Following Pentecost (chapter 2), the Apostles begin to perform miracles like their teacher, Jesus. The crippled man at the Beautiful Gate is a great story and punctuates the new spiritual authority of the Apostles. It is also an excellent lesson in Christian ministry. When something great happens (a man born crippled is now healed and is walking and leaping and praising God), Peter and John are very quick to deflect any attention away from themselves and toward Jesus. As Christian leaders we recognize that we do what we do because the Holy Spirit is working through us -- it is not because of some great virtue or power of our own.
Once again a large crowd gathers to see this new miracle and Peter, once again, takes the opportunity to invite them to a new way of living. Peter, like on Pentecost, begins as an apologist, that is he patiently explains that the power to heal was not inherently in him but is the Holy Spirit working through him. Following the explanation he proceeds to preach and to invite the gathered crowd to turn from their empty ways of living and to turn to the life giving mercy and grace that only Jesus can provide.
The author of Acts is generally believed to be Luke. Luke was a first century medical doctor and a companion of Paul (he joins the entourage around Acts 16). One of the ways that Luke reveals his training, apart from writing in excellent Greek, is that he uses medical terms that would not have been common usage. Here in Chapter 3:7 Luke uses the medical terms for feet and ankles being strengthened. Sometimes training shows in peculiar ways.