Mechanical energetics relating to rider centre of mass motion during non-seated cycling
Creators: name: Wilkinson, Ross D.
name: Cresswell, Andrew G.
name: Lichtwark, Glen A.
Publisher: name: F1000 Research Limited
Year: 2018

description: Cyclists often transition to a standing position when needing to produce high pedal torque and power (i.e. during steep climbs, accelerations and sprinting). Previous research and field observations of cyclists during these scenarios suggest that significant vertical displacement of the rider’s center of mass (COM) occurs over each pedal cycle (1). Raising and lowering the COM against gravity is significant to the energetics of cycling because the total work required to do so comes at a cost to the rider. Thus, one could assume that riders choose to incur this cost as it may increase pedaling effectiveness while minimizing energy expenditure. Presently the impact of vertical COM displacement on lower limb mechanics during standing cycling remains unresolved. We speculate that different combinations of power output and cadence impact how a rider determines the optimal level of vertical COM displacement.<br><br>PURPOSE: To test the effects of power output and cadence on vertical COM displacement during standing cycling. We hypothesize that vertical COM displacement would increase with power output. We also predict that the effect of power would be cadence dependent due to the increased cost of producing force at high velocity. Effective mechanical advantage (EMA) of the hip, knee and ankle, leg extension velocity and duty cycle were measured to compare the effects of power output and cadence on lower limb effectiveness.<br><br>METHODS: Nineteen subjects (17M; 2F) rode on an electronically braked cycle ergometer for all cycling trials. Subjects performed five maximal cycling trials of 3-s duration in a seated position to determine individual peak power output (PPO). Resistance for the sub-maximal trials was set to either 10% or 50% PPO as these levels sit either side of the reported threshold for the sit to stand transition (2). Cadence was set to either 70 rpm or 120 rpm to simulate preferred cadences during climbing and sprinting respectively. Participants performed the combinations of cadence (70 rpm: LC and 120 rpm: HC) and power output (10%: LP and 50%: HP) in a randomized order and were required to maintain these targets for a period of 10 seconds whilst kinematic and kinetic data was recorded. Crank angle and forces tangential and radial to the left and right crank were recorded at 100 Hz using instrumented cranks. The 3D trajectories of 46 passive, reflective markers placed across the lower limbs, torso and upper limbs were recorded at 200 Hz. Lower limb joint kinematics and net muscle moments were calculated using a scaled full-body model (3) within OpenSim v3.3. Data was calculated from five pedal cycles of the right leg. Processing of data was completed using custom scripts in MATLAB R2017a. A repeated measures, two-way ANOVA was performed to test for main effects of cadence and power and interaction effects (cadence x power). The alpha level for main and interaction effects was set at .0247 in order to minimize false positive risk. Whenever a main or interaction effect was found, Sidak’s multiple comparisons test was used to detect the effects of factor/s within each condition. For main and interaction effects the F-value (<i>F<sub>(dof)</sub></i>), p-value (<i>p</i>). For multiple comparisons the t-statistic (<i>t<sub>(dof)</sub>)</i>, adjusted p-value (<i>p</i>), 95% confidence intervals (95% CI [Low-High]), and effect size (Cohen’s <i>d<sub>av</sub></i>) are provided. All values are reported as mean±sd.<br><br>RESULTS: <i>Center of mass displacement:</i> There was a main effect of power (F<sub>(1,18)</sub>=33.53, p&lt;.0001) and cadence (F<sub>(1,18)</sub>=167, p&lt;.0001) on the range of vertical COM displacement. There was also an interaction effect between power and cadence (F<sub>(1,18)</sub>=7.047, p=.0161). This resulted in the largest range of vertical COM displacement occurring during LC+HP (5.1±1.5 cm) and the smallest range during HC+LP (1.1±0.03 cm). Due to the larger effect of cadence, displacement during LC+LP (3.3±1.1 cm) was significantly higher than HC+HP (2.2±1.1 cm),<i> t</i><sub>(18)</sub>=3.223, <i>p</i>&lt;.0047, 95% CI [0.387, 1.837], Cohen’s <i>d<sub>av</sub></i>=1.046. <i>Lower limb mechanics: </i>There was a main effect of cadence on the hip EMA (F<sub>(1,18)</sub>=29.69, <i>p</i>=.0002).  At low power, EMA of the hip was similar at low and high cadence. At high power, EMA at the hip was higher at low cadence (0.435±0.147) than at high cadence (0.311±0.091), <i>t</i><sub>(18)</sub>=3.116, <i>p=</i>.006, 95% CI [0.04, 0.2065], Cohen’s <i>d<sub>av</sub></i>=1.011. Increasing power output lowered peak leg extension velocity at both low (LP=1.326±0.102 v HP=1.065±0.137 m.s<sup>-1</sup>, <i>t</i><sub>(18)</sub>=6.661, <i>p</i>&lt;.0001, 95% CI [0.179, 0.343], Cohen’s <i>d<sub>av</sub></i>=2.161) and high cadence (LP=2.308±0.079 v HP=2.184±0.169 m.s<sup>-1</sup>, <i>t</i><sub>(18)</sub>=2.897, <i>p=</i>.0096, 95% CI [0.034, 0.214], Cohen’s <i>d<sub>av</sub></i>=0.94), but the effect was larger at low cadence (Fig. 1B). The ratio of time spent in extension versus flexion, known as duty cycle, was greatest during the LC+HP condition (1.43), while the only condition with a duty cycle &lt;1.0 was the HC+LP condition (0.96).<br><br>CONCLUSION: These results extend our understanding of how vertical COM displacement may impact pedaling mechanics as well as the energy fluctuations of the bicycle-rider system by highlighting the significant effects of power output and cadence. It appears that the trade-off between force applied for longer periods of time to do work on the pedal and work on the COM is favored at low cadences and high power output. The increase in EMA at the hip with higher power outputs likely also makes this strategy more effective. At higher cadences, the body tends to reduce the COM vertical fluctuations, presumably because less force is produced and there is less time per cycle to apply impulse to raise the COM. Decreasing power output is likely to have a similar effect, however decreased force likely reduced overall impulse to raise the COM. Our future research will focus on the possibility that lateral sway of the bicycle and elastic energy storage mechanisms within the lower limb also affect vertical COM displacement and lower limb mechanics during standing cycling.<br><br>REFERENCES:<br>1. Hull et al. <i>Journal of Biomechanics. </i>23(7), 1990.<br>2. Costes et al. <i>Journal of Biomechanics</i>. 48(12), 2015.<br>3. Rajagopal et al. <i>IEEE Transactions on Biomedical Engineering</i>. 63(10), 2016.