The FAA has a study on it you can find here the study is worth reading completely but the important parts of the conclusion seem to be,
Current Brace Position
This position (head against the seat back,
hands on top of the seat back) was evaluated for three common types of
This position was only successful for locked-out type seat
backs. For full break-over and energy absorbing type seat backs, the
ATD’s arms pushed the seat back away, allowing the head to accelerate
relative to the seat back, increasing the severity of the head impact.
There was, however, no evidence that the ATD interaction with any of
the seatback types resulted in hyper-extension of the shoulder joint.
The “Forward” and “Vertical” leg brace positions permitted the lower
legs to flail forward and contact the seat in front. This contact
produced femur bending and compression (below criteria limits), and
tibia injury risk in some cases.
The “Aft” leg brace position reduced
lower leg flailing and prevented femur contact with the front of the
seat frame. This position resulted in low femur and tibia injury
assessment values; however, it is only achievable for occupants whose
lower legs are long enough for their feet to firmly touch the floor.
Alternate Brace Position
To reduce the detrimental interaction
between the occupant’s arms and the seatback, the current position was
modified by placing the hands down by the lower legs instead of on the
This “Alternate” position was successful in reducing head
and neck injury risk for all of the seat back types evaluated.
This position is currently recommended when the seat
in front is too far away to support the head, or there is no seat in
front, as in a bulkhead row.
It was successful in reducing head and
neck injury risk in the row-to-row scenario, as long as the struck
seat back has a relatively soft local compliance at the point of
impact. When seated behind a bulkhead, the effectiveness of the pike
position was dependent on whether the head of the occupant struck the
bulkhead. “Head-path-reducing features,” such as Y-belts, prevented
head contact at the typical 35-in setback.
So the answer is really, it depends on the position as well as the seats in question. You find a full explanation of what the various seat types are on page 2 of the study.
Perhaps the most interesting note is that they call for a potential change in the current brace position:
...and the current positions recommended in AC 121-24B may need some
adjustment to provide an equivalent level of safety for all passenger
seat back types.