Rock and roll: the effects of centre of mass movement and bicycle lean on the biomechanics of cycling

Abstract

Cyclists frequently use a non-seated posture when accelerating, climbing steep hills, and sprinting, yet the biomechanical difference between seated and non-seated cycling remains unclear. The purpose of our first study was to test the effects of posture (seated and non-seated) and cadence (70 rpm and 120 rpm) on lower-limb joint power distribution, effective mechanical advantage, and muscle activity during very high power output cycling. Fifteen subjects rode on an instrumented ergometer at 50% of their individualised instantaneous maximal power (10.7±2.0 W/kg; above the reported threshold for seated to non-seated transition) in different postures (seated and non-seated) and at different cadences (70 rpm and 120 rpm), whilst lower-limb muscle activity, full-body motion capture, and crank radial and tangential forces were recorded. A scaled, full-body musculoskeletal model was used to solve inverse kinematics and inverse dynamics to determine joint displacements and net joint moments. Statistical comparisons were made using repeated measure, two-way analyses of variance (posture–cadence). Our results showed significant main effects of posture and cadence on the distribution of lower-limb joint power. A key finding was that the non-seated posture increased negative power at the knee, with an associated significant decrease of net power at the knee. The contribution of knee power decreased by 15% at both 70 and 120 rpm (~0.8 W/kg) when non-seated compared with seated. Subsequently, hip power and ankle power contributions were significantly higher when non-seated compared with seated at both cadences. In both postures, knee power was 9% lower at 120 rpm compared with 70 rpm (~0.4 W/kg). These results evidenced that the contribution of knee joint power to leg power was reduced by switching from a seated to non-seated posture during very high power output cycling; however, the size of the reduction is cadence dependent.Previous research and field observations also suggest that, when riding off the saddle, a rider's centre of mass (CoM) goes through a rhythmic vertical oscillation during each crank cycle. Just like in walking and running, the pattern of CoM movement may have a significant impact on the mechanical power that needs to be generated and dissipated by muscle. To date, neither CoM movement strategies during non-seated cycling, nor the limb mechanics that allow this phenomenon to occur have been quantified. In our second study we measured vertical displacement of the body's CoM and the associated changes in total mechanical energy during non-seated cycling at various combinations of power output (10%, 30%, and 50% of instantaneous maximal power output (Pmax.i) and cadence (70 rpm and 120 rpm). Our analysis revealed that cyclists increased vertical CoM motion at higher power outputs but raised and lowered their CoM during the same phases of the crank cycle under all conditions. This phasing of vertical CoM motion appears to be a movement strategy to facilitate an exchange of mechanical energy to the crank; theoretically at rates as high as 18% of peak crank power. These findings suggest that cyclists can utilise vertical motion of their CoM to reduce the contribution of the muscles to overall mechanical power output requirements.When riding off the saddle during climbing and sprinting, cyclists appear to coordinate the rhythmic, vertical oscillations of their CoM with the side-to-side lean of the bicycle. In our third study we investigated the idea that the coupling of CoM movement and bicycle lean could be a strategy to more effectively generate crank power. A combined kinematic and kinetic approach was used to understand how different constraints on bicycle lean influence CoM movement and limb mechanics during non-seated cycling on rollers. Thirteen participants cycled in a non-seated posture at a power output of 5 W/kg and a cadence of 70 rpm under three conditions: unconstrained lean on rollers, under instruction to self-restrict bicycle lean on rollers, and constrained lean in a bicycle trainer. Our results showed that riders generated higher peak crank forces and their CoM underwent greater fluctuations in total mechanical energy when leaning the bicycle a preferred amount compared to when self-restricting lean. The resultant crank force vector was also more closely aligned to the hip and knee joint when leaning the bicycle meaning that greater peak forces were produced using similar net joint moments and EMG activity within the lower limb. We interpret these findings to suggest that leaning the bicycle during non-seated cycling when no lateral support is provided allows a greater non-muscular contribution to crank force and power.In summary, these investigations have established a fundamental but new understanding of the underlying mechanics and energetics of the phenomenon of non-seated cycling, while also pointing towards the potentially detrimental influence of self-restricting bicycle lean when cycling in a non-seated posture at high-power outputs. These findings should be of interest to the fields of biomechanics, exercise physiology, and motor control, as well as those involved with optimising rider and bicycle performance.