# Cycling Biomechanics and Its Relationship to Performance
## 4 Seat and Handlebar Forces and the Role of the Upper Arms  
The mechanical actions of muscles are transmitted to the bicycle at different points of interaction (i.e., the handlebars, pedals and seat), and they can be measured using appropriate force sensors (Figure 3). The forces applied at the seat and handlebars do not produce power directly, but they are interesting to study as part of a whole-body cycling technique. It is worth noting that handlebar force sensors together with motion capture technologies allow the study of the torques and powers at the wrist, elbow and shoulder using inverse dynamics. Figure 3: Force sensors. Panels (**A–C**) show force sensors at the pedal, handlebar and saddle. These were designed by Sensix, Poitiers, France. The sensors allow the measurement of forces and moments in three dimensions at the three points of interaction between the ergometer and the cyclist. Panel (**D**) shows the forces produced at the right handlebar and the pedals when pedaling in a seated position at two intensities: 40% of the seat-to-stand transition power (SSTP) and 120% of the SSTP, which represent approximately 200 W and 600 W, respectively (at 90 RPM). Note the decrease in the Fz component during the right downstroke, which goes from positive (pushing) to negative (pulling) between 40% and 120% SSTP. Panel E provides the direction of the handle forces in Three dimensions. The seat forces are the result of the weight of the upper body, the action of the arms and the reaction of the leg forces through the hip joints [47]. When the forces produced by the legs increase the reaction forces at the hip joint (upward forces) also increase and the seat reaction forces tend to decrease. Cyclists compensate for these forces by pulling on the handlebars and the pedals until they reach a point (\(\sim\)500 N on the pedal; Figure 3) at which they tend to adopt the standing position [46; 47]. Moreover, cycling requires the alternation of right and left leg forces at the crank, which, even during seated cycling, create destabilizing motions of the trunk that must be absorbed by the arms [48; 49]. Therefore, the forces developed by the upper limbs play significant roles during cycling and might be associated non negligible energy costs [50; 51]. Thus, it is important to quantify these forces, to determine, for example, if fatigue could occur at this level and decrease the performance. Previous studies asked whether the upper limbs could transfer energy to the lower limbs via the hips. It has been shown that the power developed by the upper limbs is no more than approximately 3% to 5% of the crank power output [36; 52]. Their contribution is therefore very modest. Nonetheless, their role is important during high-intensity cycling and should not be underestimated because the power output noticeably decreases when upward forces on the handlebar cannot be produced (\(-10\%\) to \(-20\%\)) [53]. It has been suggested that upper limb muscles do not necessarily produce power, but instead facilitate the reaching of the highest power outputs by providing stable support for the leg actions [54]. However, it is not clear whether an improvement in the coordination of upper and lower limbs might improve the performance or not. These elements suggest that hand positioning may have a significant influence in the context of maximal power production. A previous study suggested, for example, that positioning the hands on the upper bar of the handlebar can improve the maximal power output during starting conditions [55]. The effect of hand positioning has also been studied in the context of whole-upper body positioning. Studies have shown that using the upright, dropped and aerodynamic postures or different trunk angles altered muscle activation levels at both the lower and upper limb muscles and also altered the index of efficiency [56; 57; 58; 59], suggesting that the lower limb joint torque distribution was influenced by upper body postures.