to IOI that (i) such Stockholder is the
beneficial and record owner of the Shares set forth on Exhibit A hereto, free
and clear of any mortgage, pledge, lien, security interest, claim, restriction
on voting or otherwise or other encumbrance, (ii) such Stockholder has the sole
right to vote such Shares free of any mortgage, pledge, lien, security interest,
claim, restriction on voting or otherwise or other

<PAGE>   2

encumbrance, (iii) such Stockholder has the full and unrestricted legal power,
authority and right to enter into, execute and deliver this Agreement without
the consent or approval of any other person, (iv) this Agreement is the valid
and binding agreement of such Stockholder and (v) no investment banker, broker
or finder is entitled to a commission or fee from such Stockholder or the
Company in respect of this Agreement based upon any arrangement or agreement
made by or on behalf of the Stockholder.

     2. IOI's Representations and Warranties. IOI hereby represents and warrants
to the Stockholders as of the date hereof that IOI has all requisite corporate
power and authority to enter into this Agreement and to consummate the
transactions contemplated hereby. The execution and delivery of this Agreement
by IOI, and the consummation of the transactions contemplated hereby, have been
duly authorized by all necessary corporate action on the part of IOI. This
Agreement has been duly executed and delivered by IOI and constitutes a valid
and binding obligation of IOI enforceable in accordance with its terms. The
execution and delivery of this Agreement do not, and the consummation of the
transactions contemplated hereby and compliance with the terms hereof will not,
conflict with, or result in any violation of, or default (with or without notice
or lapse of time or both) under any provision of, the articles of incorporation
or bylaws of IOI, any trust agreement, loan or credit agreement, note, bond,
mortgage, indenture, lease or other agreement, instrument, permit, concession,
franchise, license, judgment, order, notice, decree, statute, law, ordinance,
rule or regulation applicable to IOI or to IOI's property or assets.

     3. No Voting Trusts. Each Stockholder hereby revokes any and all proxies
and voting instructions with respect to the Shares previously given by such
Stockholder and such Stockholder agrees that it will not grant or give any other
proxies or voting instructions with respect to the voting of the Shares, enter
into any voting trust or other arrangement or agreement with respect to the
voting of the Shares (and if given or executed, such proxies, voting
instructions, voting trust or other arrangement or agreement shall not be
effective), or agree, in any manner, to vote the Shares for or against any
proposal submitted to the stockholders of the Company except in furtherance of
the proposals set forth in paragraph 4 hereof.

     4. Agreements with Respect to the Shares.

          (a) Each Stockholder agrees during the term of this Agreement:

               (i) to vote the Shares, to the extent entitled to vote, (x) in
favor of the approval of the Merger Agreement and the Merger at every meeting of
the stockholders of the Company at which such matters are considered and at
every adjournment thereof or in any other circumstances upon which a vote,
consent or other approval (including by written consent) with respect to the
Merger and the Merger Agreement is sought, and (y) against all other proposals
submitted to the stockholders of the Company which, directly or indirectly,
would reasonably be expected to prevent or

                                       2
<PAGE>   3

materially delay the consummation of the Merger or the transactions contemplated
by the Merger Agreement, in such manner as IOI may direct; and

               (ii) not to solicit, encourage or recommend to other stockholders
of the Company that (w) they vote their shares of Common Stock or Series
Preferred Stock, par value $.01 per share ("Series A Preferred Stock"), of the
Company, or any such other securities in any manner contrary to the terms of the
Merger Agreement and the transactions contemplated thereby, (x) they not vote
their shares of Common Stock or Series A Preferred Stock at all, (y) they
tender, exchange or otherwise dispose of their shares of Common Stock or Series
A Preferred Stock pursuant to a Competing Transaction (as hereinafter defined),
or (z) they attempt to exercise any statutory appraisal or other similar rights
they may have.

         (b) Unless otherwise instructed in writing by IOI, during the term of
this Agreement, each Stockholder will vote the Shares against any Competing
Transaction.

         (c) Except with the prior written consent of IOI, during the term of
this Agreement, each Stockholder agrees that such Stockholder will not, and
shall use its commercially reasonable efforts not to permit any employee,
attorney, accountant, investment banker or other agent or representative of such
Stockholder to initiate, solicit, negotiate, encourage, or provide confidential
information in order to facilitate any Competing Transaction.

         (d) For purposes of this Agreement, a "Competing Transaction" shall
mean a transaction of any kind (including, without limitation, a merger,
consolidation, share exchange, reclassification, reorganization,
recapitalization, sale or encumbrance of substantially all the assets of the
Company outside the ordinary course of business, or sale or exchange by
stockholders of the Company of all or substantially all the shares of the
Company's capital stock) proposed by any person(s).

     5. Proxies. In furtherance of the foregoing, each Stockholder is granting
to Douglas P. Badertscher, the Chief Operating Officer of IOI, and/or Laurie
Hill Gutierrez, the Chief Financial Officer of IOI, or to his or her
designee(s), irrevocable proxies and powers of attorney (which may be in the
form annexed hereto or such other form consistent with the terms hereof and
thereof as IOI may specify) to vote the Shares, to the extent such Shares are
entitled to vote, and hereby specifically agrees not to revoke such proxies
granted under any circumstances:

         (a) at any and all meetings of stockholders of the Company, notice of
which meetings are given prior to the due and proper termination of this
Agreement, with respect to matters presented to the Company's stockholders for a
vote which relates to or affects (i) the Merger or the Merger Agreement or the
approval of either thereof; and (ii) any Competing Transaction; or

                                       3
<PAGE>   4

         (b) with respect to actions to be taken by written consent of the
stockholders of the Company which relates to or affects any of the foregoing,
and which consent is solicited prior to the due and proper termination of this
Agreement.

     6. Limitation on Sales. During the term of this Agreement, except pursuant
to the Merger, each Stockholder agrees not to sell, assign, transfer, loan,
tender, pledge, hypothecate, exchange, encumber or otherwise dispose of, or
issue an option or call with respect to, any of the Shares, or impair such
Stockholder's Shares.

     7. Specific Performance. Each Stockholder acknowledges that it will be
impossible to measure in money the damage to IOI if the Stockholder fails to
comply with the obligations imposed by this Agreement, and that, in the event of
any such failure, IOI will not have an adequate remedy at law or in damages.
Accordingly, each Stockholder agrees that injunctive relief or any other
equitable remedy, in addition to any remedies at law or damages, is the
appropriate remedy for any such failure and will not oppose the granting of any
such remedy on the basis that IOI has an adequate remedy at law. Each
Stockholder agrees not to seek, and agrees to waive any requirement for, the
securing or posting of a bond in connection with IOI seeking or obtaining such
equitable relief.

     8. Reasonable Efforts. Each Stockholder will use all reasonable efforts to
cause to be satisfied the conditions to the obligations of the Company in such
Stockholder's control to effect the Closing under the Merger Agreement.

     9. Publicity. Each Stockholder agrees that, from the date hereof through
the Closing Date, such Stockholder shall not issue any public release or
announcement concerning the transactions contemplated by this Agreement and the
Merger Agreement without the prior consent of IOI.

     10. Term of Agreement; Termination.

         (a) The term of this Agreement shall commence on the date hereof and
shall terminate upon the earliest to occur of (i) the Effective Time of the
Merger and (ii) the due and proper termination of the Merger Agreement in
accordance with its terms. Upon such termination, no party shall have any
further obligations or liabilities hereunder.

         (b) The obligations of the Stockholders set forth in this Agreement
shall not be effective or binding upon any Stockholder until after such time as
the Merger Agreement is executed and delivered by IOI and the Company.

     11. Miscellaneous.

         (a) Entire Agreement. This Agreement constitutes the entire agreement
among the parties with respect to the subject matter of this Agreement and

                                        4
<PAGE>   5

supersedes all prior written and oral and all contemporaneous oral agreements
and understandings with respect to the subject matter of this Agreement.

         (b) Notices. Any notice, request, instruction or other document to be
given hereunder by any party to the others shall be in writing and shall be
deemed to have been duly given on the next business day after the same is sent,
if delivered personally or sent by telecopy or overnight delivery, or five
calendar days after the same is sent, if sent by registered or certified mail,
return receipt requested, postage prepaid, as set forth below, or to such other
persons or addresses as may be designated in writing in accordance with the
terms hereof by the party to receive such notice.

         If to IOI:

         Integrated Orthopaedics, Inc.
         1800 W. Loop South
         Suite 1030
         Houston, Texas  77027
         Attention:  Scott J. Hancock
         Facsimile No: (713) 361-2000

         with a copy to:

         Weil, Gotshal & Manges LLP
         100 Crescent Court, Suite 1300
         Dallas, Texas 75201-6950
         Attention:  Michael A. Saslaw
         Facsimile No: (214) 746-7777

         If to a Stockholder, to the address set forth below such Stockholder's
name on Exhibit A hereto,

         with a copy to:

         Locke Liddell & Sapp LLP
         3400 Chase Tower, Travis
         Houston, Texas  77002-3095
         Attention:  David F. Taylor
         Facsimile No: (713) 223-3717

         (c) Governing Law. This Agreement shall be governed by and construed in
accordance with the laws of the State of Texas as applied to contracts made and
fully performed in such state without giving effect to the principles of
conflict of laws thereof.

                                        5
<PAGE>   6

         (d) Rules of Construction. The descriptive headings in this Agreement
are inserted for convenience of reference only and are not intended to be part
of or to affect the meaning or interpretation of this Agreement. Words used in
this Agreement, regardless of the gender and number specifically used, shall be
deemed and construed to include any other gender, masculine or feminine, or
neuter, and any other number, singular or plural, as the context requires. As
used in this Agreement, the word "including" is not limiting, and the word "or"
is not exclusive. Capitalized terms used herein but not otherwise defined herein
shall have the meanings given to them in the Merger Agreement.

         (e) Parties in Interest. This Agreement shall be binding upon and inure
solely to the benefit of the parties to this Agreement and their legal
successors-in-interest, and nothing in this Agreement, express or implied, is
intended to confer upon any other person any rights or remedies of any nature
whatsoever under or by reason of this Agreement.

         (f) Counterparts. This Agreement may be executed in one or more
counterparts, and each of such counterparts shall for all purposes be deemed to
be an original, but all such counterparts together shall constitute but one
instrument.

         (g) Assignment. No party hereto shall assign its rights and obligations
under this Agreement or any part thereof, nor shall any party assign or delegate
any of its rights or duties hereunder without the prior written consent of the
other party, and any assignment made without such consent shall be void. Except
as otherwise provided herein, this Agreement shall be binding upon and inure to
the benefit of the parties hereto and their respective successors and permitted
assigns.

         (h) Amendment. This Agreement may not be amended except by an
instrument in writing signed on behalf of all the parties.

         (i) Extension; Waiver. Any party to this Agreement may extend the time
for the performance of any of the obligations or other acts of any of the other
parties to this Agreement or waive compliance by any other party with any of the
agreements or conditions contained herein or any breach thereof. Any agreement
on the part of any party to any such extension or waiver shall be valid only if
set forth in an instrument in writing signed on behalf of such party.

         (j) Severability. The provisions of this Agreement are severable and,
if any thereof are invalid or unenforceable in any jurisdiction, the same and
the other provisions hereof shall not be rendered otherwise invalid or
unenforceable.

                                        6
<PAGE>   7

               IN WITNESS WHEREOF, the parties hereto, intending to be legally
bound hereby, have duly executed this Voting Agreement as of the date first
above written.

                                  INTEGRATED ORTHOPAEDICS, INC.

                                  By:
                                     ------------------------------------------
                                     Name:
                                     Title:

                                  ---------------------------------------------
                                  Robert W. Ohnesorge

                                  ---------------------------------------------
                                  K. Wade Bennett

<PAGE>   8

                                    EXHIBIT A

                HOLDINGS OF COMMON STOCK AS OF SEPTEMBER 15, 2000

<TABLE>
<CAPTION>
<S>                                            <C>
Name of Stockholder:                           Robert W. Ohnesorge

Address:                                       c/o PowerBrief, Inc.
                                               5858 Westheimer
                                               Houston, Texas 77057

Telephone No.:                                 (713) 526-4600

Facsimile No.:                                 (713) 586-4790

Number of Shares of Common Stock:              5,625,000

Name of Stockholder:                           K. Wade Bennett

Address:                                       c/o PowerBrief, Inc.
                                               5858 Westheimer
                                               Houston, Texas 77057
Telephone No.:                                 (713) 526-4600

Facsimile No.:                                 (713) 586-4790

Number of Shares of Common Stock:              5,625,000
</TABLE>

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