The purpose of the study was to compare the seasonal changes (preparation period [PP] and competition period [CP]) of vertical jumping performance and knee muscle strength in a team of under-19 women volleyball players (N = 12, 16.2 ± 1.5 years). The countermovement jump was used to evaluate jumping performance. The isometric knee extension moment at 150 ms from the onset of contraction (T150) and at a maximum of contraction (TMAX) were determined at 9 knee angles (from 10° to 90°, full knee extension = 0°). The peak isokinetic knee extension (TISOK-EXT) and flexion (TISOK-FLEX) moment were determined at 60, 180, and 240°·s. Repeated-measures analysis of variance was applied to the differences between PP and CP (p ≤ 0.05). Significant increases in jumping performance were found for jump height, peak impulse, total impulse, peak power, and takeoff velocity (p ≤ 0.05). At the knee flexion angles from 40° to 90°, T150 was significantly increased (p ≤ 0.05), whereas the increase was not significant at the rather extended knee angles of 10°, 20°, and 30° (p > 0.05). Only at 90° of knee flexion (p ≤ 0.05), TMAX was significantly increased. With the exception of TISOK-FLEX at 60°·s (p ≤ 0.05), the increases of TISOK-EXT and TISOK-FLEX were not significant (p > 0.05). The TISOK-EXT/TISOK-FLEX ratios were not significantly changed (p > 0.05). The main application of the study is that it provides performance standards and potential criteria for variable selection for jumping performance and knee muscle strength seasonal evaluation.

Field jumpers (n = 20) and volleyball players (n = 21). The knee extensor muscular strength measured at 9 knee angles was correlated with jumping height and peak power at the squat (SJ) and the countermovement (CMJ) vertical jump tests. Pearson product coefficient of correlation was used to test the significance of these relationships (p ≤ 0.05). The relationships were strong to moderate, but only in the volleyball players (p ≤ 0.05 and power of analysis >0.80). Specifically, in the volleyball players, the strong relationships were noted for muscular strength at the knee angle range of 40° to 90° and CMJ jumping height as well as SJ peak power. Results indicate the dissimilarity in the relationships between the knee extensor muscular strength and jumping performance in the young female track and field jumpers and volleyball players. In addition, it appears that the measure selected to evaluate jumping performance alters the correlational results....

The purpose of this study was to examine the differences in the ground reaction force (GRF) patterns between elite and novice players during two types of handball shots, as well as the relationships between throwing performance and the GRF variables. Ball velocity and throwing accuracy were measured during jump shots and 3-step shots performed by 15 elite and 15 novice players. The GRF pattern was recorded for the vertical and the anterior-posterior GRF components (Kistler forceplate type-9281, 750Hz). One-way ANOVA was used for the group differences and the Pearson coefficient for the correlation between throwing performance and GRF variables (SPSS 21.0, p ≤ 0.05). The elite players performed better in both types of shot. Both groups developed consistent and similar GRF patterns, except for the novices’ inconsistent Fz pattern in the 3-step shot. The GRF variables differed significantly between groups in the 3-step shot (p ≤ 0.05). Significant correlations were found only for ball velocity and predominantly for the novice players during the 3-step shot (p ≤ 0.05). The results possibly highlight a shortage in the novice ability to effectively reduce their forward momentum so as to provide a stable base of support for the momentum transfer up the kinetic chain, a situation that may predispose athletes to injury.

Standard moment-angle relations are typically obtained for values at maximum of contraction. Muscle properties for contraction times, similar to the short times of powerful athletic activities, may be more important than muscle properties that are obtained at maximum (TMAX) of contraction. The research question was whether moment-angle relations obtained during the initial time course of contraction are linearly scaled reflections of moment-angle relations obtained at maximum of contraction. The isometric moment of the knee extensor and the electromyographic activity (EMG) of the knee extensor and flexor muscles were measured at 9 knee angles in 22 elite female track and field jumpers. Absolute moments (Nm), relative moments (%TMAX) and EMG integrals were determined at 30 ms, 50 ms, 150 ms and 200 ms from the onset of contraction and at maximum. For moment-angle relations obtained during the initial time course of contraction, there was a shift of the optimum knee angle to a more extended knee. During the initial time course of the contraction, %TMAX moment was greater near full knee extension. The EMG activity was the same across knee angles. The results may have important practical implications for muscular force evaluation for activities where maximum force production is coupled to a short activation time.

In popular fitness modalities, the participants often perform abdominal breathing while maintaining stable or rather unstable as well as inverted body stances that may challenge the respiratory muscles’ activation. This study aimed to examine the abdominal breathing effect on postural stability and the respiratory muscles’ activation during four body stances: the Upright Stance, the Quadrupled Inverted V, the Elbow Side-Bridge, and the Headstand. Participants (n = 29) maintained (40 s) the body stances under regular and abdominal breathing (the latter verified through visual inspection and 3D inertial sensing of the abdominal wall angular displacements, LORD-MicroStrain^®, 100 Hz, MicroStrain, Inc., Williston, VT, USA). The trajectory of the center of pressure (CoP) (Kistler force plate, 100 Hz, Kistler Group, Winterthur, Switzerland) was recorded in synchronization with the respiratory muscles’ (sternocleidomastoid, external intercostals, diaphragm, rectus abdominis) vibromyographic activation (Biopac VMG sensors, 2000 Hz, Biopac Systems, Inc., Santa Barbara, CA, USA). Abdominal breathing had a significant (p ≤ 0.05) deteriorating effect on postural stability and an increasing one on the respiratory muscles’ activation; however, this was not consistent across body stances. The body stance specificity of the abdominal breathing effect justifies the purpose of the present study. Thus, before the request for abdominal breathing in popular fitness modalities, one should acknowledge the postural and the breathing demands of each particular stance, particularly for the inverted ones.

Sit-to-Walk (STW) is a critical task for daily independence, yet its two inherent destabilizing events (seat-off, walking initiation) may diminish postural stability under fast motion speed (FS). This study aimed at the FS effect on the STW spatial and temporal patterns, with a specific interest in the relative STW temporal pattern. The STW kinetics and kinematics were recorded (n=18 men, 20.7±2.0 years) at preferred and FS. Statistics included One-Way repeated measures ANOVA (SPSS 25.0, p≤0.05). The FS spatial pattern reveals a discontinuous mode of the forward ground reaction force, indicating a balance rather than a propulsive strategy during the Rising phase. The FS relative temporal pattern reveals the prolongation of the Leaning phase (most possibly due to the feet repositioning), the shortening of the Rising and the Walking phases, and a relative delay in the spatial variables (p≤0.05). Overall, the results do not allow the STW consideration at FS as a “magnified” with respect to force, or a “shrinked-in” with respect to time, copy of the preferred motion speed. As more generic and versatile than the absolute one, the relative temporal pattern may be used as a reference for a variety of populations.