Frédérique Hintzy

A simple method for measuring force, velocity and power output during squat jump

Our aim was to clarify the relationship between power output and the different mechanical parameters influencing it during squat jumps, and to further use this relationship in a new computation method to evaluate power output in field conditions. Based on fundamental laws of mechanics, computations were developed to express force, velocity and power generated during one squat jump. This computation method was validated on eleven physically active men performing two maximal squat jumps. During each trial, mean force, velocity and power were calculated during push-off from both force plate measurements and the proposed computations. Differences between the two methods were not significant and lower than 3% for force, velocity and power. The validity of the computation method was also highlighted by Bland and Altman analyses and linear regressions close to the identity line (P<0.001). The low coefficients of variation between two trials demonstrated the acceptable reliability of the proposed method. The proposed computations confirmed, from a biomechanical analysis, the positive relationship between power output, body mass and jump height, hitherto only shown by means of regression-based equations. Further, these computations pointed out that power also depends on push-off vertical distance. The accuracy and reliability of the proposed theoretical computations were in line with those observed when using laboratory ergometers such as force plates. Consequently, the proposed method, solely based on three simple parameters (body mass, jump height and push-off distance), allows to accurately evaluate force, velocity and power developed by lower limbs extensor muscles during squat jumps in field conditions.

Optimal pedalling velocity characteristics during maximal and submaximal cycling in humans

Abstract The aim of this study was to compare optimal pedalling velocities during maximal (OVM) and submaximal (OVSM) cycling in human, subjects with different training backgrounds. A group of 22 subjects [6 explosive (EX), 6 endurance (EN) and 10 non-specialised subjects] sprint cycled on a friction-loaded ergometer four maximal sprints lasting 6 s each followed by five 3-min periods of steady-state cycling at 150 W with pedalling frequencies varying from 40 to 120 rpm. The OVM and OVSM were defined as the velocities corresponding to the maximal power production and the lowest oxygen consumption, respectively. A significant linear relationship (r2 = 0.52, P < 0.001) was found between individual OVM [mean 123.1 (SD 11.2) rpm] and OVSM [mean 57.0 (SD 4.9) rpm, P < 0.001] values, suggesting that the same functional properties of leg extensor muscles influence both OVM and OVSM. Since EX was greater than EN in both OVM and OVSM (134.3 compared to 110.9 rpm and 60.8 compared to 54.0 rpm, P < 0.01 and P < 0.05, respectively) it could be hypothesised that the distribution of muscle fibre type plays an important role in optimising both maximal and submaximal cycling performance.

Force–velocity characteristics of upper limb extension during maximal wheelchair sprinting performed by healthy able-bodied females

The aim of this study was to determine the relationship between force and velocity parameters during a specific multi-articular upper limb movement – namely, hand rim propulsion on a wheelchair ergometer. Seventeen healthy able-bodied females performed nine maximal sprints of 8 s duration with friction torques varying from 0 to 4 N · m. The wheelchair ergometer system allows measurement of forces exerted on the wheels and linear velocity of the wheel at 100 Hz. These data were averaged for the duration of each arm cycle. Peak force and the corresponding maximal velocity were determined during three consecutive arm cycles for each sprint condition. Individual force–velocity relationships were established for peak force and velocity using data for the nine sprints. In line with the results of previous studies on leg cycling or arm cranking, the force–velocity relationship was linear in all participants (r = −0.798 to −0.983, P <0.01). The maximal power output (mean 1.28 W · kg−1) and the corresponding optimal velocity (1.49 m · s−1) and optimal force (52.3 N) calculated from the individual force–velocity regression were comparable with values reported in the literature during 20 or 30 s wheelchair sprints, but lower than those obtained during maximal arm cranking. A positive linear relationship (r = 0.678, P <0.01) was found between maximal power and optimal velocity. Our findings suggest that although absolute values of force, velocity and power depend on the type of movement, the force–velocity relationship obtained in multi-articular limb action is similar to that obtained in wheelchair locomotion, cycling and arm cranking.

Non-circular chainring improves aerobic cycling performance in non-cyclists

Abstract Non-circular chainrings alter the crank velocity profile over a pedalling cycle. The aim of this study was to investigate the effect of this altered crank velocity profile on the aerobic performance compared to a circular chainring (CC). Ten male non-cyclists performed two incremental maximal tests at 80 rpm on a cycle ergometer: one with a circular (Shimano) and the other with a non-circular chainring Osymetric® (Somovedi), at least 50 h apart. Each test started with a workload of 100 W lasting 3 min. During the first 12 min, the workload was increased by 30 W every 3 min. Thereafter, the workload was increased by 30 W every 2 min until exhaustion. The power output, the intra-cycle crank angular velocity and the physiological parameters were monitored continuously, averaged over the last 30 s of each increment and at exhaustion, and compared for the two chainrings. Results showed a higher maximal aerobic power attained with the non-circular chainring (362.6 ± 37.9 vs. 338.8 ± 32.6 W, p < .001; moderate effect), which could be explained by a significantly lower energy expenditure during the first increment at 100 W. It could be hypothesised that the use of the non-circular chainring allowed saving a small part of energy expenditure throughout the test, allowing the exhaustion of the subject at a higher increment for a similar maximal energy expenditure, in comparison with a CC. Although this improvement is obtained only for non-cyclists, it allowed highlighting the link between cycling equipment modifying the pedalling motion and physiological responses.

Influence of pedalling rate on the energy cost of cycling in humans. Answer to Piero Mognini, Franco Saibene, and Brian R. Umberger

First we agree with Mognoni that it could also be said that the data presented mainly demonstrate that the cost per revolution is minimum at a pedalling frequency that was very close to that freely chosen by road cyclists. In which case Fig. 1 better illustrates this concept than the Fig. 1 of the original article. fCr, min is then defined as the pedalling rate that minimizes the energy cost per pedal revolution (JÆkgÆrevolution) and will have the same value [101.1 (3.2) rpm] as calculated in the paper. This approach further supports the hypothesis that the preferred pedalling cadence is associated with lowerextremity net joint moments (see, for instance, March et al. 1998) and more generally with muscular effort exerted in cycling as well as in human gait (Saibene and Minetti 2003). Therefore we thank Piero Mognoni for his additional view and fruitful comments concerning the data presented in our paper. As mentioned by Brian R. Umberger, the differences in cadence set during friction ergometer tests would simply represent, in field conditions, the different gear ratio chosen by the riders at the selected speed. In agreement with him and with the equation given by di Prampero et al. (1979), the road velocity corresponding to 150 W is nearly 9 mÆs in zero wind speed and zero road grade conditions. It is then necessary to verify that the gear ratio usually selected and available on road bicycles is compatible with the cadence and friction force combinations used in the present study. Bicycles are usually equipped with front chainrings (at the pedal level) of 32 to 52 teeth and with rear sprockets of 11 to 28 teeth. It is then possible to obtain gear ratios ranging from 1.14 (32/28) to 4.73 (52/11). Considering that the diameter of the wheels of a standard bicycle are 0.7 m (measured at the rim of the tyres), one pedal revolution will then correspond to a displacement (at the road level) ranging from 2.5 m (32/ 28p0.7) to 10.4 m (32/28p0.7). At 9 ms velocity and according to the different cadences used in our experiment, one pedal revolution will correspond to the values given in Table 1. According to the data of Table 1, except for the 40 rpm condition, the cadence of the present study is compatible with 9 ms road displacement and with the gear ratios available on road bicycles. Because 40 rpm corresponds to the cadence that is the most different from the value of fCr, min, it does not significantly affect the discussion concerning fCr, min (the average value of fCr, min was 105.2 rpm when the 40 rpm cadence was not taken into account in the regression). We also agree that the method used to compute the energy cost of cycling (in JÆmÆkgand with a constant gear ratio corresponding to 6 m per pedal revolution) at 150 W is limited to very specific and narrow cases; for instance, with the ‘‘appropriate’’ changes in wind and/or grade nicely computed by Brian Umberger in his letter. However, there are some cycling competitions, especially on the track, where a zero grade, a zero wind speed and a constant gear ratio are encountered. For instance during the 1-h cycling world record reported by Padilla et al. (2000) the cyclist used a constant gear ratio of 59·14 (8.77 m per pedal revolution). The average velocity, power and cadence were respectively 53 kmÆh, 510 W, and 101 rpm. Although the average power was much higher than the 150 W of our study, the selected cadence was comparable to fCr, min, suggesting that in the 1-h cycling world record the gear ratio and the cadence selected by the cyclist and his coach corresponded to minimal energy cost per pedal revolution and possibly per meter. It would then be of interest in future studies to calculate fCr, min at power levels corresponding to track competitions. Therefore, we thank also Brian Umberger for his stimulating comments. Eur J Appl Physiol (2003) 90: 221–222 DOI 10.1007/s00421-003-0886-3

Effect of using poles on foot-ground kinetics during stance phase in trail running.

Abstract The aim of this study was to investigate the effect of using poles on foot–ground interaction during trail running with slopes of varying incline. Ten runners ran on a loop track representative of a trail running field situation with uphill (+9°), level and downhill (−6°) sections at fixed speed (3.2 m.s−1). Experimental conditions included running with (WP) and without (NP) the use of poles for each of the three slopes. Several quantitative and temporal foot–ground interaction parameters were calculated from plantar pressure data measured with a portable device. Using poles induced a decrease in plantar pressure intensity even when the running velocity stayed constant. However, the localisation and the magnitude of this decrease depended on the slope situations. During WP level running, regional analysis of the foot highlighted a decrease of the force time integral (FTI) for absolute (FTIabs; −12.6%; P<0.05) and relative values (FTIrel; −14.3%; P<0.05) in the medial forefoot region. FTIabs (−14.2%; P<0.05) and duration of force application (Δt; −13.5%; P<0.05) also decreased in the medial heel region when WP downhill running. These results support a facilitating effect of pole use for propulsion during level running and for the absorption phase during downhill running.

A soft ankle brace increases soleus Hoffman reflex amplitude but does not modify presynaptic inhibition during upright standing.

External ankle supports, such as ankle braces, may improve postural stability by stimulating cutaneous receptors. It remains unknown whether these supports have an effect on the posture central regulation. The aim of this study was to determine the effect of wearing a soft ankle brace on soleus H-reflex amplitude and presynaptic inhibition during standing. Sixteen subjects stood on a rigid floor with their eyes opened, either barefoot or wearing a soft ankle brace. H-reflex amplitude was measured on the soleus muscle by stimulating the tibial nerve electrically. Modulation of presynaptic inhibition was assessed by conditioning the H-reflex with fibular nerve (D1 inhibition) and femoral nerve (heteronymous facilitation) electrical stimulations. The unconditioned H-reflex amplitude was significantly greater when wearing the ankle brace than barefoot, whereas D1 and HF conditioned soleus H-reflex did not differ significantly between bracing conditions. These results suggest that the ankle brace increased the soleus motoneuron excitability without altering presynaptic mechanisms, potentially because of increased cutaneous mechanoreceptors afferent signals provided by the soft ankle brace.

The Effect of a Non-Circular Chainring on Cycling Performance

The aim of this work was to analyse the effect of a non-circular chainring during sub and supra-maximal cycling conditions on physiological, mechanical and muscular data. Results showed that the use of the non-circular chainring was beneficial during top and bottom dead centres by decreasing the effective force for a same external force for sub-maximal condition and by increasing the crank angular velocity for supra-maximal condition. However, this non-circular chainring was without effect during the pedal downstroke.