Variability observed. Overall, the similarity of muscle tensions is essentially owing to the similarity of fibre structure and thick filament length across muscles and species, in contrast with the variability of muscle speeds which are affected by the variability of thin filament lengths (e.g. [210]). It is remarkable that tension is smaller in flight locomotors (median 79 kPa) than in terrestrial locomotors (median 187 kPa) and in swim locomotors (183 kPa), although only the difference for terrestrial locomotors is significant according to ANOVA at level 5 (figure 3e). Despite the high power needed for flight, the high frequencies required may impose a large concentration of mitochondria and, at least in birds, of sarcoplasmic reticulum at the expense of myofibrils. Solving this issue will need further investigation.rsos.royalsocietypublishing.org R. Soc. open sci. 3:…………………………………………4.5. Absence of large-scale trend with cell’s or body’s massGiven the constancy in both central value (mean or median) and dispersion (s.d. or interquartile range) of f in molecular and non-molecular motors, it is not surprising that the regressions in a log og plot of f against M, the mass of the cell (for subcellular motors) or body (for cellular and supracellular motors) from which the motor is extracted, give no evidence of overall trend (figure 4a,b). Other variables for the mass might be used, but their implementation is difficult because they are often ill-defined or unknown. This is the reason why we chose for the horizontal axis a proxy of the mass that the motor moves– the mass of the next higher hierarchical level, i.e. the cell’s mass for subcellular motors (M1, M2, MF) and the animal’s mass for cellular and supracellular motors (FI, MU, MS). This definition is simple, unambiguous, known in almost all cases and discriminant with a range extending over 18 orders of magnitude. If we had chosen the motor’s mass m for the horizontal axis, the range would have been still wider since the minimum mass would be 10-22 kg (kinesin) and the maximum mass > 1 kg (muscle), so that as the overall range of f would remain the same, the slope of the regression line would become still closer to zero.The absence of global trends does not preclude the existence of `local’ trends, i.e. regression lines with slope significantly different from zero, for GSK2256098 supplier specific classes of motors extending on a narrower mass range. Several examples of such significant trends were found (see the electronic supplementary material, NS-018 web tables S8 12) but their slopes are small and difficult to interpret. These small-scale relationships are outside the scope of this paper which focuses on a large-scale study. The wide range of size, mass and area considered allows one to transcend the possible variations specific to certain categories.rsos.royalsocietypublishing.org R. Soc. open sci. 3:…………………………………………4.6. Scaling with motor’s massA different approach based on force F and motor mass m strengthens this conclusion. Indeed, Marden Allen [18] studied the scaling of forces with motor’s mass for two classes of animal- and humanmade motors and found that one of them, `Group 1′ motors, producing translational motion, scale allometrically with motor mass m, as F 103 m2/3 (with F in Newtons and m in kilograms). We show below that this scaling, expressed in terms of specific tension f, is in good agreement with the typical specific tens.Variability observed. Overall, the similarity of muscle tensions is essentially owing to the similarity of fibre structure and thick filament length across muscles and species, in contrast with the variability of muscle speeds which are affected by the variability of thin filament lengths (e.g. [210]). It is remarkable that tension is smaller in flight locomotors (median 79 kPa) than in terrestrial locomotors (median 187 kPa) and in swim locomotors (183 kPa), although only the difference for terrestrial locomotors is significant according to ANOVA at level 5 (figure 3e). Despite the high power needed for flight, the high frequencies required may impose a large concentration of mitochondria and, at least in birds, of sarcoplasmic reticulum at the expense of myofibrils. Solving this issue will need further investigation.rsos.royalsocietypublishing.org R. Soc. open sci. 3:…………………………………………4.5. Absence of large-scale trend with cell’s or body’s massGiven the constancy in both central value (mean or median) and dispersion (s.d. or interquartile range) of f in molecular and non-molecular motors, it is not surprising that the regressions in a log og plot of f against M, the mass of the cell (for subcellular motors) or body (for cellular and supracellular motors) from which the motor is extracted, give no evidence of overall trend (figure 4a,b). Other variables for the mass might be used, but their implementation is difficult because they are often ill-defined or unknown. This is the reason why we chose for the horizontal axis a proxy of the mass that the motor moves– the mass of the next higher hierarchical level, i.e. the cell’s mass for subcellular motors (M1, M2, MF) and the animal’s mass for cellular and supracellular motors (FI, MU, MS). This definition is simple, unambiguous, known in almost all cases and discriminant with a range extending over 18 orders of magnitude. If we had chosen the motor’s mass m for the horizontal axis, the range would have been still wider since the minimum mass would be 10-22 kg (kinesin) and the maximum mass > 1 kg (muscle), so that as the overall range of f would remain the same, the slope of the regression line would become still closer to zero.The absence of global trends does not preclude the existence of `local’ trends, i.e. regression lines with slope significantly different from zero, for specific classes of motors extending on a narrower mass range. Several examples of such significant trends were found (see the electronic supplementary material, tables S8 12) but their slopes are small and difficult to interpret. These small-scale relationships are outside the scope of this paper which focuses on a large-scale study. The wide range of size, mass and area considered allows one to transcend the possible variations specific to certain categories.rsos.royalsocietypublishing.org R. Soc. open sci. 3:…………………………………………4.6. Scaling with motor’s massA different approach based on force F and motor mass m strengthens this conclusion. Indeed, Marden Allen [18] studied the scaling of forces with motor’s mass for two classes of animal- and humanmade motors and found that one of them, `Group 1′ motors, producing translational motion, scale allometrically with motor mass m, as F 103 m2/3 (with F in Newtons and m in kilograms). We show below that this scaling, expressed in terms of specific tension f, is in good agreement with the typical specific tens.