Consequence of feedback-based learning of an effective hand rim wheelchair force production on mechanical efficiency

Abstract

Objective. Investigation of the effect of visual feedback on effective hand rim wheelchair force production and the subsequent effect on gross mechanical efficiency.

Design. Ten subjects in an experimental group and 10 subjects in a control group practised three weeks ($\text{3·}\text{wk}{}^{\mathrm{-1}}$, i.e., a pre-test and 8 trials) on a computer-controlled wheelchair ergometer. Every trial consisted of two blocks of 4 min at 0.15 and $\text{0.25}\text{W}\text{·}\text{kg}{}^{\mathrm{-1}}$ at $\text{1.11}\text{m.s}{}^{\mathrm{-1}}$. On three trials an additional block at $\text{0.40}\text{W}\text{·}\text{kg}{}^{\mathrm{-1}}$ was performed. The experimental group practised with and the control group practised without visual feedback on the effectiveness of force production.

Background. In mechanical terms, the low gross mechanical efficiency of hand rim wheelchair propulsion may be the result of ineffective force production.

Methods. During all trials oxygen uptake, power output, forces and torque on the hand rims were measured.

Results. In comparison with the control group, the experimental group at trial 8 had a significantly more effective force production compared to the control group (90–97% vs. 79–83%, respectively), but showed a significantly lower mechanical efficiency (5.5–8.5% vs. 5.9–9.9%, respectively).

Conclusion. Findings indicate that the most effective force production from a mechanical point of view is not necessarily the most efficient way – in terms of energy cost – from a biological point of view and that force direction is based on an optimization of cost and effect.
Relevance

Learning a more effective force production by visual feedback is not useful for increasing the mechanical efficiency of hand rim wheelchair propulsion.

Introduction

Hand rim wheelchair propulsion is a way of locomotion with a low gross mechanical efficiency (ME). Gross ME of wheelchair propulsion rarely exceeds 11% and is much lower than in arm cranking (16%) [1], [2] or cycling (18–23%) [3]. As a consequence, hand rim wheelchair propulsion is associated with a high physical strain in daily life [4] and leads most likely to a high mechanical load on the upper extremity. The latter may lead to a high prevalence of overuse injuries in shoulder and wrist [5]. It was suggested that propulsion technique plays a role in the low ME [6]. Therefore, it is important to study which aspects of propulsion technique are associated with ME and how hand rim wheelchair propulsion technique can be improved in terms of efficiency and mechanical strain.

What is known about the gross ME of hand rim wheelchair propulsion is that, at least partially, it is the result of non-optimal tuning of the wheelchair to the physical characteristics of the user [7]. The low ME can also be due to the occurrence of, so-called, ineffective propulsion technique characteristics, such as braking torques at the start and end of the push phase [6], [8], and/or a propulsion force whose direction is – at least from a mechanical viewpoint – not fully optimal, as it would be when tangential to the hand rims [6].

From a purely mechanical standpoint, the greater the portion of force directed tangentially to the hand rim and the more positive the torque around the hand, the greater the moment developed around the wheel hub. Individuals who apply large non-tangential forces will require larger total forces to produce the same effective torque [5]. Veeger and colleagues [6], [8], [9], [10] have described the tangential vs. total force produced and developed the fraction of effective force (FEF). This measure is defined as the ratio of effective (tangential) force and total force, expressed as a percentage, and was used to describe how effective an individual was in applying forces to the hand rim. The FEF is dependent on the direction of the propulsion force that is applied and on the direction and magnitude of torque around the hand [11]. The FEF was found to be low (between 57% and 81%) in able-bodied, low-level spinal cord injured subjects [6], [9], [10], [11], [12], [18], as well as in wheelchair athletes [13]. A low FEF generally indicates a more downward direction of the total force vector. Boninger et al. [5] using a comparable but not identical measure (squared tangential force/squared resultant force of the force components in three directions, expressed as a percentage) found equally low values (52–54%) in experienced wheelchair users on a wheelchair dynamometer. Since able-bodied as well as experienced wheelchair-dependent subjects appear to direct the force always more downward, this may indicate that the force is directed to the best of abilities – regarding joint mechanics and muscle coordination – when directed non-tangentially [14].

Besides a simulation experiment of force direction [15], which suggested that experienced users optimize the force pattern by balancing the mechanical effect as well as the musculoskeletal cost, no literature is yet available concerning the consequences of a high FEF on gross ME in hand rim wheelchair propulsion. Therefore, a visual feedback computer program on FEF was developed. This was implemented in a practice period to obtain a group, who could apply a high FEF, for studying the effect on ME. Although feedback on force application was found to be effective in changing pedal force patterns in cycling [16], [17], it was not certain whether subjects could indeed improve FEF with help of visual feedback. Therefore, the first purpose of the study was to investigate the effect of visual feedback on FEF. The second and main purpose of the present study was to investigate the consequences of a – learned – high FEF on gross ME, compared to a freely chosen FEF in two otherwise comparable novice, able-bodied subject groups. Novice able-bodied wheelchair users were included in this study since experienced wheelchair users already found a balance between mechanical effect and musculoskeletal cost.