A cycle ergometer mounted on a standard force platform for three-dimensional pedal forces measurement during cycling
Introduction
Cycle ergometers are used in laboratories for training and testing purposes (e.g. Denis et al., 1982; Hautier et al., 2000; Lepers et al., 2000; Linossier et al., 1993) and also for basic investigations concerning muscular efficiency (Gaesser and Brooks, 1975; Sidossis et al., 1992; Whipp and Wasserman, 1969). In these studies, mechanical work done when pedaling is of interest and can be accurately measured (Arsac et al., 1996; Lakomy, 1986). Moreover cycle ergometers are also used for research questions where the knowledge of pedal forces is useful, as for pedal forces asymmetry investigations (Daly and Cavanagh, 1976) or comparison of shoe-pedal interfaces (Wheeler et al., 1992). So it is important to accurately measure pedal forces. For that purpose, different systems were proposed.
Previous studies analyzed pedal force in one dimension, using either strain gauges fixed on the crank (Daly and Cavanagh, 1976; Künstlinger et al., 1984; Sargeant and Davies, 1977) or directly under the pedal to measure the normal force (Brooke et al., 1981; Hoes et al., 1968). However, these did not allow for an inverse analysis where intersegmental loads can be computed.
Then pedal forces measurements in two dimensions (Beelen et al., 1994; Gregor et al., 1985; Newmiller et al., 1988; Patterson et al., 1983; Rohmert and Krell, 1980; Soden and Adeyefa, 1979) and complete three-dimensional (3D) pedal force investigations (Boyd et al., 1996; Hull and Davis, 1981; Ruby and Hull, 1993; Ruby et al., 1992; Stone and Hull, 1993) were performed using strain gauges fixed under the pedals. Furthermore piezo electric transducers fixed under the pedal for 3D pedal force measurement were proposed in the literature (Broker and Gregor, 1990; Ericson et al., 1985). However, these methods based on forces transducers fixed under the pedal are not widely available and pedal force investigations could be more accessible without the need of these specialized force pedals.
The aim of this study was then to develop and validate a new measurement system that allows 3D pedal force measurement with often an accessible force platform. For that purpose, the pedals and gear mechanism were separated from the bicycle frame and were fixed on a force platform.
Section snippets
Mechanical device
A typical friction-loaded cycle ergometer (Monark type 818E, Stockholm, Sweden) was equipped with specific transducers according to Arsac et al. (1996). The friction force applied by the tension of the belt that surrounded the flywheel was measured by means of a strain gauge (FGP Instrumentation type FN3030 0–20 daN, les Clayes sous Bois, France) previously calibrated by a known mass (5 kg) hung on the friction belt and in an unloaded condition to give the 0 value The strain gauge non-linearity
Results
The orientation of the force given by the system was always correct during static evaluations. The measurement errors between the knowing force applied (what represented the normalizing quantity expressed in N) and the force calculated by the system for the pedal and dimension concerned were −7.61±7.8 N, [range: −18.2 N to +5.59 N] corresponding to a relative error of −3.06±3.30%, [range: −8.04% to +2.45%] for the vertical dimension, −3.37±4.02 N, [−7.4 N to+4.53 N] or −2.60±3.09%, [−7.09% to +3.32%]
Discussion
The accuracy of the static measurements and especially of the combined forces test was satisfactory when looking at the maximal error of −4% in medial–lateral direction. Even if the pedals’ moments were considered negligible in the Eqs. (7), (8), (9) used to determine pedals force, the measurement errors were typical because those were not influenced by this assumption as no pedal moment was generated during the static evaluations while applying a force in a single dimension. The negative mean
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