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Nicole A. Dinn and David G. Behm

Purpose:

Studies have both supported and refuted the concept that it is the intent to perform ballistic contractions that determines velocity-specific gains in resistance training. The purpose of this investigation was to determine whether ballistic intent is as effective as ballistic movement in improving muscle activation, force, movement time, and reaction time.

Methods:

Subjects completed 8 wk of punch training. A dynamic (DYN) group trained with elastic resistance bands, and the isometric (ISO) group trained with an unyielding strap. A control (CTRL) group was also tested. Pretesting and posttesting measures included isometric force; electromyography (EMG) of triceps, biceps, pectoralis major, and latissimus dorsi; movement and reaction time of both arms; and a quick-hands test of coordination.

Results:

Triceps iEMG increased by 63% in the ISO group (P = .03). Pectoralis major iEMG increased by 65% in the DYN group (P = .007). Movement time decreased 17.6% in the DYN training group (P = .001). Isometric force did not improve in either training group or in the CTRL group.

Conclusions:

Because of its specificity of movement, dynamic training might be a more appropriate method to improve punching speed for martial artists and boxers. The intent to contract explosively over a short duration does not appear to be beneficial in increasing force production or speed of movement in punching.

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Jeni R. McNeal, William A. Sands and Michael H. Stone

Purpose:

The aim of this study was to investigate the effects of a maximal repeated-jumps task on force production, muscle activation and kinematics, and to determine if changes in performance were dependent on gender.

Methods:

Eleven male and nine female athletes performed continuous countermovement jumps for 60 s on a force platform while muscle activation was assessed using surface electromyography. Performances were videotaped and digitized (60 Hz). Data were averaged across three jumps in 10-s intervals from the initial jump to the final 10 s of the test.

Results:

No interaction between time and gender was evident for any variable; therefore, all results represent data collapsed across gender. Preactivation magnitude decreased across time periods for anterior tibialis (AT, P < .001), gastrocnemius (GAS, P < .001) and biceps femoris (BF, P = .03), but not for vastus lateralis (VL, P = .16). Muscle activation during ground contact did not change across time for BF; however, VL, G, and AT showed significant reductions (all P < .001). Peak force was reduced at 40 s compared with the initial jumps, and continued to be reduced at 50 and 60 s (all P < .05). The time from peak force to takeoff was greater at 50 and 60 s compared with the initial jumps (P < .05). Both knee fexion and ankle dorsifexion were reduced across time (both P < .001), whereas no change in relative hip angle was evident (P = .10). Absolute angle of the trunk increased with time (P < .001), whereas the absolute angle of the shank decreased (P < .001).

Conclusions:

In response to the fatiguing task, subjects reduced muscle activation and force production and altered jumping technique; however, these changes were not dependent on gender.

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Tobias Kalenscher, Karl-Theodor Kalveram and Jürgen Konczak

This study investigated force adaptation in humans during goal-directed flexion forearm motion. The ability of the motor system to adapt to changes in internal or external forces is essential for the successful control of voluntary movement. In a first experiment, we examined how under- or overdamping differentially affected the length of the adaptation and the arm kinematics between force transitions. We found that transitions diverging from a null-force produced larger transition effects than transitions converging to a null force condition, indicating that re-adaptation was less error-prone. Whether the subjects had previously experienced underdamping or the null-force had no significant impact on the spatial trajectory after switching to overdamping. That is, prior force experience had no differential effect on the spatial transition kinematics. However, the transitions underdamping-to-overdamping and underdamping-to–null force did produce differently strong transition effects. These results indicate that exposure to the new force rather than previous force-field experience is responsible for transition- and after-effects. In a second experiment, we investigated whether learning was law-like—that is, whether it generalized to unvisited workspace. Subjects were tested in new, unvisited workspaces in the null-force condition after sufficient training in either force condition. The occurrence of transferred after-effects indicated that adaptation to both positive and negative damping was mediated by rule-based rather than exclusive associative processes.

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Ewald M. Hennig and Mario A. Lafortune

Using data from six male subjects, this study compared ground reaction force and tibial acceleration parameters for running. A bone-mounted triaxial accelerometer and a force platform were employed for data collection. Low peak values were found for the axial acceleration, and a time shift toward the occurrence of the first peak in the vertical force data was present. The time to peak axial acceleration differed significantly from the time to the first force peak, and the peak values of force and acceleration demonstrated only a moderate correlation. However, a high negative correlation was found for the comparison of the peak axial acceleration with the time to peak vertical force. Employing a multiple regression analysis, the peak tibial acceleration could be well estimated using vertical force loading rate and peak horizontal ground reaction force as predictors.

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Xiaogang Hu and Karl M. Newell

The purpose of this study was to investigate the mechanisms contributing to the different scaling functions between force and force variability in continuous and discrete isometric forces. Muscle forces were simulated with the Fuglevand et al. (1993) model of motor unit recruitment and rate coding, and a range of recruitment and firing properties were manipulated. The influence of time-to-peak force on the discrete force variability was also examined. The results revealed that the peak firing rate, the synchrony between motoneurons, and the recruitment range contributed to the different variability functions in continuous and discrete forces. The shorter time-to-peak force led to higher variability in the peak force. The findings show that the model can produce the distinct properties of the force variability scaling functions in continuous and discrete forces. The simulation results provide preliminary insight into the neuromuscular mechanisms of the different force variability functions in continuous and discrete isometric forces.

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Jacob J. Sosnoff, Sae Young Jae, Kevin Heffernan and Bo Fernhall

The purpose of the current investigation was to examine the relation between cardioballistic impulse and the fluctuations in continuous isometric force production. Subjects produced isometric force via index finger flexion to constant force targets (0.5, 1 and 2 N) with and without visual feedback while beat to beat blood pressure of their middle finger was recorded. Force fluctuations were quantified using distributional statistics. The association between blood pressure oscillations and fluctuations in force output were quantified with coherence analysis. Overall, it was found that force variability (i.e., SD) increased with force level and removal of visual feedback. Coherence values between blood pressure oscillations and force fluctuations were significant and the greatest in the 8–12 Hz bandwidth. There was no effect of force magnitude on the coupling strength between blood pressure oscillations and force production. This coupling was greater in the visual condition. These data suggest that peripheral alterations in blood pressure are related to fluctuations in isometric force production independent of force level and that this interaction is influenced by visual feedback.

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Travis J. Peterson, Rand R. Wilcox and Jill L. McNitt-Gray

Our aim was to determine how skilled players regulate linear and angular impulse while maintaining balance during the golf swing. Eleven highly-skilled golf players performed swings with a 6-iron and driver. Components contributing to linear and angular impulse generated by the rear and target legs (resultant horizontal reaction force [RFh], RFh-angle, and moment arm) were quantified and compared across the group and within a player (α = .05). Net angular impulse generated by both the rear and target legs was greater for the driver than the 6-iron. Mechanisms used to regulate angular impulse generation between clubs varied across players and required coordination between the legs. Increases in net angular impulse with a driver involved increases in target leg RFh. Rear leg RFh-angle was maintained between clubs whereas target leg RFh became more aligned with the target line. Net linear impulse perpendicular to the target line remained near zero, preserving balance, while net linear impulse along the target line decreased in magnitude. These results indicate that the net angular impulse was regulated between clubs by coordinating force generation of the rear and target legs while sustaining balance throughout the task.

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Jefferson W. Streepey, M. Melissa Gross, Bernard J. Martin, Sundravalli Sudarsan and Catherine M. Schiller

The relationship between playing surface and muscle fatigue was examined in 22 male subjects performing a simulated basketball task on a conventional wood floor and less stiff composite floor. Force and electromyographic activity (EMG) were measured during maximum and submaximum (10% of maximum) voluntary contractions of knee extensor and ankle plantarflexor muscles before and after completion of the simulated basketball task. Jump height was evaluated during the task, and perceived fatigue was assessed at the end of the task. Although not all subjects jumped significantly higher on the composite floor compared to the wood floor. competitive basketball players showed a significant improvement in jump height (3.4 cm. 6%) when jumping on the composite floor. Perceived fatigue was significantly lower for the composite floor (21.7%) than the wood floor (30.2%). The objective measures indicated the occurrence of fatigue; however, force and EMG magnitudes obtained during maximum exertions were not sensitive lo floor types. Post-task increase in EMG magnitude indicated a significant fatigue effect for the soleus muscle on the wood floor only. These findings suggest that the composite floor may benefit human performance without increasing fatigue during basketball-related activities.

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Lars Janshen, Klaus Mattes and Günter Tidow

In sweep-oar rowers, asymmetrical force production of the legs is a known phenomenon. The purpose of this study was to investigate the muscular activity of the legs that may cause this asymmetry even when oarsmen perform a symmetrical endurance task. Seven male young elite oarsmen performed an all-out 2000-m test on a rowing ergometer. During stroke kinematics, myoelectric activity of six muscles of each leg and pressure distribution under both feet were measured. Data were collected over two 30-s time windows starting 1 and 5 min after the test started. No significant differences were observed between legs and time windows for the range of motion of the hip, knee, and ankle joint as well as for the onset/offset timing of muscles. However, in the drive phase, the knee and hip muscles of the leg on the oar side (inside leg) showed 20–45% (both p < .05) higher activation intensities compared with the leg opposite the oar (outside leg). Corresponding to this, 56–91% (both p < .05) higher mean pressure values under the ball of the inside foot compared with the outside foot indicated an asymmetrical force production of the legs even under kinematically symmetrical working conditions.

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Robert W. Meyers, Jon L. Oliver, Michael G. Hughes, Rhodri S. Lloyd and John B. Cronin

Purpose:

The aim of this study was to examine the influence of age and maturation upon magnitude of asymmetry in the force, stiffness and the spatiotemporal determinants of maximal sprint speed in a large cohort of boys.

Methods:

344 boys between the ages of 11 and 16 years completed an anthropometric assessment and a 35 m sprint test, during which sprint performance was recorded via a ground-level optical measurement system. Maximal sprint velocity, as well as asymmetry in spatiotemporal variables, modeled force and stiffness data were established for each participant. For analysis, participants were grouped into chronological age, maturation and percentile groups.

Results:

The range of mean asymmetry across age groups and variables was 2.3–12.6%. The magnitude of asymmetry in all the sprint variables was not significantly different across age and maturation groups (p > .05), except relative leg stiffness (p < .05). No strong relationships between asymmetry in sprint variables and maximal sprint velocity were evident (rs < .39).

Conclusion:

These results provide a novel benchmark for the expected magnitude of asymmetry in a large cohort of uninjured boys during maximal sprint performance. Asymmetry in sprint performance is largely unaffected by age or maturation and no strong relationships exist between the magnitude of asymmetry and maximal sprint velocity.