¶1Following a jury trial in Superior Court, the . defendant appeals from his convictions on seven counts of *304statutory rape of a child, two counts of disseminating matter harmful to a minor, and one count of indecent assault and battery. He urges two grounds for overturning the convictions. First, pointing to his pretrial police statement, in which he provided an explanation as to why the victim had accused him, he contends that his remarks constituted comment on the credibility of a witness, and that it was reversible error to permit the introduction at trial of that unredacted commentary during the police officerâs testimony. He argues that this error was compounded when he was cross-examined by the prosecutor. Second, he claims error in an expert opinionâs reference on cross-examination to a scientific study. We conclude that the contentions are not meritorious, and we affirm the convictions.
¶21. Background. The victim testified that over a fourteen-month period, beginning when she was seven years old, the defendant initiated about ten offensive sexual encounters with her. During that time, the victim lived with her mother, her motherâs boyfriend, her brother and sister, her aunt, and her auntâs daughter. The defendant, who had an intermittent relationship with the victimâs aunt, also lived in the same house as the victim.
¶3The victimâs mother and her boyfriend ordered the defendant to leave the residence when the victim informed them that she was being abused. The victim later repeated her allegations to a police officer, who testified at trial as a fresh complaint witness. The victim stated that on about ten occasions, the defendant had taken her into his room at night, locked the door, undressed her, and penetrated her, either with his finger or his penis. She also alleged that the defendant had shown her pornographic magazines and a videotape. A search of the defendantâs room by police officers produced pornographic magazines and videotapes, as well as a vibrator. Two days after the search, the defendant provided a statement to the police.
¶4At trial, the victim repeated the substance of her earlier allegations. In addition, she stated that she had been abused in both her room and the defendantâs, that he had also used a vibrator that he kept in his room, and that oral and anal intercourse had taken place. On cross-examination, she testified that she wanted to make her parents and the police proud of *305her, that she had practiced her testimony with the prosecution, and that she sometimes forgot things, which the prosecutor helped her remember.
¶5The balance of the prosecutionâs case consisted of the physical evidence collected from the defendantâs room, and the testimony of several witnesses. Besides the police officer who testified as a fresh complaint witness, another police officer, William Lawrence, testified regarding the statement the defendant had given him. A forensic pediatrician, Dr. Christine Barron, testified about the physical examination she performed on the victim. She described the injury she found, and opined that it was consistent with a penetrating injury to the vagina, and that it was exceedingly rare for such an injury to be accidental. The victimâs mother also testified. She stated that the victim began having nightmares in December, 2001 (which would have been shortly after the defendant allegedly had begun abusing her). She also described an incident where she found the defendant in the victimâs bedroom, and heard the defendant telling her daughter that â[y]ou are going to do it now or go downstairs or go in my room.â She testified that, later that night, she opened her bedroom door to find the defendant lying on the floor in front of the door, and the victim standing in her doorway crying.
¶6The defense was based primarily on the defendantâs testimony. He denied any form of sexual contact with the victim. He admitted to owning some of the videotapes in his room, but stated that the magazines belonged to the motherâs boyfriend and that he had seen the other videotapes in the attic. The defendant testified that the vibrator belonged to the victimâs aunt. He also explained that he was simply saying goodbye to the victim when the mother found him in the victimâs room, because he had decided to leave that night. He did not remember being found lying on the floor later that night. The defense also sought to show through cross-examination of the Commonwealthâs witnesses that the victimâs testimony might have been suggested by others, and that the victimâs mother, who had been abused herself, was concerned that her daughter might have been abused.
¶72. The defendantâs statement. The defendant urges reversible erior flowing from the admission of the unredacted statement *306he voluntarily made after surrendering himself into police custody.
¶8The contested question and response of the defendant in the statement, admitted during the testimony of Detective Lawrence, were:
Q: âWhy would [the victim] accuse you of sexually abusing her?â
A: âI came up with two conclusions. Maybe sheâs mad at me. Sheâs kind of evil and she lies a lot. The other is maybe somebody else did it.â
¶9During his defense, on direct examination, the defendant stated that he had âcooperatedâ with the police, that they had asked him questions, and that he had answered them. On cross-examination, the prosecutor asked the defendant to explain what he meant when he told Detective Lawrence that the complainant was âkind of evil.â The defendant testified that, as he had told the police officer, he meant that the complainant âlied a lot.â
¶10It is well-established, and we have repeatedly stated, that it is *307improper to ask a witness at trial to assess the credibility of other witnessesâ testimony. See, e.g., Commonwealth v. Triplett, 398 Mass. 561, 567 (1986) (Triplett); Commonwealth v. Long, 17 Mass. App. Ct. 707, 708 (1984) (Long). The purpose of the bar on credibility testimony is that â[t]he factfinder, not the witness, must determine the weight and credibility of testimony.â Triplett, 398 Mass. at 567. Questions, and the testimony they elicit, run afoul of this prohibition when they suggest to the jury that âdifferences in the testimony of the witness and any other witness âcould only be the result of lying and not because of misrecollection, failure of recollection or other innocent reason.â â Commonwealth v. Ward, 15 Mass. App. Ct. 400, 402 (1983), quoting from United States v. Narciso, 446 F. Supp. 252, 321 (E.D. Mich. 1977). Commonwealth v. Morris, 20 Mass. App. Ct. 114, 119-120 (1985). Such inquiry is irrelevant, and âtransforms the interrogation stage of the trial into the phase traditionally reserved for argument and summation.â Long, 17 Mass. App. Ct. at 709-710 & n.6. Moreover, such inquiry inappropriately diverts the jury from the matter at hand â the consideration of the evidence â by pitting witness against witness for the purpose of casting the proponent in a derogatory or unsympathetic light. See, e.g., Triplett, 398 Mass. at 567 (â[t]he relationship between the two witnesses, that of mother and son, magnifies the prejudice caused by the improper questioningâ). See also Commonwealth v. DeMars, 42 Mass. App. Ct. 788, 794 (1997) (conduct âsought to cause the jury to loathe the defendantâ).
¶11In this case, the Triplett-Long line of cases is not implicated. Here, the police query and the resulting comment on the victimâs credibility were made before trial during police interrogation, and only ultimately came before the jury during the interrogating officerâs testimony. When the defendant gave his statement to the police, he was not testifying as a witness at trial, and he was not commenting on the testimony of another witness. This was not the equivalent of commenting on the *308credibility of live testimony given by another witness under oath during the trial. The defendantâs explanation to the police, as to why the victim had accused him, was properly considered by the jury as an admission, another bit of information to be used in its assessment of the charges and its ultimate determination of guilt or innocence. As such, it was properly the subject of cross-examination. Commonwealth v. Bibby, 35 Mass. App. Ct. 938, 940-941 (1993).
¶12In any event, even if there were error, it was harmless. The record here is distinguishable from the harmful error in Long, where the improper cross-examination spanned over one hundred pages of transcript, see Long, 17 Mass. App. Ct. at 708, and closer in tenor to cases where the isolated nature of the questioning and testimony was considered to be harmless error. Compare Ward, 15 Mass. App. Ct. at 402; Commonwealth v. Flanagan, 20 Mass. App. Ct. 472, 478 (1985). See also Commonwealth v. Kirkpatrick, 26 Mass. App. Ct. 595, 603 (1988) (no miscarriage of justice in two improper questions to defendant). Nor was the testimony mentioned in closing argument. Commonwealth v. Krepon, 32 Mass. App. Ct. 945, 948 (1992).
¶13Moreover, the substantial and corroborative evidence in the instant case distinguishes it from Triplett, where reversible error was found because the critical issue before the jury was the credibility of the defendant and his mother, and where he was asked repeatedly to call his mother a liar. See Triplett, 398 Mass. at 566-567. The weight of the rest of the Commonwealthâs evidence supports a conclusion of harmless error. Along with the consistent testimony of the victim, the Commonwealth presented physical and testimonial evidence to corroborate the victimâs story: the defendant admitted that the victim had access to his room and that she once slept in his room; the victimâs mother observed two strange interactions between the defendant and the victim; the victim suffered from nightmares during the period in which she claimed the abuse occurred; police uncovered various pornographic magazines, videotapes, and a vibrator described by the victim in the defendantâs room; the vibrator was found where the victim stated that it was kept; and the victim had physical injuries consistent with sexual abuse. *309Taken together, the strength of the evidence supports a conclusion of harmless error. See Commonwealth v. Mahdi, 388 Mass. 679, 696-697 (1983).
¶143. Dr. Barronâs testimony. The defendant takes issue with the cross-examination of the prosecutionâs expert witness, Dr. Barron.
Defense Counsel: âBut this, in your opinion, was either an exceedingly rare accident or an intentional act?â
Dr. Barron: âWhat I mean by exceedingly rare, in fact not just myself, but thereâs even a study in 1996 Pediatrics that identifies trauma to the hymen should be consistent and be considered as sexual because otherwise proven ââ
¶15*310Defendant contends that it was prejudicial error to refuse to strike the testimony.
¶16As a prehminary matter, we note that the defendant has raised two of these three objections for the first time on appeal. At trial, the defendantâs objection was grounded only upon unresponsiveness. We examine that contention for the presence of prejudicial error. Commonwealth v. Peixoto, 430 Mass. 654, 660 & n.6 (2000). Because the other two arguments were not preserved at trial, we consider them only âto avoid a substantial risk of a miscarriage of justice.â See, e.g., Commonwealth v. Clark, 378 Mass. 392, 397 (1979).
¶17To set the cross-examination in context, we note that on direct examination, Dr. Barron was qualified as an expert,
¶18The cold transcript, devoid of the possible inflection in the voice of the cross-examining defense counsel, does not readily resolve whether Dr. Barronâs answer to defense counselâs question â âBut this, in your opinion, was either an exceedingly rare accident or an intentional act?â â was unresponsive to the question put to her. This question was part of a series of queries that recounted the opinion Dr. Barron had just proffered on direct examination, and more than one of those queries included the words âin your opinion.â Merely by stressing the word âyour,â *311defense counselâs question might reasonably have been understood as: âBut this, in your opinion, was either an exceedingly rare accident or an intentional act?â It is conceivable that taken together, these questions gave Dr. Barron the impression that defense counsel was questioning the basis of her opinion by making it seem as if this opinion had no foundation or was unique to her. Viewed in this light, her answer concerning the Pediatrics study was simply a response to his challenge to the basis of her opinion. We further note that the judge, who did have the benefit of hearing defense counsel, overruled the objection.
¶19In any event, we conclude that even if Dr. Barronâs answer was unresponsive, the mention of the Pediatrics study was harmless error. Even if the jury completely accepted Dr. Barronâs fleeting description of the contents of the Pediatrics study, it did not induce the jury to conclude that it was the defendant who had abused the victim. More importantly, Dr. Barron properly stated numerous times that the injuries she observed could only have been the result of either an accident or sexual abuse. She emphasized that the chance that the injuries were caused by an accident was âexceedingly rare.â Thus, the Pediatrics study did little more than reinforce a proper medical opinion, an opinion that went uncontested by the defense. Moreover, the statement regarding the study was made only once, and was not repeated or referred to during closing argument. These facts, particularly in the context of the substantial inculpatory evidence noted previously, suggest that any error âdid not influence the jury, or had but very slight effect.â
¶20Turning to the other objections to Dr. Barronâs statement, newly raised on appeal, we find no substantial risk of a miscarriage of justice. With respect to the claim that the testimony impermissibly touched on an ultimate issue of fact, we take *312note of several guiding principles. First, â[e]xpert testimony âis admissible whenever it will aid the jury in reaching a decision, even if the expertâs opinion touches on the ultimate issues that the jury must decide.â â Commonwealth v. Federico, 425 Mass. at 847, quoting from Commonwealth v. Dockham, 405 Mass. 618, 628 (1989). Second, although â[ejvidence of physical injury of abuse is not always available in child sexual abuse cases, . . . [it] is admissible when available.â Federico, 425 Mass. at 851 n.12. Third, that while an expert may not opine as to whether a particular child has been raped or sexually abused, an expert may opine, after a physical examination of the victim, that a childâs vaginal injuries are âconsistent withâ penetration. See Commonwealth v. Montmeny, 360 Mass. 526, 530 (1971) (permitting testimony that vaginal injuries were consistent with penetration, as distinguished from assertion that rape occurred); Commonwealth v. Lewandowski, 22 Mass. App. Ct. 148, 150-151 (1986). See generally Commonwealth v. Federico, 425 Mass. at 851 n.12 (discussing Montmeny, supra); Commonwealth v. Colin C., 419 Mass. 54, 60 n.7 (1994) (discussing Montmeny, supra).Considered in light of these principles, and given the strength of the evidence, the expertâs answer here did not produce a substantial risk of a miscarriage of justice.
¶21Judgments affirmed.
¶22 The voluntariness of the statement was not contested by the defendant. On the morning of trial, counsel filed in court a motion in limine seeking redaction of portions of the defendantâs statement that commented on âthe credibility of a witness.â At side bar, counsel stated that he was concerned about questions referring to the defendantâs âimpression of the complaining witnessâs or other peopleâs reactions and why they might have said what they said, in terms of their credibility.â The Commonwealth informed the court that it understood counselâs concern, and opined that the parties might âwell be able to come to an agreement.â The judge thereupon directed that they âwork that out [themjselves.â When the statement was about to be offered during the police officerâs testimony, and after the statement was introduced, defense counsel renewed his objection. The judge, initially interpreting the objection as pertaining to the voluntariness of the statement, ruled that the statement was admissible, but did not specifically address the merits of the redaction request. Ultimately, after the unredacted statement was introduced, the judge determined that the comment relating to credibility was âan admission, and I will let the jury assess it, and whatever significance they attach to it.â
¶23 When asked by the prosecution to describe the lies, the defendant said that on ânumerousâ occasions, not exceeding â15 or 20,â he had heard the *307complainant falsely tell her parents that she had not snatched toys from other children. The defendant claimed that he had observed her do so. When asked if she lied about anything else, the defendant replied, âNot that I know of, no.â
¶24 The cross-examination proceeded as follows:
¶25Defense Counsel: âDoctor, you testified this was a penetration injury?â
¶26Dr. Barron: âThatâs correct.â
¶27Defense Counsel: âDone by some act?â
¶28Dr. Barron: âYes.â
¶29Defense Counsel: âLikely an intentional act?â
¶30Dr. Barron: âYes. Itâs exceedingly rare to have an accidental penetration injury.â
¶31Defense Counsel: âIt can happen.â
¶32Dr. Barron: âIt can happen.â
¶33Defense Counsel: âIt can happen by accident, but itâs exceedingly rare in your opinion.â
¶34Dr. Barron: âExceedingly rare, yes.â
¶35Defense Counsel: âBut this, in your opinion, was either an exceedingly rare accident or an intentional act?â
¶36Dr. Barron: âWhat I mean by exceedingly rare, in fact not just myself, but thereâs even a study in 1996 Pediatrics that identifies trauma to the hymen should be consistent and be considered as sexual because until otherwise proven ââ
¶37 Dr. Barron testified on direct examination that her credentials include certification as a pediatrician by the American Academy of Pediatrics. She also completed a fellowship in forensic pediatrics, that is the diagnosis and treatment of child abuse (sexual and physical) and neglect, at Brown University in Rhode Island. In addition, Dr. Barron is the division director for the child protection program at the University of Massachusetts Memorial Medical Center.
¶38 We also note that the judge gave a standard expert witness jury instruction, telling the jury that an expert witnessâs opinion was âas good as you find it to beâ and that in the final analysis, applying credibility rules used to assess the credibility of a witness, âyou make your own determination as to how much weight you are going to assign to her testimony.â
¶39 Defense counselâs closing argument reinforces the conclusion that the expertâs testimony did not impermissibly intrude on the juryâs fact-finding function. He told the jury that Dr. Barron had said that the childâs injury could have been an âaccident,â â[i]t could also happen by another intentional act, one not done necessarily by Mr. Colon or anyone we have heard from. Could have been anyone, at any time. . . . She didnât find any other injuries.â