Electromechanical delay in isometric actions initiated from nonresting levels.

PURPOSE: The purpose of this study was to determine whether electromechanical delay (EMD) was associated exclusively with the onset of tension from a resting state and whether EMD remained constant across different rates of force development. METHODS: Twenty-four subjects (23.9 +/- 5.4 yr, 171.7 +/- 7.3 cm, 72.9 +/- 12.8 kg) performed isometric elbow flexion trials in the transverse plane by using the dominant arm during which isometric force data and surface EMG activity were collected. Subjects completed three trials to establish a maximal force (MF) reference. Subjects then completed trials in which pulse forces of varying magnitudes were elicited at a frequency of 1 Hz from different baseline intensities. All forces were expressed relative to MF. Three trials of the following conditions (baseline-pulse) were performed in random order: 0-25%, 25-50%, 50-75%, 0-50%, and 0-75%. EMG and force data were collected for 10 pulse cycles during these trials. EMD was defined as the temporal shift that maximized a normalized cross-correlation function. RESULTS: EMD for a 25% pulse force developed from rest (83.5 +/- 12.9 ms) was significantly longer than that developed from 25% (66.3 +/- 11.5 ms) or 50% (60.6 +/- 16.6 ms) baselines. EMD values were not different when force was developed from 25% and 50% baselines. EMD associated with a 25% pulse force from rest was significantly longer than 50% (70.3 +/- 10.0 ms) and 75% (68.9 +/- 8.7 ms) pulse forces from rest. EMD for 50% and 75% pulse forces from rest were not statistically different. CONCLUSION: It was concluded that EMD is present during exertions initiated from both resting and nonresting states but is reduced when exertions are initiated from non-resting states and with higher rates of force development.

Effects of varying central command and muscle mass on the cardiovascular responses to isometric exercise.

To determine if the central command signal associated with isometric exercise is mass-dependent, 20 subjects (nine male, 11 female; 23 +/- 1 years) performed four 5-min bouts of supine isometric exercise with a large (quadriceps; LEG) and small (forearm; ARM) muscle mass. For each extremity, one bout entailed maintaining a constant force (CF; 20% maximal voluntary contraction) and the other constant electromyographic activity (CE; approximately 20% MVC initially). Central command was assumed to increase with CF and remain unchanged with CE. Heart rate increased more with LEG than ARM (P<0.001) and, in LEG, was higher in CF than CE at min 5 (P<0.001). Mean arterial pressure was higher in LEG (P<0.001) by min 2 and 10 +/- 3 mmHg higher in LEG CF than LEG CE by min 5 (P<0.001). Ratings of perceived exertion were highest in LEG CF (P<0.001); LEG CE did not differ from ARM CE (P<0.001) by min 4. The ARM responses did not differ between CF and CE in any variable. These data suggest that muscle mass influences the central command signal during isometric exercise and central command modulates this response in larger muscle masses.

Sex differences in the centre of buoyancy location of competitive swimmers.

The aims of this study were to identify differences in the centre of buoyancy (CB) and centre of mass (CM) locations of male and female collegiate swimmers, and to assess the influence that buoyancy has on freestyle kicking performance. Sixteen female collegiate swimmers (mean +/- s: age 19.1 +/- 1.2 years) had significantly more adipose tissue (20.2 +/- 4.4%) than 15 male collegiate swimmers (19.9 +/- 1.0 years, 12.6 +/- 3.8%). The ratio of the sum of abdominal and suprailiac skinfolds to the thigh skinfold was significantly greater for the males (2.07 +/- 0.37) than the females (1.31 +/- 0.32), implying that females had proportionately more fatty tissue caudally than males. The distance d between the centres of buoyancy and mass was significantly larger for the males (0.79 +/- 0.43 cm) than the females (0.16 +/- 0.34 cm). Both points were more caudal in the female subjects (59.9 +/- 0.7% and 59.8 +/- 0.7% of body height respectively) than in the male subjects (61.7 +/- 0.8% and 61.2 +/- 0.9% respectively). These data suggest that the difference in d may be attributed to the difference in the location of the centre of buoyancy, because the centre of mass difference was not significant and was characterized by a smaller effect size. The amount and distribution of adipose tissue accounted for a significant proportion of variance in d (R2 = 0.25 and 0.29 respectively). Males had a significantly higher proportional kick time, defined as the ratio of times to complete a 22.9 m sprint when kicking and swimming respectively, than females (1.57 +/- 0.09 and 1.51 +/- 0.13 respectively). This shows that the male swimmers kicked proportionally more slowly than the female swimmers. However, the distance d did not account for a significant proportion of variance in the proportional kick time. Therefore, our results do not support the notion that skilled male swimmers are at a performance disadvantage in terms of natural buoyancy characteristics.

NLT and extrapolated DLT:3-D cinematography alternatives for enlarging the volume of calibration.

This study investigated the accuracy of the direct linear transformation (DLT) and non-linear transformation (NLT) methods of 3-D cinematography/videography. A comparison of standard DLT, extrapolated DLT, and NLT calibrations showed the standard (non-extrapolated) DLT to be the most accurate, especially when a large number of control points (40-60) were used. The NLT was more accurate than the extrapolated DLT when the level of extrapolation exceeded 100%. The results indicated that when possible one should use the DLT with a control object, sufficiently large as to encompass the entire activity being studied. However, in situations where the activity volume exceeds the size of one's DLT control object, the NLT method should be considered.