Various examples pertaining to efficient trigger-based (TB) multi-user (MU) uplink (UL) transmissions in wireless local area networks (WLANs) are described. A stations (STA) receives a trigger frame from an access point (AP), with the trigger frame assigning one or more resource units (RUs) corresponding to one or more subchannels of a plurality of subchannels. The STA then performs a transmission using the assigned one or more RUs responsive to receiving the trigger frame. In an event that at least one subchannel of the plurality of subchannels is detected as being busy, the one or more subchannels may include a subset (e.g., some but not all) of the plurality of subchannels.