20 CFR § 725.457 - Witnesses.
(a) Witnesses at the hearing shall testify under oath or affirmation. The administrative law judge and the parties may question witnesses with respect to any matters relevant and material to any contested issue. Any party who intends to present the testimony of an expert witness at a hearing, including any physician, regardless of whether the physician has previously prepared a medical report, shall so notify all other parties to the claim at least 10 days before the hearing. The failure to give notice of the appearance of an expert witness in accordance with this paragraph, unless notice is waived by all parties, shall preclude the presentation of testimony by such expert witness.
(b) No person shall be required to appear as a witness in any proceeding before an administrative law judge at a place more than 100 miles from his or her place of residence, unless the lawful mileage and witness fee for 1 day's attendance is paid in advance of the hearing date.
(1) In the case of a witness offering testimony relevant to the liability of the responsible operator, in the absence of extraordinary circumstances, the witness must have been identified as a potential hearing witness while the claim was pending before the district director.
(2) In the case of a physician offering testimony relevant to the physical condition of the miner, such physician must have prepared a medical report. Alternatively, in the absence of a showing of good cause under § 725.456(b)(1) of this part, a physician may offer testimony relevant to the physical condition of the miner only to the extent that the party offering the physician's testimony has submitted fewer medical reports than permitted by § 725.414. Such physician's opinion shall be considered a medical report subject to the limitations of § 725.414.
(d) A physician whose testimony is permitted under this section may testify as to any other medical evidence of record, but shall not be permitted to testify as to any medical evidence relevant to the miner's condition that is not admissible.