Effects of replica running shoes upon external forces and muscle activity during running.

Twelve participants ran (9 km · h(-1)) to test two types of running shoes: replica and original shoes. Ground reaction force, plantar pressure and electromyographic activity were recorded. The shoes were tested randomly and on different days. Comparisons between the two experimental conditions were made by analysis of variance (ANOVA) test (P ≤ 0.05). The time to first peak, loading rate of the first peak and impulse of the first 75 ms of stance were significantly different between the shoes (P ≤ 0.05), revealing an increase of impact forces for the replica shoes. The peak plantar pressure values were significantly higher (P ≤ 0.05) when wearing replica shoes. During running, the contact area was significantly smaller (P ≤ 0.05) for the replica shoe. The electromyographic activity of the analysed muscles did not show changes between the two shoes in running. These findings suggest that the use of replica running shoes can increase the external load applied to the human body, but may not change the muscle activity pattern during locomotion. This new mechanical situation may increase the risk of injuries in these movements.

Perception of impact is affected by stimulus intensity.

Perception of external loads may be a central topic to understand adjustments to the mechanical demands during movement. Nevertheless, the association between the perceived and the real load received is still controversial. This study aimed to correlate vertical ground reaction force (vGRF) to the perception of impact in different regimens of stimulus application. Ten physically active men performed drop jumps from four different heights (0.20, 0.40, 0.60 and 0.80 m). A force plate measured the vGRF, while perception of impact was evaluated through Borg's Ratings of Perceived Exertion. Higher values of maximum vGRF (Fy_max) and impulse of the first 50 ms (I_50), and reduced time to reach Fy_max indicate increased external forces as drop jump height raised. Perception of impact increased gradually with increasing jump height for I_50. Fy_max and I_50 showed moderate to strong correlations to perceived load for 70% and 90% of participants, respectively. Higher and different intensity of stimulus facilitated the perception of impact, presenting moderate to strong correlations to kinetic parameters related to external load during landing from drop jump. Perception of higher impacts could be used as a surrogate to monitor 'real' impacts and possibly also for managing impact-related injury risk.

Análise da percepção sobre estímulos mecânicos durante a corrida e os saltos / Analysis of the perception of mechanical stimuli during running and jumping

The aim of this study is to verify the correlation between load perception and component of Ground Reaction Force (GRF) during running and jumping. GRF and subjective data (Borg's Ratings of Perceived Exertion) were measured during two different conditions: condition 1 (P1) during running on a treadmill, and condition 2 (P2) during landing. Although load impact was not an experimental factor for P1, dynamical variables related to GRE were affected by time (p<0.0001). However, some variables (Fy 1, Dt Fy 1, GC, and 1_75) and load perception were not correlated. Small range of mechanical load variation during P1 did mot allow the quantification of any effect of load impact heights (0.20 up to 0.80 m). There is correlation between load perception and GC (r =.72 and r=0.53, for S1 e S2, respectively), 1_75 (r = 0.74 and r = 0.83, for S1 and S2, respectively), AND Fy 1 (r = 0.56, and r = 0.62). These results show that P2 provides a better resolution for the relation between load perception and mechanical loads applied to locomotor system. As a conclusion, mechanical load intensity affects how quantify load perception.

Efeito da normalização na força de reação do solo durante a locomoção / Effect of standardization in the ground reaction forces during gait

Normalization is a process to remove the effect of a variable into another to decrease its variability. Normalization is also a process to scale a variable. For gait analysis, the most common normalization is to divide Ground Reaction Force by body weight. The objective of this paper is to analyze the effects of normalizalizing a set of ground reaction force parameters. Thirteen adults were the subjects of this study. A force plataform was used to measure the vertical component of ground reaction (vGRF) during stance phase. Twenty samples for each subject were obtained (10 for walking with and other 10 for walking barefoot). For each condition, 5 samples were due to the left support and other 5 were due to the rigth support. Some parameters were calculated using vGRF: intensity, impulse and inclination. The raw signal was filtered with low-pass, 2nd order, zero lag, Butterworth filter at 100 Hz. For statistical analysis, different produres were used: to compare groups, we used two-way analysis of variance; to study the effect of normalization into the signal's variability, we used the principal component analysis. The main results suggest that normalization affects those paraeters, and it is not affected by shoes condiction, chaging the metric among them. However, this is not a pure lenear effect and probably it is also affected by a covariable. We suggest that time is one covariate. We also suggest that other experiments should be run to analyze such effect, considering as covariables gait speed, time, shoes and frequency components.