LENDER JOINDER AND ASSUMPTION AGREEMENT

Exhibit 10.13

LENDER JOINDER AND ASSUMPTION AGREEMENT

THIS LENDER JOINDER AND ASSUMPTION AGREEMENT (this “Joinder”) is made as of February 13, 2009 (the “Effective Date”) by Manufacturers and Traders Trust Company, (the “Additional Lender”).

Background

Reference is made to the Credit Agreement dated as of January 28, 2009 among Under Armour, Inc., a Maryland corporation (the “Borrower”), each of the Guarantors party thereto, the Lenders party thereto, PNC Bank, National Association, as administrative agent (the “Administrative Agent”), SunTrust Bank, as Syndication Agent, and Compass Bank, as Documentation Agent (as the same has been and may hereafter be modified, supplemented, amended or restated from time to time, the “Credit Agreement”). Capitalized terms defined in the Credit Agreement are used herein as defined therein.

Agreement

In consideration of the Lenders’ permitting the Additional Lender to become a Lender under the Credit Agreement to provide a Post-Closing Loan pursuant to Section 2.4 of the Credit Agreement, the Additional Lender agrees that effective as of the Effective Date hereof it shall become, and shall be deemed to be, a Lender under the Credit Agreement and each of the other Loan Documents and agrees that from the Effective Date hereof and so long as the Additional Lender remains a party to the Credit Agreement, such Additional Lender shall assume the obligations of a Lender under and perform, comply with and be bound by each of the provisions of the Credit Agreement which are stated to apply to a Lender and shall be entitled to the benefits, rights and remedies set forth therein and in each of the other Loan Documents. The Additional Lender hereby acknowledges that it has heretofore received a true and correct copy of the Credit Agreement (including any modifications thereof or supplements or waivers thereto) as in effect on the Effective Date hereof and the executed original of its Note dated the Effective Date hereof issued by the Borrower under the Credit Agreement in the face amount of $20,000,000.

The Commitments and Ratable Shares of the Additional Lender and each of the other Lenders are as set forth on Schedule 1.1(B) to the Credit Agreement. Schedule 1.1(B) to the Credit Agreement is being amended and restated effective as of the Effective Date hereof to read as set forth on Schedule 1.1(B) hereto. Schedule 1 hereto lists as of the date hereof the amount of Loans under each outstanding Borrowing Tranche. Notwithstanding the foregoing, on the date hereof, the Borrower shall repay all outstanding Loans to which either the Base Rate Option or the LIBOR Rate Option applies and simultaneously reborrow a like amount of Loans under each such Interest Rate Option from the Lenders (including the Additional Lender) according to the Ratable Shares set forth on attached Schedule 1.1(B) and shall be subject to breakage fees and other indemnities provided in Section 4.12 [Indemnity].

The Additional Lender is executing and delivering this Joinder as of the Effective Date and acknowledges that it shall: (A) share ratably in all Loans subject to the Base Rate Option borrowed by the Borrower on and after the Effective Date hereof; and (B) participate in all new Loans subject to the LIBOR Rate Option borrowed by the Borrower on and after the Effective Date hereof according to its Ratable Share.

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Signatures follow on next page.

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[SIGNATURE PAGE TO LENDER JOINDER AND ASSUMPTION AGREEMENT]

IN WITNESS WHEREOF, the Additional Lender has duly executed and delivered this Joinder as of the Effective Date hereof.

[ACKNOWLEDGEMENT PAGE TO LENDER JOINDER AND ASSUMPTION AGREEMENT]

SCHEDULE 1.1 (B)

COMMITMENTS OF LENDERS

Revised Schedule 1.1(B) to Credit Agreement attached.

SCHEDULE 1.1(B)

COMMITMENTS OF LENDERS AND ADDRESSES FOR NOTICES

Part 1 - Commitments of Lenders and Addresses for Notices to Lenders