# Cycling Biomechanics and Its Relationship to Performance
## 7 Measures of Muscle Mechanical Performance  
Intrinsic muscle mechanical properties are critical determinants of any motor performance. Characterizing in vivo muscle mechanics is possible in a laboratory environment using isokinetic devices (e.g., Contrex, Biodex) or adapted set-ups using force sensors, myo-meter devices, electrical stimulators and/or kinematic measurements. Classic mechanical properties include musculo-tendinous stiffness, force-length relationships, force-velocity relationships and rate of force development during voluntary contraction or muscle activation dynamics (i.e., electromechanical delay, force development and relaxation times) [75; 76; 77]. These measures are generally performed for groups of muscles, such as the knee extensors or ankle plantar flexors, rather than for individual muscles. These measures are essential for diagnosis and for understanding basic science of movement. However, the relation between individual joint performance and whole-body performance is not strong [78; 75; 33; 79], [PERSON] et al. (2017) [78] found that at best, isolated maximal joint moments can account for approximately 47% of the variance in peak power output in recreational cyclists (for the knee joint). This lack of correlation might reflect the need to coordinate the multiple joints and/or the limitation associated with isolated joint torque evaluations [80]. In agreement with the former hypothesis, muscle activation and joint torque during maximal cycling exercises are often less than during maximal voluntary contractions performed for each joint separately [79; 33], suggesting that the pedaling movement imposes certain constraints on the organization of the joint torque and muscle activity. In agreement with the latter hypothesis, the peak power output has been found to be better correlated with muscle volume (i.e., r\({}^{2}\) = 0.67) [81] than with maximal joint moments (r\({}^{2}\) = 0.47) [78], suggesting that isolated joint torque measurements do not provide a good estimate of the maximal capacity of the muscles. As a consequence, measurements performed directly on a bike or a cycling ergometer are in general more representative of the actual cycling performance [78]. By changing the position of the seat (e.g., seat tube angle, seat height), angle of the trunk or length of the crank, the segment configuration changes, making the lower-limb muscles work at different portions of their force-length relationships. Positioning is known to affect cycling performance [1], suggesting that force-length characteristics significantly influence the pedaling performance. For example, an upright position of the trunk appears to be an advantage in terms of peak crank torque and global efficiency, although its advantage decreases with increasing velocity in the field, because of aerodynamic resistances [59; 82]. Optimal positioning can also be determined as the one the minimize a certain cost function based on the VO\({}_{2}\), joint moment or EMG measures [42; 44; 83]. Contraction velocity has major effects on muscle mechanics and energy expenditure. During cycling, the muscle contraction velocity is related to the pedal velocity, which depends on the crank length and pedaling frequency [84]. A classic characterization of muscle performance in cycling relies on the relationship between cadence and crank power. This relation derives from [PERSON]'s well known force-velocity relationship for a single muscle [85] (i.e., when the contraction velocity increases, the maximal force that a muscle can produce decreases and eventually reaches zero at its maximal contraction velocity). Therefore, the muscle power (force \(\times\) velocity) is maximal at an optimal contraction velocity. An easy way to perform this test while cycling is to fix the resistive load at different intensities and measure the maximal pedaling rate during short sprints [52]. The result is a linear relationship between the angular velocity (rad/s) and the torque (N\(\cdot\)m), which can be used to compute the power output.