〈膝／ひざ／ヒザ〉〈打／討／撃／射／う〉ち

Google N-gram Corpus Counts
╭─ーーーー─┬─────┬───────╮
│ 膝撃ち　 │ 329 │ 48.8% │ - add
│ 膝うち　 │ 159 │ 23.6% │
│ 膝射ち　 │  72 │ 10.7% │
│ ひざ撃ち │  47 │  7.0% │
│ ひざうち │  67 │  9.9% │
├─ーーーー─┼─────┼───────┤
│ 膝打ち　 │ 728 │  N/A  │
│ ひざ打ち │ 133 │  N/A  │
│ ヒザ打ち │ 104 │  N/A  │
╰─ーーーー─┴─────┴───────╯

I assume 打 has a different meaning, like slapping one's knee. That's what google images seems to show.

撃 is surely okay (compare 立ち撃ち), but not in the refs.

@@ -5,0 +6,3 @@
+</k_ele>
+<k_ele>
+<keb>膝撃ち</keb>

GG5, Koj, Daijirin, etc. 「しっしゃ（膝射）」に同じ。
膝射ち	72
膝射	693