Number of Trials Necessary to Achieve Performance Stability of Selected Ground Reaction Force Variables During Landing

The objectives were to determine the number of trials necessary to achieve performance stability of selected ground reaction force (GRF) variables during landing and to compare two methods of determining stability. Ten subjects divided into two groups each completed a minimum of 20 drop or step-off landings from 0.60 or 0.61 m onto a force platform (1000 Hz). Five vertical GRF variables (first and second peaks, average loading rates to these peaks, and impulse) were quantified during the initial 100 ms post-contact period. Test-retest reliability (stability) was determined using two methods: (1) intra-class correlation coefficient (ICC) analysis, and (2) sequential averaging analysis. Results of the ICC analysis indicated that an average of four trials (mean 3.8 ± 2.7 Group 1; 3.6 ± 1.7 Group 2) were necessary to achieve maximum ICC values. Maximum ICC values ranged from 0.55 to 0.99 and all were significantly (p < 0. 05) different from zero. Results of the sequential averaging analysis revealed that an average of 12 trials (mean 11.7 ± 3.1 Group 1; 11.5 ± 4.5 Group 2) were necessary to achieve performance stability using criteria previously reported in the literature. Using 10 reference trials, the sequential averaging technique required standard deviation criterion values of 0.60 and 0.49 for Groups 1 and 2, respectively, in order to approximate the ICC results. The results of the study suggest that the ICC might be a less conservative, but more objective method for determining stability, especially when compared to previous applications of the sequential averaging technique. Moreover, criteria for implementing the sequential averaging technique can be adjusted so that results closely approximate the results from ICC. In conclusion, subjects in landing experiments should perform a minimum of four and possibly as many as eight trials to achieve performance stability of selected GRF variables. Researchers should use this information to plan future studies and to report the stability of GRF data in landing experiments.

Key points

• The number of trials obtained from a subject in an experiment influences the stability (test-retest reli-ability) and thus validity of the data.
• One trial might not be representative of a subject''s more general performance.
• Multiple-trial protocols have been recommended by several researchers for a variety of activities, but the number of trials necessary to achieve stabil-ity of ground reaction force variables during land-ing has not been examined.
• Researchers have used different criteria and meth-odologies for determining stability, making com-parisons among studies and activities difficult.
• In the current study, test-retest intra-class correla-tion coefficient revealed that on average four trials were necessary for stability, while the more con-servative sequential averaging analysis suggested that 12 trials were necessary for stability.
• Researchers should be aware of the stability of landing data and collect enough trials from each subject within a single testing session to maximize reliability of their data.

Key words: Reliability, variability, sequential averaging, intra-class correlation coefficient.     

Reliability and Validity of the Polar V800 Sports Watch for Estimating Vertical Jump Height

This study aimed to assess the reliability and validity of the Polar V800 to measure vertical jump height. Twenty-two physically active healthy men (age: 22.89 ± 4.23 years; body mass: 70.74 ± 8.04 kg; height: 1.74 ± 0.76 m) were recruited for the study. The reliability was evaluated by comparing measurements acquired by the Polar V800 in two identical testing sessions one week apart. Validity was assessed by comparing measurements simultaneously obtained using a force platform (gold standard), high-speed camera and the Polar V800 during squat jump (SJ) and countermovement jump (CMJ) tests. In the test-retest reliability, high intraclass correlation coefficients (ICCs) were observed (mean: 0.90, SJ and CMJ) in the Polar V800. There was no significant systematic bias ± random errors (p > 0.05) between test-retest. Low coefficients of variation (<5%) were detected in both jumps in the Polar V800. In the validity assessment, similar jump height was detected among devices (p > 0.05). There was almost perfect agreement between the Polar V800 compared to a force platform for the SJ and CMJ tests (Mean ICCs = 0.95; no systematic bias ± random errors in SJ mean: -0.38 ± 2.10 cm, p > 0.05). Mean ICC between the Polar V800 versus high-speed camera was 0.91 for the SJ and CMJ tests, however, a significant systematic bias ± random error (0.97 ± 2.60 cm; p = 0.01) was detected in CMJ test. The Polar V800 offers valid, compared to force platform, and reliable information about vertical jump height performance in physically active healthy young men.Key points

• The Polar V800 sports watch was shown to be a reliable and valid tool for measuring jumping ability.
• The Polar V800 is a simple, multifunctional, inexpensive and practical device that offers adequate information about the vertical jump height performance in physically active healthy young men.
• It would also be interesting to demonstrate the reliability and validity of the Polar V800 sports watch to assess vertical jump height in highly trained athletes or other populations.

Key words: Jumping ability, squat jump, countermovement jump, pulsometer, Bland Altman plot, intraclass correlation coefficient     

The Influence of Vibration on Muscle Activation and Rate of Force Development during Maximal Isometric Contractions

At present there appears to be a need for research conducted on the effects of vibration on the contractile ability of skeletal muscle tissue. The aim of this study was to address this issue by examining the effects of a superimposed muscle/tendon vibration at 50.42±1.16 Hz (acceleration 13.24 ± 0.18ms^-2: displacement ≈5mm) on muscular activation and maximal isometric contraction. Sixteen participants with a mean age, body mass, and height of 22 ± 4.4 years, 73.2 ± 11.7 kg and 173.1 ± 9.7 cms, respectively, were recruited for this study. Electromyography and accelerometry from the rectus femoris, and maximal isometric force data characteristics were collected from the dominant limb under conditions of vibration, and no-vibration. A superimposed 50 Hz vibration was used during the contraction phase for the maximal isometric leg extension for the condition of vibration. A one-way ANOVA revealed no significant (p > 0.05) differences between the vibration and no-vibration conditions for peak normalized EMG_RMS (84.74% Vs 88.1%) values. An ANOVA revealed significant (p > 0.05) differences between the peak fundamental frequencies of the FFT between the conditions vibration (27.1 ± 12.2 Hz) and no-vibration (9.8 ± 3.5 Hz). Peak isometric force, peak rate of force development, rate of force development at times 0.05, 0.01, 0.1, 0.5 seconds, and rate of force development at 50, 75, and 90% of peak force were not significantly different. The results of this study suggest that the application of vibration stimulation at 50 Hz during the contraction does not contribute to muscle activation, or enhance force production for maximal isometric contractions.

Key Points

• The application of a vibratory stimulation to the human body increases the normal acceleration resulting in an increase in force and a change in performance
• This study was to address this issue by examining the effects of a direct superimposed muscle/tendon vibration at 50 Hz on isometric strength characteristics
• No improvement or change in isometric force or rate of force development
• No changes to peak normalized EMG_RMS values

Key words: Strength, oscillations, isometric, peak, muscle activation     

Flexibility is not Related to Stretch-Induced Deficits in Force or Power

Previous studies have demonstrated that an acute bout of static stretching may cause significant performance impairments. However, there are no studies investigating the effect of prolonged stretch training on stretch-induced decrements. It was hypothesized that individuals exhibiting a greater range of motion (ROM) in the correlation study or those who attained a greater ROM with flexibility training would experience less stretch-induced deficits. A correlation study had 18 participants (25 ± 8.3 years, 1.68 ± 0.93 m, 73.5 ± 14.4 kg) stretch their quadriceps, hamstrings and plantar flexors three times each for 30 s with 30 s recovery. Subjects were tested pre- and post-stretch for ROM, knee extension maximum voluntary isometric contraction (MVIC) force and drop jump measures. A separate training study with 12 subjects (21.9 ± 2.1 years, 1.77 ± 0.11 m 79.8 ± 12.4 kg) involved a four-week, five-days per week, flexibility training programme that involved stretching of the quadriceps, hamstrings and plantar flexors. Pre- and post-training testing included ROM as well as knee extension and flexion MVIC, drop and countermovement jump measures conducted before and after an acute bout of stretching. An acute bout of stretching incurred significant impairments for knee extension (-6.1% to -8.2%; p < 0.05) and flexion (-6.6% to -10.7%; p < 0.05) MVIC, drop jump contact time (5.4% to 7.4%; p < 0.01) and countermovement jump height (-5.5% to -5.7%; p < 0.01). The correlation study showed no significant relationship between ROM and stretch-induced deficits. There was also no significant effect of flexibility training on the stretch-induced decrements. It is probable that because the stretches were held to the point of discomfort with all testing, the relative stress on the muscle was similar resulting in similar impairments irrespective of the ROM or tolerance to stretching of the muscle.

Key Points

• A correlation and training study were used to examine the effects of increased range of motion on stretch-induced changes in force and jump measures
• An acute bout of stretching incurred significant impairments for knee extension and flexion MVIC, drop jump contact time and countermovement jump height.
• Neither study showed any significant relationship between ROM and stretch-induced deficits.

Key words: Flexibility, force, jumps, static stretching     

Does Acute Vibration Exercise Enhance Horizontal Jump Performance?

The aim of this study was to investigate the acute effect of vibration exercise (VbX) on repetitive horizontal jumping performance and to examine the duration of the rest interval between the horizontal jump sets following acute VbX. Fourteen track athlete males (age 20.8 ± 1.8 yr; height 1.80 ± 0.05 m; body mass 73.1 ± 7.5 kg) performed four conditions in a randomised order; (a) VbX with 1 min rest between repetitive horizontal jump (RHJ) sets [VbX-1min]; (b) VbX with 2 min rest between RHJ sets [VbX-2min]; (c) No VbX with 1 min rest between RHJ sets [Con-1min]; (d) No VbX with 2 min rest between RHJ sets [Con-2min]. Intermittent VbX (six 60 s exposures with 30 s rest) at 26 Hz (6 mm peak-to-peak displacement) was performed in an isometric squat position (120° of knee flexion). The mean values of distance, velocity and time taken of RHJ from the four conditions were used in repeated measures [condition (VbX and Control) and rest period (1min; and 2min)] ANOVA. There was a condition effect such that VbX significantly increased RHJ distance (p < 0.05) compared to control (no VbX). Furthermore, VbX significantly increased RHJ velocity (p < 0.05) compared to no VbX and there was an interaction effect (condition x rest) where the velocity was significantly higher in VbX-2min compared to VbX-1 min, Con-2min, and Con-1min respectively. Acute intermittent VbX has the ability to enhance repetitive horizontal jump distance and velocity, which could be used as an additional method for warm-up intervention to increase explosive power performance.

Key points

• Acute intermittent VbX can enhance repetitive horizontal jump distance and velocity.
• Acute intermittent VbX may be used as an additional method for warm-up intervention to increase explosive power performance.

Key words: Repetitive jump, sets, rest, recovery, velocity     

Characterisation of the Mechanical Loads and Metabolic Intensity of the CAPO Kids Exercise Intervention for Healthy Primary School Children

Sedentarism is associated with obesity and other chronic diseases at all ages. Increasing physical activity with in-school interventions, focusing on energy expenditure and bone loading reduces risk of a number of costly chronic diseases. The aim of the current study was to characterise the metabolic and musculoskeletal load intensity of the recent successful CAPO Kids exercise intervention. Pre and early pubertal children (10.4 ± 0.5 years old) from the CAPO Kids trial wore an armband sensor to estimate energy expenditure during a 10-minute CAPO Kids session. Eleven participants performed manoeuvres from the session on a force platform to determine vertical ground reaction forces. In total, 28 boys and 20 girls had armband measures and 11 boys and girls undertook GRF testing. The energy expenditure associated with the 10-minute session was 39.7 ± 9.3 kcal, with an average of 4 kcal·min-1. The intensity of physical activity was ‘vigorous’ to ‘very vigorous’ for 34% of the session. Vertical ground reaction forces of the CAPO Kids manoeuvres ranged from 1.3 ± 0.2 BW (cartwheels) to 5.4 ± 2.3 BW (360° jump). CAPO Kids generates adequate load intensity to stimulate positive health adaptations in both metabolic and musculoskeletal systems of pre and early pubertal children.

Key points

• Energy expenditure of a single bout of CAPO Kids yields 39.7±9.3 kcal and includes activities performed at a vigorous and very vigorous intensity.
• Mechanical loads associated with CAPO Kids surpass five times bodyweight and more than 140 bodyweights per second.
• CAPO Kids intervention represents a viable approach to stimulate musculoskeletal and metabolic adaptation in children.

Key words: Pediatrics, energy expenditure, ground reaction forces, physical activity     

Jump Kinetic Determinants of Sprint Acceleration Performance from Starting Blocks in Male Sprinters

The purpose of this research was to identify the jump kinetic determinants of sprint acceleration performance from a block start. Ten male (mean ± SD: age 20 ± 3 years; height 1.82 ± 0.06 m; weight 76.7 ± 7.9 kg; 100 m personal best: 10.87 + 0.36 s {10.37 - 11.42}) track sprinters at a national and regional competitive level performed 10 m sprints from a block start. Anthropometric dimensions along with squat jump (SJ), countermovement jump (CMJ), continuous straight legged jump (SLJ), single leg hop for distance, and single leg triple hop for distance measures of power were also tested. Stepwise multiple regression analysis identified CMJ average power (W/kg) as a predictor of 10 m sprint performance from a block start (r = 0.79, r² = 0.63, p<0.01, SEE = 0.04 (s), %SEE = 2.0). Pearson correlation analysis revealed CMJ force and power (r = -0.70 to -0.79; p = 0.011 - 0.035) and SJ power (r = -0.72 to -0.73; p = 0.026 - 0.028) generating capabilities to be strongly related to sprint performance. Further linear regression analysis predicted an increase in CMJ average and peak take-off power of 1 W/kg (3% & 1.5% respectively) to both result in a decrease of 0.01 s (0.5%) in 10 m sprint performance. Further, an increase in SJ average and peak take-off power of 1 W/kg (3.5% & 1.5% respectively) was predicted to result in a 0.01 s (0.5%) reduction in 10 m sprint time. The results of this study seem to suggest that the ability to generate power both elastically during a CMJ and concentrically during a SJ to be good indicators of predicting sprint performance over 10 m from a block start.

Key Points

• The relative explosive ability of the hip and knee extensors during a countermovement jump can predict 10 m sprint performance from a block start.
• The relative power outputs of male competitive sprinters during a squat jump can predict 10 m sprint performance from a block start.

Key words: Anthropometry, horizontal jumps, sprint performance, vertical jumps     

Spinal Cord Injury and Contractile Properties of the Human Tibialis Anterior

The purpose of this study was to evaluate contractile properties of the tibialis anterior of paralyzed and non-paralyzed subjects. The contractile properties and the fatigability of the tibialis anterior muscle (TA) were tested in 8 spinal cord injured (SCI) and 8 control individuals. The TA was stimulated at frequencies from 10 to 100 Hz to determine a force-frequency curve. A fatigue bout was also performed by stimulating the muscle at 40 Hz every two seconds for three minutes. The SCI muscles produced lower forces overall, but higher forces relative to maximal force at lower frequencies, shifting the force-frequency curve of the SCI group to the left. The half-relaxation time and rate of relaxation at 40 Hz was slower in the SCI muscles than in the control muscles (127 ± 18.4 ms vs. 78 ± 8.7 ms, 6 ± 1.5 kg·s^-1 20 ± 4.1 kg·s^-1 respectively). In addition, force loss and slowing of relaxation during the fatigue protocol were not significantly different between the two groups due to high variability in the SCI group. The TA of the SCI group had slower contractile properties than the control group and fatigability was not significantly different between the SCI and control group. The protocol may be useful to assess training effects during rehabilitation of paralyzed muscle.

Key Points

• Stimulated contractions were tested on controls and spinal cord injured subjects to determine differences in contractile characteristics of the tibialis anterior (ta) muscle.
• Forces were lower in the ta of the spinal cord injured subjects compared to the controls.
• All indices of contractile speed were slower in the spinal cord injured subjects than in the controls.
• The reason for possible differences in contractile capabilities and other biochemical indices of contractile speed in disused muscle need to be further evaluated.

Key words: Muscle, contractility, fatigue, paralysis, paraplegia     

Kinematic and Kinetic Profiles of Trunk and Lower Limbs during Baseball Pitching in Collegiate Pitchers

The purpose of this study was to clarify differences in the kinematic and kinetic profiles of the trunk and lower extremities during baseball pitching in collegiate baseball pitchers, in relation to differences in the pitched ball velocity. The subjects were 30 collegiate baseball pitchers aged 18 to 22 yrs, who were assigned to high- (HG, 37.4 ± 0.8 m·s^-1) and low-pitched-ball-velocity groups (LG, 33.3 ± 0.8 m·s^-1). Three-dimensional motion analysis with a comprehensive lower-extremity model was used to evaluate kinematic and kinetic parameters during baseball pitching. The ground-reaction forces (GRF) of the pivot and stride legs during pitching were determined using two multicomponent force plates. The joint torques of hip, knee, and ankle were calculated using inverse-dynamics computation of a musculoskeletal human model. To eliminate any effect of variation in body size, kinetic and GRF data were normalized by dividing them by body mass. The maxima and minima of GRF (Fy, Fz, and resultant forces) on the pivot and stride leg were significantly greater in the HG than in the LG (p < 0.05). Furthermore, Fy, Fz, and resultant forces on the stride leg at maximum shoulder external rotation and ball release were significantly greater in the HG than in the LG (p < 0.05). The hip abduction, hip internal rotation and knee extension torques of the pivot leg and the hip adduction torque of the stride leg when it contacted the ground were significantly greater in the HG than in the LG (p < 0.05). These results indicate that, compared with low-ball-velocity pitchers, high-ball-velocity pitchers can generate greater momentum of the lower limbs during baseball pitching.

Key points

• High-ball-velocity pitchers are characterized by greater momentum of the lower limbs during pitching motion.
• For high-pitched-ball velocity, stabilizing lower limbs during pitching plays an important role in order to increase the rotation and forward motion of the trunk.
• Computation of the lower-extremity kinetics and measurement of lower-extremity strength may help clarify the role of muscle strength in determining knee and hip function in baseball pitching.

Key words: Throwing movement, pitching ball velocity, ground-reaction force, lower limbs, pivot and stride legs     

Assessment of Isometric Trunk Strength – The Relevance of Body Position and Relationship between Planes of Movement

The aim of the study was to assess the differences in maximal isometric trunk extension and flexion strength during standing, sitting and kneeling. Additionally, we were interested in correlations between the maximal strength in sagittal, frontal and transverse plane, measured in the sitting position. Sixty healthy subjects (24 male, 36 female; age 41.3 ± 15.1 yrs; body height 1.70 ± 0.09 m; body mass 72.7 ± 13.3 kg) performed maximal voluntary isometric contractions of the trunk flexor and extensor muscles in standing, sitting and kneeling position. The subjects also performed lateral flexions and rotations in the sitting position. Each task was repeated three times and average of maximal forces was used for data analysis. RANOVA with post-hoc testing was applied to the flexion and extension data. The level of statistical significance was set to p < 0.05. Overall, in both genders together, the highest average force for trunk extension was recorded in sitting posture (910.5 ± 271.5 N), followed by kneeling (834.3 ± 242.9 N) and standing (504.0 ± 165.4 N), compared with flexion, where we observed the opposite trend (508.5 ± 213.0 N, 450.9 ± 165.7 N and 443.4 ± 153.1 N, respectively). Post-hoc tests showed significant differences in all extension positions (p < 0.0001) and between sitting/standing (p = 0.018) and kneeling/standing (p = 0.033) flexion exertions. The extension/flexion ratio for sitting was 2.1 ± 0.4, for kneeling 1.9 ± 0.4, followed by standing, where motion forward approximately equals motion backward (1.1 ± 0.6). Trunk sagittal-transverse strength showed the strongest correlation, followed by frontal-transverse and sagittal-frontal plane correlation pairs (R² = 0.830, 0.712 and 0.657). The baseline trunk isometric strength data provided by this study should help further strength diagnostics, more precisely, the prevention of low back disorders.

Key points

• Maximal voluntary isometric force of the trunk extensors increased with the angle at the hips (highest in sitting, medium in kneeling and lowest in upright standing).
• The opposite trend was true for isometric MVC force of trunk flexors (both genders together and men only).
• In the sitting position, the strongest correlation between MVC forces was found between sagittal (average flexion/extension) and transverse plane (average left/right rotation).
• In order to increase the validity of trunk strength testing the letter should include: specific warm-up, good pelvic fixation and visual feedback.

Key words: Voluntary force, testing, hip angle, low back pain     

Ground Reaction Force Differences Between Running Shoes,Racing Flats,and Distance Spikes in Runners.

Various shoes are worn by distance runners throughout a training season. This study measured the differences in ground reaction forces between running shoes, racing flats, and distance spikes in order to provide information about the potential effects of footwear on injury risk in highly competitive runners. Ten male and ten female intercollegiate distance runners ran across a force plate at 6.7 m·s^-1 (for males) and 5.7 m·s^-1 (for females) in each of the three types of shoes. To control for differences in foot strike, only subjects who exhibited a heel strike were included in the data analysis. Two repeated-measures ANOVAs with Tukey’s post-hoc tests (p < 0.05) were used to detect differences in shoe types among males and females. For the males, loading rate, peak vertical impact force and peak braking forces were significantly greater in flats and spikes compared to running shoes. Vertical stiffness in spikes was also significantly greater than in running shoes. Females had significantly shorter stance times and greater maximum propulsion forces in racing flats compared to running shoes. Changing footwear between the shoes used in this study alters the loads placed on the body. Care should be taken as athletes enter different phases of training where different footwear is required. Injury risk may be increased since the body may not be accustomed to the differences in force, stance time, and vertical stiffness.

Key points

• To determine the differences in ground reaction forces between regular running shoes and competitive footwear, force plate data was obtained from 10 males (6.7 m·s^-1) and 10 females (5.7 m·s^-1) for each of three shoe types.
• Data from men and women were analyzed in two separate groups, and significant differences were found for various GRF components between the three types of shoes.
• The significant increases in GRF components in competitive footwear suggest that the body must deal with greater impact forces in these shoes than in running shoes at the same running speed.
• The results from this study warrant the recommendation that runners transition gradually from periods when most or all of their training is done in running shoes to more competitive seasons when more of their training is done in racing flats and spikes.

Key words: Footwear, impact, track     

Season-to-Season Variations of Physiological Fitness Within a Squad of Professional Male Soccer Players

The purpose of this study was to examine season-to-season variations in physiological fitness parameters among a 1^st team squad of professional adult male soccer players for the confirmatory purposes of identifying normative responses (immediately prior to pre-season training (PPS), mid-season (MID), and end-of-season (EOS)). Test-retest data were collected from a student population on the primary dependent variables of anaerobic threshold (AT) and maximal aerobic power (VO₂ max) to define meaningful measurement change in excess of test-retest technical error between test-to-test performances. Participants from a pool of 42 professional soccer players were tested over a set sequence of tests during the 3-year period: 1) basic anthropometry, 2) countermovement jump (CMJ) tests 3) a combined AT and VO₂ max test. Over the 3-year period there were no test-to-test changes in mean VO₂ max performance exceeding pre-defined limits of test agreement (mean of eight measures: 61.6 ± 0.6 ml·kg^-1·min^-1). In contrast, VO₂ at AT was significantly higher at the MID test occasion in seasons 2 (+4.8%; p = 0.04, p < 0.05) and 3 (+6.8%; p = 0.03, p < 0.05). The CMJ tests showed a test-to-test improvement of 6.3% (best of 3 jumps) (p = 0.03, p < 0.05) and 10.3% (20-s sustained jumping test) (p = 0.007, p < 0.01) between PPS2 and MID2 and thereafter remained stable. Anthropometrics were unaffected. In summary, despite some personnel changes in the elite cohort between test-to-test occasions, VO₂ max values did not vary significantly over the study which supports previous short-term observations suggesting a general ‘elite’ threshold of 60 ml·kg^-1 min. Interestingly, AT significantly varied where VO₂ max was stable and these variations also coincided with on- and off-seasons suggesting that AT is a better indication of acute training state than VO₂ max.

Key points

• Maximal aerobic power remains fairly stable across inter- and intra-season measurements.
• Anaerobic threshold appears more sensitive of training state confirming our earlier observations.
• The professional players tended to attain optimal performances at the mid-season interval over the 3 seasons, presumably prior to the development of accumulative fatigue.

Key words: Aerobic power, anaerobic threshold, countermovement jump, elite athletes     

Kinetic Consequences of Constraining Running Behavior

It is known that impact forces increase with running velocity as well as when stride length increases. Since stride length naturally changes with changes in submaximal running velocity, it was not clear which factor, running velocity or stride length, played a critical role in determining impact characteristics. The aim of the study was to investigate whether or not stride length influences the relationship between running velocity and impact characteristics. Eight volunteers (mass=72.4 ± 8.9 kg; height = 1.7 ± 0.1 m; age = 25 ± 3.4 years) completed two running conditions: preferred stride length (PSL) and stride length constrained at 2.5 m (SL2.5). During each condition, participants ran at a variety of speeds with the intent that the range of speeds would be similar between conditions. During PSL, participants were given no instructions regarding stride length. During SL2.5, participants were required to strike targets placed on the floor that resulted in a stride length of 2.5 m. Ground reaction forces were recorded (1080 Hz) as well as leg and head accelerations (uni-axial accelerometers). Impact force and impact attenuation (calculated as the ratio of head and leg impact accelerations) were recorded for each running trial. Scatter plots were generated plotting each parameter against running velocity. Lines of best fit were calculated with the slopes recorded for analysis. The slopes were compared between conditions using paired t-tests. Data from two subjects were dropped from analysis since the velocity ranges were not similar between conditions resulting in the analysis of six subjects. The slope of impact force vs. velocity relationship was different between conditions (PSL: 0.178 ± 0.16 BW/m·s^-1; SL2.5: -0.003 ± 0.14 BW/m·s^-1; p < 0.05). The slope of the impact attenuation vs. velocity relationship was different between conditions (PSL: 5.12 ± 2.88 %/m·s^-1; SL2.5: 1.39 ± 1.51 %/m·s^-1; p < 0.05). Stride length was an important factor that determined impact force magnitude. It is likely that lower extremity posture is a determining factor influencing impact characteristics.

Key Points

• As running velocity increased, the magnitude of the vertical ground reaction impact force increased as expected.
• As running velocity increased, stride length increased as expected.
• When stride length was constrained to be 2.5 m for all running velocities, the magnitude of the vertical ground reaction impact force did not increase as expected.
• When running different velocities, the changes in the magnitude of the vertical ground reaction impact force was related to stride length changes.

Key words: Ground reaction force, impact attenuation, shock     

Multi-Axis Prosthetic Knee Resembles Alpine Skiing Movements of an Intact Leg

The purpose of the study was to analyse the flexion angles of the ski boot, ankle and knee joints of an above-knee prosthesis and to compare them with an intact leg and a control group of skiers. One subject with an above-knee amputation of the right leg and eight healthy subjects simulated the movement of a skiing turn by performing two-leg squats in laboratory conditions. By adding additional loads in proportion to body weight (BW; +1/3 BW, +2/3 BW, +3/3 BW), various skiing regimes were simulated. Change of Flexion Angle (CoFA) and Range of Motion (RoM) in the ski boot, ankle and knee joints were calculated and compared. An average RoM in the skiing boot on the side of prosthesis (4.4 ± 1.1°) was significantly lower compared to an intact leg (5.9 ± 1.8°) and the control group (6.5 ± 2.3°). In the ankle joint, the average RoM was determined to be 13.2±2.9° in the prosthesis, 12.7 ± 2.8° in an intact leg and 14.8±3.6 in the control group. However, the RoM of the knee joint in the prosthesis (42.2 ± 4.2°) was significantly larger than that of the intact leg (34.7 ± 4.4°). The average RoM of the knee joint in the control group was 47.8 ± 5.4°. The influences of additional loads on the kinematics of the lower extremities were different on the side of the prosthesis and on the intact leg. In contrast, additional loads did not produce any significant differences in the control group. Although different CoFAs in the ski boot, ankle and knee joints were used, an above-knee prosthesis with a built-in multi-axis prosthetic knee enables comparable leg kinematics in simulated alpine skiing.

Key points

• The RoM in the ski boot on the side of the prosthetic leg was smaller than the RoM of the intact leg and the control group of healthy subjects.
• The RoM in the ankle joint of prosthetic leg was comparable to that of the intact leg and the control group of healthy subjects.
• The RoM in the prosthetic knee joint was greater than the RoM in the knee joint of the intact leg and smaller than that of the control group.
• The total knee flexions in the laboratory measurements were comparable with field measurements.
• Additional load affects the RoM of the ski boot, ankle and knee joints for the amputated skier in both legs. No significant influence from the additional load was found on the RoM in the control group of healthy subjects.
• The above-knee prosthesis with a multiple-axis prosthetic knee reproduces the alpine skiing kinematics of an intact leg.

Key words: Above-knee amputation, Alpine skiing, impairment, kinematics, prosthesis     

Effects of Combined Foot/Ankle Electromyostimulation and Resistance Training on the In-Shoe Plantar Pressure Patterns during Sprint in Young Athletes

Several studies have already reported that specific foot/ankle muscle reinforcement strategies induced strength and joint position sense performance enhancement. Nevertheless the effects of such protocols on sprint performance and plantar loading distribution have not been addressed yet. The objective of the study is to investigate the influence of a 5-wk foot/ankle strength training program on plantar loading characteristics during sprinting in adolescent males. Sixteen adolescent male athletes of a national training academy were randomly assigned to either a combined foot/ankle electromyostimulation and resistance training (FAST) or a control (C) group. FAST consisted of foot medial arch and extrinsic ankle muscles reinforcement exercises, whereas C maintained their usual training routine. Before and after training, in-shoe loading patterns were measured during 30-m running sprints using pressure sensitive insoles (right foot) and divided into nine regions for analysis. Although sprint times remained unchanged in both groups from pre- to post- training (3.90 ± 0.32 vs. 3.98 ± 0.46 s in FAST and 3.83 ± 0.42 vs. 3.81 ± 0.44 s in C), changes in force and pressure appeared from heel to forefoot between FAST and C. In FAST, mean pressure and force increased in the lateral heel area from pre- to post- training (67.1 ± 44.1 vs. 82.9 ± 28.6 kPa [p = 0.06]; 25.5 ± 17.8 vs. 34.1 ± 14.3 N [p = 0.05]) and did not change in the medial forefoot (151.0 ± 23.2 vs. 146.1 ± 30.0 kPa; 142.1 ± 29.4 vs. 136.0 ± 33.8; NS). Mean area increased in FAST under the lateral heel from pre- to post- (4.5 ± 1.3 vs. 5.7 ± 1.6 cm² [p < 0.05]) and remained unchanged in C (5.5 ± 2.8 vs. 5.0 ± 3.0 cm²). FAST program induced significant promising lateral and unwanted posterior transfer of the plantar loads without affecting significantly sprinting performance.

Key points

• We have evaluated the effects of a foot/ankle strength training program on sprint performance and on related plantar loading characteristics in teenage athletes, and this have not been examined previously.
• Our results showed no significant pre- to post- changes in sprint performance.
• This study revealed initially a lateral transfer and secondly a posterior transfer of the plantar loads after the foot/ankle strength training program.

Key words: Track and field, medial arch, reinforcement, injury prevention     

Short-Term Bone Biochemical Response to a Single Bout of High-Impact Exercise

Bone response to a single bout of exercise can be observed with biochemical markers of bone formation and resorption. The purpose of this study was to examine the response of bone biochemical markers to a single bout of exhaustive high-impact exercise. 15 physically active young subjects volunteered to participate. The subjects performed continuous bilateral jumping with the ankle plantarflexors at 65 % of maximal ground reaction force (GRF) until exhaustion. Loading was characterized by analyzing the GRF recorded for the duration of the exercise. Venous blood samples were taken at baseline, immediately after, 2h and on day 1 and day 2 after the exercise. Procollagen type I amino terminal propeptide (P1NP, marker of bone formation) and carboxyterminal crosslinked telopeptide (CTx, marker of bone resorption) were analyzed from the blood samples. CTx increased significantly (32 %, p = 0.015) two days after the exercise and there was a tendensy towards increase seen in P1NP (p = 0.053) one day after the exercise. A significant positive correlation (r = 0.49 to 0.69, p ≤ 0.038) was observed between change in P1NP from baseline to day 1 and exercise variables (maximal slope of acceleration, body weight (BW) adjusted maximal GRF, BW adjusted GRF exercise intensity and osteogenic index). Based on the two biochemical bone turnover markers, it can be concluded that bone turnover is increased in response to a very strenuous single bout of exhaustive high-impact exercise.

Key points

• Studies on bone acute biochemical response to loading have yielded unequivocal results.
• There is a paucity of research on the biochemical bone response to high impact exercise.
• An increase in bone turnover was observed one to two days post exercise.

Key words: Bone biochemical marker, jumping, bone turnover, osteogenic index     

The Effect of Patellar Taping on Some Landing Characteristics During Counter Movement Jumps in Healthy Subjects

The aim of the present study was to determine the effect of patellar taping (PT) on landing characteristics of the vertical ground reaction force (VGRF) and on flight time during a counter movement jump (CMJ). Eleven healthy male subjects (age: 31.1 ± 4.2 years) volunteered for the study. Each subject performed six CMJs under two different jumping conditions: with PT and without PT (WPT). The order of the two conditions was randomized. All of the measured variables had fair-to-good reliability (intra-class correlation coefficient > 0.75). When we compared the PT and WPT groups, we did not find a significant difference in the magnitude of the first (F1) and second (F2) peaks of the VGRF. We also did not find a significant difference in the time to production of these peaks (T1 and T2), and the time to stabilization (TTS) (p < 0. 05). Furthermore, the flight time was similar in the two groups (0.475 ± 0.046 and 0.474 ± 0.056 s, respectively, for PT and WPT). These results suggest that PT does not jeopardize performance during CMJ. Furthermore, it also does not soften the VGRF generated during the landing, indicating that PT may be of limited utility in preventing injuries associated with this type of movement.

Key points

• We investigated whether patellar taping interferes with athletic performance, as has been suggested by previous studies.
• We also explored the effect of patellar taping on the forces generated during the landing phase of counter movement jumps.
• Patellar taping had no effect on the flight time during counter movement jumps.
• Patellar taping also had no effect on the vertical ground reaction force variables measured during the landing phase of counter movement jumps.
• This information may be relevant to athletes and trainers who are concerned about the effects of patellar taping on performance.

Key words: Biomechanics, force platform, vertical ground reaction force, landing phase     

Familiarisation and Reliability of Sprint Test Indices During Laboratory and Field Assessment

The aim of the study was to assess the reliability of sprint performance in both field and laboratory conditions. Twenty-one male (mean ± s: 19 ± 1 years, 1.79 ± 0.07 m, 77.6 ± 7.1 kg) and seventeen female team sport players (mean ± s: 21 ± 4 years, 1.68 ± 0. 07 m, 62.7 ± 4.7 kg) performed a maximal 20-metre sprint running test on eight separate occasions. Four trials were conducted on a non-motorised treadmill in the laboratory; the other four were conducted outdoors on a hard-court training surface with time recorded by single-beam photocells. Trials were conducted in random order with no familiarisation prior to testing. There was a significant difference between times recorded during outdoor field trials (OFT) and indoor laboratory trials (ILT) using a non-motorised treadmill (3.47 ± 0.53 vs. 6.06 ±1.17s; p < 0.001). The coefficient of variation (CV) for time was 2.55-4.22% for OFT and 5.1-7.2% for ILT. During ILT peak force (420.9 ± 87.7N), mean force (147.2 ± 24.7N), peak power (1376.8 ± 451.9W) and mean power (514.8 ± 164.4W), and were measured. The CV for all ILT variables was highest during trial 1-2 comparison. The CV (95% confidence interval) for the trial 3-4 comparison yielded: 9.4% (7.7-12. 1%), 7.9% (6.4-10.2%), 10.1% (8.2-13.1%) and 6.2% (5.1-8.0%) for PF, MF, PP and MP and respectively. The results indicate that reliable data can be derived for single maximal sprint measures, using fixed distance protocols. However, significant differences in time/speed over 20-m exist between field and laboratory conditions. This is primarily due to the frictional resistance in the non- motorised treadmill. Measures of force and power during ILT require at least 3 familiarisations to reduce variability in test scores.

Key points

• Reliable data can be derived from single maximal sprint measures in both indoor and outdoor environments using fixed distance protocols.
• There may be significant time differences to complete fixed distance trials between the two environments.
• Measures of mean force, peak force and peak power during indoor trials may require multiple trials to reduce variability in test scores.

Key words: Non-motorised treadmill, force, power, familiarisation, sprint running     

The Influence of Musical Cadence into Aquatic Jumping Jacks Kinematics

The aim of this study was to analyze the relationships between the head-out aquatic exercise “Jumping jacks” kinematics and the musical cadence in healthy and fit subjects. Five young women, with at least one year of experience conducting head- out aquatic programs were videotaped in the frontal plane, with a pair of cameras providing a double projection (above and below the water surface). Subjects performed an incremental protocol of five bouts (120 b·min^-1, 135 b·min^-1, 150 b·min^-1, 165 b·min^-1 and 180 b·min^-1) with 16 full cycles of the “Jumping jacks” exercise. Data processing and calculation of upper limbs’ (i.e. hands), lower limbs’ (i.e. feet) and center of mass’ 2D linear velocity and displacement were computed with the software Ariel Performance Analysis System and applying the 2D-DLT algorithm. Subjects decreased the cycle period during the incremental protocol. Significant and negative relationships with the musical cadence were verified for the center of mass and upper limbs vertical displacement. On the other hand, for the lower limbs lateral velocity, a significant and positive relationship was observed. It is concluded that expert and fit subjects increase the lower limb’s velocity to maintain the range of motion, while the upper limb’s displacement is reduced to coupe the music cadence.

Key points

• While performing the Jumping Jacks, expert and fit subjects increase their lower limbs segmental velocity to maintain the range of motion.
• The upper limbs displacement is reduced to maintain the music cadence.
• Expert and fit subjects present similar response for alternating or simultaneously head-out aquatic exercises when increasing the music cadence.

Key words: Aquatic jumping exercises, simultaneously actions, music rhythm, range of motion, segmental velocity