You could monitor page network activity with browser development console (F12 - Network in Chrome) to see what request does the page do when you scroll down, use that data and reproduce the request with requests. As an alternative, you can use selenium to control a browser programmatically to scroll down until page is ended, then save its HTML.
I guess I found the right request
Request URL:http://store.nike.com/html-services/gridwallData?country=US&lang_locale=en_US&gridwallPath=mens-shoes/7puZoi3&pn=3
Request Method:GET
Status Code:200 OK
Remote Address:87.245.221.98:80
Request Headers
Provisional headers are shown
Accept:application/json, text/javascript, */*; q=0.01
Referer:http://store.nike.com/us/en_us/pw/mens-shoes/7puZoi3?ipp=120
User-Agent:Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/56.0.2924.87 Safari/537.36
X-NewRelic-ID:VQYGVF5SCBAJVlFaAQIH
X-Requested-With:XMLHttpRequest
Seems that query parameter pn means the current "subpage". But you still need to understand the response correctly.