Around
90% of manual wheelchairs users (MWUs) employ a hand-rim propelled
wheelchairs (HRW) as primary mode of locomotion for daily living [1].
However, HRW propulsion is a straining and mechanically inefficient
mode of transportation. The combination of daily hand-rim wheelchair
propulsion and transfers into and out of the wheelchair are thought to
be responsible for high upper-limb strains and consequent prevalence of
musculoskeletal complaints, oftentimes leading to inactivity [2,3,4,5].
Conventional
HRW propulsion has a relatively low gross mechanical efficiency (GME);
GME varies somewhere between 3 and 12%, depending among others on
environmental conditions, speed, power output, wheelchair design,
wheeling experience and propulsion technique [6, 7]. Compared to regular cycling (18–23%) [8] and handcycling (8–20%) [9, 10], hand-rim propulsion is an inefficient mode of locomotion.
The
hand-rim wheelchair propulsion cycle is bi-phasic: an active push phase
(~ 30% cycle time) and an inactive recovery phase, necessitating a
continuous coupling and de-coupling of the hands to the hand-rims. The
low fraction of the time for power transfer creates peak levels of force
and work [11].
Spatial orientation of the humerus and scapula during HRW propulsion as
well as in transfers, increase the musculoskeletal strain and risk of
shoulder complaints, especially when they are paired with continued high
external loads in daily life (e.g. floor surface, slopes, curb
negotiation) [12].
The
abovementioned factors suggest that any improvements in the propulsion
mechanism and interfacing, may aid in reducing upper-body strains and
complaints, while eventually promoting a physically active lifestyle [1, 13,14,15,16,17]. Apart from handcycling [17],
few alternative propulsion mechanisms successfully entered and stayed
in the marketplace. With ups and downs, lever-propelled wheelchairs have
been available over the past 70 years in several different lever
propulsion designs. Although crank-lever mechanisms exist [16], the most common designs involve two different levers each propelling one of the rear wheels individually [13].
In such a design, a push to the levers is similar to a push to the
hand-rim and the handles can be pulled back during the recovery phase of
the propulsion cycle. When tested on physiological performances, these
types of wheelchairs proved to be more energy-efficient and less
straining than HRWs [14,15,16, 18].
While bi-lever propulsion systems have been experimentally evaluated [1, 13, 14, 18, 19],
a bi-manually driven single push–pull lever propulsion mechanism (PPLM)
has very scarcely been tested on its physiological and biomechanical
merit. The only studies that featured a design with a single lever
originated in the 1970s and 1980s [15, 20, 21].
These early crank lever designs were different from the current design,
as they were connected to the rear wheels and their cycle frequency was
directly fixed to the speed, compared to the freewheel of the current
design.
The current study evaluates a new single central-lever push–pull design (rotamobility.com, Fig. 1A).
This particular wheelchair design features a gearing system and a
free-wheel, with which power can be transferred into forward motion in
both the push and pull phases of the propulsion cycle, while it allows
steering through the central lever mechanism. Utilisation of both phases
of the cycle increases the portion of the propulsion cycle that can be
used to exert force, thus theoretically reducing peak strain and
potentially energy expenditure (EE) [22,23,24].
Moreover, the different directions of force application allow different
muscle groups to be recruited. Therefore, the work is also divided over
a larger total muscle mass, reducing strain on individual muscles.
Because
of the novelty of the system, the current study does not yet compare
experienced users of the PPLM to manual wheelchair users. Since early
handrim wheelchair use is accompanied with motor learning processes [25, 26],
it is assumed that the novel lever propulsion mechanism may require
motor adaptation, skill acquisition and learning as well. Therefore,
three research questions were addressed in this study. Firstly, does
PPLM have physiological advantages over HRW, looking at EE, GME and HR?
Secondly, are the peak forces needed to use an PPLM indeed lower than
those needed for propelling a HRW? And finally, does practice change the
outcomes of these questions with inexperienced users? These questions
will be tested by comparing two inexperienced groups, either using the
PPLM and a HRW before and after a practice intervention.
Based on previous studies on lever-propulsion [1, 13, 14, 18, 19],
it was hypothesised that the PPLM will be less physiologically
straining than a conventional HRW. This is expected to be shown in a
higher GME and lower EE and HR. Also, forces that are applied on the
lever of the PPLM are hypothesised to be lower than the forces applied
on the handrim of the HRW, because the applied forces can be spread over
a longer period of time of each full cycle and is divided in a push and
pull action, involving different muscle sets. Therefore, equal power
can be produced with lower peak forces.
These questions and
hypotheses have been tested in a 5-day between-group experiment. In this
experiment, 30 able-bodied novices performed 60 min of practice in
either the push–pull central lever wheelchair (n = 15) or the hand-rim
wheelchair (n = 15). At the first and final sessions cardiopulmonary
stain, propulsion kinematics and force production were determined in
both instrumented propulsion mechanisms.
More at link.
