The effect of plyometric training on power and kicking distance in female adolescent soccer players.

The purpose of this study was to measure the effects of low-frequency, low-impact plyometric training on vertical jump (VJ) and kicking distance in female adolescent soccer players. Sixteen adolescent soccer players were studied (age 13.4 ± 0.5 years) across 14 weeks. The control group (general soccer training only) had 6 subjects, and the plyometric training (general soccer training plus plyometric exercise) group had 10 subjects. All subjects were tested for VJ and kicking distance on 3 occasions: pre-test, 7 weeks, and 14 weeks. Data were analyzed using a 2 (Training) × 3 (Test) analysis of variance (ANOVA) with repeated measures on the factor test. No significant difference in kicking distance was found between groups at pre-test (p = 0.688) or 7 weeks (p = 0.117). The plyometric group had significantly greater kicking distance after 14 weeks (p < 0.001). No significant difference in VJ height was found between groups at pre-test (p = 0.837) or 7 weeks (p = 0.108). The plyometric group had a significantly higher VJ after 14 weeks (p = 0.014). These results provide strength coaches with a safe and effective alternative to high-intensity plyometric training. Based on these findings, to increase lower-body power resulting in increased VJ and kicking distance, strength coaches should implement once-weekly, low-impact plyometric training programs with their adolescent athletes.

The magnitude of tissue cooling during cryotherapy with varied types of compression.

CONTEXT: Certified athletic trainers can choose different types of external compression (none, Flex-i-Wrap, and elastic wrap) when applying an ice bag to the body. However, which type facilitates the greatest magnitude of tissue cooling is unclear. OBJECTIVE: To compare the effects of 2 common types of external compression on the magnitude of surface and intramuscular cooling during an ice-bag treatment. DESIGN: Randomized controlled trial. SETTING: University research laboratory. PATIENTS OR OTHER PARTICIPANTS: Fourteen college students (10 women, 4 men; age = 22.4 +/- 1.8 years, height = 169.1 +/- 8.2 cm, mass = 73.3 +/- 18.5 kg, skinfold = 13.14 +/- 1.61 mm) with previous cryotherapy experience and a posterior lower leg skinfold equal to or less than 15 mm. INTERVENTION(S): On 3 different days separated by 24 to 48 hours, an ice bag was applied to the posterior lower leg surface of each participant for 30 minutes with no compression, with elastic wrap, or with Flex-i-Wrap. MAIN OUTCOME MEASURE(S): Posterior lower leg surface and intramuscular (2 cm) temperatures were recorded for 95 minutes. RESULTS: At 15 minutes, the elastic wrap produced greater surface temperature reduction than no compression (P = .03); this difference remained throughout the protocol (P range, .03 to .04). At 30 minutes, surface temperatures were 14.95 degrees C, 11.55 degrees C, and 9.49 degrees C when an ice bag was applied with no external compression, Flex-i-Wrap, and elastic wrap, respectively. Surface temperatures between Flex-i-Wrap and elastic wrap and between Flex-i-Wrap and no compression were never different. At 10 minutes, Flex-i-Wrap (P = .006) and elastic wrap (P < .001) produced greater intramuscular temperature reduction than no compression produced; these differences remained throughout the protocol. At 10 minutes, no compression, Flex-i-Wrap, and elastic wrap decreased intramuscular temperature by 1.34 degrees C, 2.46 degrees C, and 2.73 degrees C, respectively. At 25 minutes, elastic wrap (8.03 degrees C) produced greater temperature reduction than Flex-i-Wrap (6.65 degrees C) (P = .03) or no compression (4.63 degrees C) (P < .001 ). These differences remained throughout ice application and until 50 minutes after ice-bag removal. CONCLUSIONS: During an ice-bag application, external compression with elastic wrap was more effective than Flex-i-Wrap at reducing intramuscular tissue temperature. Elastic wraps should be used for acute injury care.

A description of shock attenuation for children running.

CONTEXT: A growing number of children are participating in organized sport activities, resulting in a concomitant increase in lower extremity injuries. Little is known about the impact generated when children are running or how this impact is attenuated in child runners. OBJECTIVE: To describe shock attenuation characteristics for children running at different speeds on a treadmill and at a single speed over ground. DESIGN: Prospective cohort study. SETTING: Biomechanics laboratory. PATIENTS OR OTHER PARTICIPANTS: Eleven boys (age = 10.5 +/- 0.9 years, height = 143.7 +/- 8.3 cm, mass = 39.4 +/- 10.9 kg) and 7 girls (age = 9.9 +/- 1.1 years, height = 136.2 +/- 7.7 cm, mass = 35.1 +/- 9.6 kg) participated. INTERVENTION(S): Participants completed 4 running conditions, including 3 treadmill (TM) running speeds (preferred, fast [0.5 m/s more than preferred], and slow [0.5 m/s less than preferred]) and 1 overground (OG) running speed. MAIN OUTCOME MEASURE(S): We measured leg peak impact acceleration (LgPk), head peak impact acceleration (HdPk), and shock attenuation (ratio of LgPk to HdPk). RESULTS: Shock attenuation (F(2,16) = 4.80, P = .01) was influenced by the interaction of speed and sex. Shock attenuation increased across speeds (slow, preferred, fast) for boys (P < .05) but not for girls (P > .05). Both LgPk (F(1,16) = 5.04, P = .04) and HdPk (F(1,16) = 6.04, P = .03) were different across speeds, and both were greater for girls than for boys. None of the dependent variables were influenced by the interaction of setting (TM, OG) and sex (P >or= .05). Shock attenuation (F(1,16) = 33.51, P < .001) and LgPk (F(1,16) = 31.54, P < .001) were different between TM and OG, and each was greater when running OG than on the TM, regardless of sex. CONCLUSIONS: Shock attenuation was between 66% and 76% for children running under a variety of conditions. Girls had greater peak impact accelerations at the leg and head levels than boys but achieved similar shock attenuation. We do not know how these shock attenuation characteristics are related to overuse injuries.

Acute effects of static and ballistic stretching on measures of strength and power.

Preactivity stretching is commonly performed by athletes as part of their warm-up routine. However, the most recent literature questions the effectiveness of preactivity stretching. One limitation of this research is that the stretching duration is not realistic for most athletes. Therefore, the purpose of this study was to determine the effects of a practical duration of acute static and ballistic stretching on vertical jump (VJ), lower-extremity power, and quadriceps and hamstring torque. Twenty-four subjects performed a 5-minute warm-up followed by each of the following three conditions on separate days with order counterbalanced: static stretching, ballistic stretching, or no-stretch control condition. Vertical jump was determined with the Vertec VJ system and was also calculated from the ground-reaction forces collected from a Kistler force plate, which also were used to calculate power. Torque output of the quadriceps and hamstrings was measured through knee extension and flexion on the Biodex System 3 Dynamometer at 60 degrees x s(-1). Data normalized for body weight were analyzed using five separate, 3 (stretch condition) x 2 (gender) analysis-of-variance procedures with repeated measures on the factor of stretch condition. The gender x stretch interaction was not significant for any of the four measures, suggesting that the stretching conditions did not affect men and women differently. The results of this study reveal that static and ballistic stretching did not affect VJ, or torque output for the quadriceps and hamstrings. Despite no adverse effect on VJ, stretching did cause a decrease in lower-extremity power, which was surprising. Because of the mixed results, strength coaches would be better served to use dynamic stretching before activity; this has been consistently supported by the literature.

Subcutaneous tissue thickness alters the effect of NMES.

CONTEXT: No direct research has been conducted on the relationship between subcutaneous tissue thickness and neuromuscular electrical stimulation (NMES). OBJECTIVE: The purpose of this study was to determine the effects of subcutaneous tissue thickness on NMES amplitude and NMES force production of the quadriceps. DESIGN: Simple fixed design, testing the independent variable of subcutaneous thickness (skinfold) groups with the dependent variables of NMES amplitude and force production. SETTING: Athletic Training Laboratory. PARTICIPANTS: 29 healthy women. INTERVENTION: NMES to produce at least 30% of maximal voluntary isometric contractions (MVIC) of the quadriceps. MAIN OUTCOME MEASURE: Maximal NMES amplitude and percentage of MVIC using NMES. RESULTS: A significant skinfold category difference F2,28=3.92, P=.032 on NMES amplitude was found. Post hoc revealed the thinnest skinfold category tolerated less amplitude compared to the thickest category. A significant correlation was found between NMES amplitude skinfold category R=.557, P=.002. CONCLUSION: Higher NMES amplitudes are needed for the thickest skinfold category compared to the thinnest skinfold category.

Effect of hamstring-emphasized resistance training on hamstring:quadriceps strength ratios.

A decreased hamstring:quadriceps (H:Q) ratio may put the hamstrings and anterior cruciate ligament (ACL) at increased risk of injury. Therefore, the purpose of this study was to evaluate H:Q ratios of 12 female National Collegiate Athletic Association soccer players, and to test the effects of a 6-week strength training program on these ratios. Each subject completed 2 practice sessions before a pretest. Subjects then completed 6 weeks of strength training that included the addition of 2 hamstring specific exercises, followed by a posttest. Peak torque during concentric and eccentric actions for both hamstrings and quadriceps was measured with an isokinetic dynamometer. Each muscle action was tested at 3 angular velocities in the following order: concentric 240, 180, and 60 degrees x s(-1) and eccentric 60, 180, and 240 degrees x s(-1). The H:Q strength ratio was evaluated using concentric muscle actions (concentric hamstrings:concentric quadriceps). This method is commonly used and is thus called the conventional ratio. Because concentric actions do not occur simultaneously in opposing muscles, a more functional assessment compares eccentric hamstring actions to concentric quadriceps actions. This functional ratio was also analyzed. Mean conventional and functional H:Q ratio data were analyzed using separate analysis of variance procedures with repeated measures on all factors (2 [Test] x 2 [Leg] x 3 [Angular Velocity]). The results revealed a significant main effect for factor (F test) with the functional ratio (p < 0.05) but not for the conventional ratio. The mean functional ratio increased from 0.96 +/- 0.09 in pretest to 1.08 +/- 0.11 in posttest. These results suggest that 6 weeks of strength training that emphasizes hamstrings is sufficient to significantly increase the functional ratio. The functional ratio after training exceeded 1.0, which is specifically recommended for prevention of ACL injuries.

Effect of the simultaneous application of NMES and HVPC on knee extension torque.

CONTEXT: Electrical stimulation using simultaneous application of two current types for multiple effects is a current treatment option, but the effect of this treatment is not currently known. OBJECTIVES: To compare isometric knee extension torque when using neuromuscular electrical stimulation (NMES) in combination with High Voltage Pulsed Current (HVPC) versus NMES alone during three contraction conditions of quadriceps. DESIGN: Counterbalanced, within-subjects design to test independent variables, stimulation protocol, and contraction condition; ANOVA to analyze dependent variable, peak torque. SETTING: Athletic Training Research Laboratory. PARTICIPANTS: 14 healthy subjects (7 male and 7 female, age = 21.9 +/- 2.0 yr, height = 173.4 +/- 10.1 cm, weight = 76.1 +/- 16.7 kg). INTERVENTION: Participants performed three contraction conditions during two stimulation protocols. MAIN OUTCOME MEASURE: Peak isometric knee extension torque. RESULTS: The main effect for Stimulation Protocol was not significant: F1,26 = .01, P = .94. CONCLUSION: Simultaneous application of HVPC with NMES does not facilitate the neuromuscular response but may provide an efficient treatment when managing atrophy, strength loss, pain, and edema associated with reconstructive surgery.

An 18-day stretching regimen, with or without pulsed, shortwave diathermy, and ankle dorsiflexion after 3 weeks.

CONTEXT: The amount of retained ankle flexibility gains and the effects of diathermy on those gains are unclear. OBJECTIVE: To determine the retention of flexibility 3 weeks after an 18-day stretching regime and the effect of pulsed, shortwave diathermy on that retention. DESIGN: We used a 2x4 factorial with repeated measures on day (1, 19, 24, and 39). The other independent variable was treatment (stretch only versus diathermy and stretch). The dependent variable was ankle-dorsiflexion angular displacement as measured on a digital inclinometer. SETTING: Therapeutic Modality Research Laboratory. PATIENTS OR OTHER PARTICIPANTS: 23 healthy college-aged volunteers (8 males, 15 females; age = 22.7 +/- 2.1 years, height = 171.1 +/- 8.8 cm, mass = 70.4 +/- 13.5 kg). INTERVENTIONS: All subjects performed 3 weeks (not including weekends) of low-load, prolonged, long-duration stretching. One group performed stretching only; the other group also received diathermy. MAIN OUTCOME MEASURE(S): After an 18-day stretching regime and 7-day retention study, subjects returned 14 days later for the 3-week retention measure. The angle of inclination from the posterior Achilles tendon to the sole of the shoe near the heel was measured on each treatment and test day. RESULTS: Regardless of group (F(1,21) = 0.74, P = 0.40), the flexibility gained between days 1 (99.7 +/- 4.0 degrees), 19 (102.9 +/- 5.8 degrees), and 24 (105.0 +/- 6.2 degrees) were maintained at day 39 (104.8+/- 7.2 degrees) (P < .05). CONCLUSIONS: Flexibility gains in normal ankles with 3 weeks of training were retained for at least 3 weeks after training ceased. The application of pulsed, shortwave diathermy during stretching did not appear to influence the chronic retention of flexibility gains in normal subjects.

Cryotherapy, Sensation, and Isometric-Force Variability.

OBJECTIVE: To determine the changes in sensation of pressure, 2-point discrimination, and submaximal isometric-force production variability due to cryotherapy. DESIGN AND SETTING: Sensation was assessed using a 2 x 2 x 2 x 3 repeated-measures factorial design, with treatment (ice immersion or control), limb (right or left), digit (finger or thumb), and sensation test time (baseline, posttreatment, or postisometric-force trials) as independent variables. Dependent variables were changes in sensation of pressure and 2-point discrimination. Isometric-force variability was tested with a 2 x 2 x 3 repeated-measures factorial design. Treatment condition (ice immersion or control), limb (right or left), and percentage (10, 25, or 40) of maximal voluntary isometric contraction (MVIC) were the independent variables. The dependent variables were the precision or variability (the standard deviation of mean isometric force) and the accuracy or targeting error (the root mean square error) of the isometric force for each percentage of MVIC. SUBJECTS: Fifteen volunteer college students (8 men, 7 women; age = 22 +/- 3 years; mass = 72 +/- 21.9 kg; height = 183.4 +/- 11.6 cm). MEASUREMENTS: We measured sensation in the distal palmar aspect of the index finger and thumb. Sensation of pressure and 2-point discrimination were measured before treatment (baseline), after treatment (15 minutes of ice immersion or control), and at the completion of isometric testing (final). Variability (standard deviation of mean isometric force) of the submaximal isometric finger forces was measured by having the subjects exert a pinching force with the thumb and index finger for 30 seconds. Subjects performed the pinching task at the 3 submaximal levels of MVIC (10%, 25%, and 40%), with the order of trials assigned randomly. The subjects were given a target representing the submaximal percentage of MVIC and visual feedback of the force produced as they pinched the testing device. The force exerted was measured using strain gauges mounted on an apparatus built to measure finger forces. RESULTS: Sensation of pressure was less (ie, it took greater pressure to elicit a response) after ice immersion, thumbs were more affected than index fingers, and the decrease was greater in the right limb than the left. Two-point discrimination was not affected by cryotherapy but was higher in the finger than in the thumb under all conditions. Isometric-force variability (standard deviation of mean isometric force) was greater as percentage of force increased from 10% to 40% of MVIC. Targeting accuracy (root mean square error) was decreased at 40% of MVIC. Accuracy and force variability were not affected by cryotherapy. CONCLUSIONS: The application of cryotherapy and reduced sensation of pressure appear to have little effect on motor control of the digits. These results support the hypothesis that the use of cold is not contraindicated for use as an analgesic before submaximal rehabilitative exercise focusing on restoring neuromuscular control to injured tissues.