Relationship between the increase of effectiveness indexes and the increase of muscular efficiency with cycling power

Abstract

We determined the index of effectiveness (IE), as defined by the ratio of the tangential (effective force) to the total force applied on the pedals, using a new method proposed by Mornieux et al. (J Biomech, 2005), while simultaneously measuring the muscular efficiency during sub-maximal cycling tests of different intensities. This allowed us to verify whether part of the changes in muscular efficiency could be explained by a better orientation of the force applied on the pedals. Ten subjects were asked to perform an incremental test to exhaustion, starting at 100 W and with 30 W increments every 5 min, at 80 rpm. Gross (GE) and net (NE) efficiencies were calculated from the oxygen uptake and W(Ext) measurements. From the three-dimensional force's measurements, it was possible to measure the total force (F(Tot)), including the effective (F(Tang)) and ineffective force (F (Rad + Lat)). IE has been determined as the ratio between F(Tang) and F(Tot), applied on the pedals for three different time intervals, i.e., during the full revolution (IE(360 degrees)), the downstroke phase (IE(180 degrees Desc)) and the upstroke phase (IE(180 degrees Asc)). IE(360 degrees) and IE(180 degrees Asc) were significantly correlated with GE (r = 0.79 and 0.66, respectively) and NE (r = 0.66 and 0.99, respectively). In contrast, IE(180 degrees Desc) was not correlated to GE or to NE. From a mechanical point of view, during the upstroke, the subject was able to reduce the non-propulsive forces applied by an active muscle contraction, contrary to the downstroke phase. As a consequence, the term 'passive phase', which is currently used to characterize the upstroke phase, seems to be obsolete. The IE(180 degrees Asc) could also explain small variations of GE and NE for a recreational group.