In a laboratory study, 17 participants tested (whereby 2 datasets had to be excluded later on) different backrest steering-motion mechanics in a stationary steerable wheelchair prototype by controlling movement in a virtual environment through the backrest. 
Results suggest that good control of the steering interface does not directly depend on a particular minimum level of trunk control but, rather, that good motion control through sideways leaning is possible for wheelchair users across a wide functional spectrum - if the backrest mechanism is individually calibrated.
These results show greater than expected potential of a backrest-controlled steering system on manual wheelchairs as it is clearly not limited to people with intact trunk function. A future backrest system, however, needs to be modular and easily customisable to meet the vastly different needs of users.

Togni R, Disch K, Plüss M, Lerch I, Zemp R, Taylor WR. Leaning for Turning: Viability of Utilising Dynamic Backrests to Control Steering in Manual Wheelchairs. (currently under review)