Official Journal of the American College of Sports Medicine Vol. 38 No. 5 Supplement S443

METHODS: Forty healthy male and female college students (20.10 5 2.02 yrs, 67.60 2 3.61cm. 70.50+ 12.29 kg) volunteered to participate in the shldy. Subjects were placed in a supine position with their dominant leg and foot strapped into the custom made JPS device by the use of an elastic foam strap. Following the orientation session at 15" of inversion and 5" of eversion, the blindfolded subjects with headphones on their ears were passively placed at 5" and 10" of eversion and 10" and 20" of inversion in a randomized order. The subjects were held at the test angle for 15 seconds, then taken to the opposite end range of motion, and subsequently returned to predetermined neutral. Once at neutral the subject manually returned to a perceived test angle. Three trials were averaged at each test angle for the nonfatigi~ed and fatigued conditions. The output was recorded on a differential amplifier and displayed on the Biopac EMG system. The difference between the test angle and the perceived angle was recorded as the absolute error (AE)measurement. Immediately after thc pretest, subjects underwent a fatiguing protocol consisting of concentric-eccentric eversion exercises at maximal effort on a isokinetic dynamometer at a rate of 60"Is. Muscular fatigue was reached when each subject was unable to complete 3 consecutive repetitions at 50% or greater of their maximum joint torque. The subjects then undenvent post fatigue testing following protocol described earlier.

RESULTS: There was a significant main effect for condition (F,,,,, P<.OOI. ES = ,552) and angle (F,,,,, P=.011, ES = .155). There was no significant interaction between condition and angle (F,,?,, P=.272, ES = ,031). All pairwise comparisons between pre and post fatigue for AE were significant with thc greatest difference noted for 20"of inversion (P=.003), followed by 1 0 o f inversion (P<.OOI), 10" of eversion (P=.005) and 5"of eversion (P-,005).

CONCLUSIONS: When the ankle everters were fatigued, the AE In JPS was significantly higher at all four test angles. If fatigue acts as a paradigm for injury, future research is necessary in understanding the interacttons among fatlgue and JPS deficits in the ankle following injury.

2402 Board #I34 9:30 AM - 10:30 AM Post-Activation Potentiation following Electrical Stimulation of Different Frequencies Viviana A. Cintolesi, Lisa Griffin. Department ofKinesiology and Health Education, The Universih of Terar, Austin, TX Email: vcintolesi@mail.utexas.edu

Enhancement of force output following muscular activlty termed post-activation potentiation (PAP) is commonly observed during human performance. PAP likely plays a significant role in the activity of single motor unit firing rate modulation during voluntary contractions and should be taken into consideration in the design of functional electrical stimulation protocols. However, few studies have performed systematic analysis to determine how PAP responds to different frequencies of stimulation.

PURPOSE: The purpose of this study was to compare post-activation potentiation of the human adductor pollicis muscle in response to a fill1 range of simulat~on frequencies.

METHODS: Six healthy individuals participated in an experiment In whlch six, 100 pulse test trains of frequencies 7.5, 15,25, 30, 50 and 100 Hz were delivered in random order via supramax~mal stimulation of the ulnar nerve. Each participant's hand was placed in a pronated position w ~ t h the thumb against a force transducer. The duration of the trains were 13.34s. 6.67s, 4s. 3.33s, 2s and 1s respectively. Four additional trains, not matched in pulse number, were incorporated into the random sequence: two of 30 Hz (with durations of 1.6s and 6.67s) and two of 50 Hz ( I s and 4s). Potentiated twitch forces were mcasured as a percentage of unpotentiated twitch forces and then compared with a one-way repeated measures ANOVA with Tukey- Kramer post-hoc analysis.

RESULTS: There was no significant difference in twitch force potentiation between any of the pulse-matched trains (p>0.78). The mean i standard error for the percentages of twitch force potentiation in pulse-matched trains ranged from 34.15 + 3.58% to 44.63 i 3.35%. W~thin a set frequency (30 Hz or 50 Hz), twitch force potentiation increased significantly as the number of the stimuli ~ncreased. For the three 30 Hz trials, the PAPwas 17.57 + 1.43%, 3 8 . 6 0 i 7.41% and 85.88 + 6.47% for the 1.67s, 3.33s and 6.67s trains respectively. Similarly in the 50 Hz trains, PAP twitch force ~ncreased by 10.70 = 4.13%. 40.05 + 8.18% and 73.04 .t 8.59% for the Is, 2s and 4s trains respectively.

CONCLUSION: In able-bodied individuals, the number of pulses delivered, rather than the frequency of stimulation, determines the magnitude of post activation potentiation.

2403 Board #I35 8:30 AM - 9:30 AM The Effect of Fall Prevention Exercise Program for the Elderly Byungsung Kim', Seungwoo Nam', Hyunrim Choi', Changwon Won', Younghee Chae2. 'Kjrunghee UniversiQ, Seoul, Republic of Korea. 2Seongbuk elder!)^ Wegare Centei: Seoul, Repzrblic ofKorea. Email: byungskim@naver.com

PURPOSE: Fall is a frequent problem for elderly and it could give rise to fracture, decreased mobility, fear for recurrent falls, and medical complications. The authars tried to find out the effect of regular physical exercise for the prevention of recurrent falls.

METHODS: This shldy was randomized controlled trial of 12 week course of exercise for the community dwelling elderly aged 65 or over who had experienced at least one fall during the last year. The exercise consisted of lower extremities muscle strengthening of plantar flexors, dorsiflexors, inverters, and everters using theraband and proprioceptive exercise using stability trainers. Each exercise session continued for 90 minutes including flexibility exercises and was performed twice weekly for 12 weeks. For the control group, we did not glve any ~nformation about fall prevention. We compared the one-leg standing time of each legs and the occurrence rate of falls for 8 months after intervention using Mann-Whitney U-test.

RESULTS: The one-leg standing time increased significantly for the exercise group in both sides compared to control group(left: 35.9+39.7 vs. -3.4*10.8 seconds, p<0.001; right: 45.2k49.5 vs. -2.6114.4 seconds, p<0.001). After the exercise intervention, there were 3 cases(14.3%) of falls out of 21 persons for the exercise group compared to 7 cases(36.8%) out of I9 person for the control group(p=0.212). The frequency of falls tended to be lower for those with high compliance among exercise group.

CONCLUSIONS: Twelve weeks of ankle muscles strengthening and balance exercise program was effective to increase one-leg standing time and tended to decrease the frequency of falls among the elderly.

2404 Board #I36 9:30 AM - 10:30 AM Muscle Fatigue Alters Neuromuscular Performance during Resisted Weight Bearing Exercise Bryon T. Ballantyne', Richard K. Shields2. 'St. Amhmse University, Davenport, IA. 'The Unrversip oflowa, Iowa City, IA. Email: BallantyneBryonT@sau.edu

Neuromuscular control is essential to the maintenance of lower extremity stability during functional activities. Numerous studies suggest that muscle fatigue contributes to deteriorat~on of neuromuscular performance and may increase the risk of injury. However, limited information is currently available regarding muscle synergistic patterns and triggered reflex responses during dynamic weight bearing activities in the presence of muscle fatigue.

PURPOSE: To examine the effect of muscle fatigue on I) voluntary and reflexive patterns of muscle activation and 2) performance accuracy while tracking a visual target during the weight bearing lateral step down exercise.

METHODS: A specially designed apparatus was used to provide controlled resistance to knee motlon during the lateral step down exercise. Motion of the knee was measured as subjects were asked to track a sinusoidal target waveform as accurately as possible during the exercise. Absolute, constant, variable, and endpoint errors were calculated to evaluate tracking performance. Random perturbations were delivered in 20% of the trials by unexpectedly releasing the resistance during the flexion phase of the exercise. Quadriceps and hamstring muscle activity was recorded during both perturbed and unperturbed tnals. Tests were conducted in a group of 12 healthy females before and after completing a repetitive submaximal eccentric quadriceps fatigue protocol. A second group of 12 females served as controls.

RESULTS: Muscle fatigue increased the level of quadriceps muscle activity throughout most of the exercise. In the flexion phase, average reChls femoris (RF) and vastus lateralis (VL) EMG increased by 2.5% (p=.008) and 3.7% (p=.002) MVIC, respectively. In the extenslon phase, RF activity increased by 5.5% MVlC (p<.001), while VL activity increased by 8.5% MVIC (p< .001). Muscle fatigue also resulted in a reversal in the direction of the constant errors during the extenslon phase of the exercise, causing subjects to lag behind the target. Unexpected perturbations elicited long latency responses characterized by facilitation of the quadriceps and inhibition of the hamstrings. Muscle fatigue increased the amplitude of this response in VL by 4.3% MVIC (p=.004). Despite the increased amplitude of the response, errors at the endpoint of the flexion phase increased by .67 cm (p=.034) after fatigue.

CONCLUSION: These results ~ndicate that muscle fatigue alters patterns of coordinated muscle activity, which may render subjects less able to cope with unexpected perturbations during activities that require a high degree of accuracy.

2405 Board #I37 8:30 AM - 9:30 AM Effects of the Rate of Force Development on Fatigue Onset and Location Mark D. Ricard', Carlos Ugrinowitsch2, Sterling Hilton3, Allen C. Parcel13, lain Hunter', Kenneth Knight3, Valmor Tricoli2. 'University of T a a . at Arlington, Arlington, TX 'University ofSao Paulo, Sao Paulo, Brazil. 'Brigham Young University, Provo, UT Email: ugrinowi@usp.br

Increases in the rate of force development (RFD) seems to change the pattern of motor unit activation. Thus, the pattern of fatigue onset and location may also change as the RFD changes.

PURPOSE: This study was designed to determine if the RFD affects fatigue onset and location.

METHODS: Ten untrained males had their knee extensors fatigued through maximal isometric contractions with distinct RFDs: 30%, 60%, 90%, and 100% maxRFDi sec. Central activation ratio (twitch interpolation) was measured before and after the fatigue protocol. A~nplitude (RMS) and median frequency (MF) of the surface

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S444 Vol. 38 No. 5 Supplement MEDICINE & SCIENCE IN SPORTS & EXERCISEa

electromyographic (EMG) signal were estimated during the fatigue protocol on each contraction at four relative torque intervals (20%-30% 45%-55%, 70%-80%, and loo%), for the vastus lateralis and vastus medialis muscles.

RESULTS: The slow RFD condition (30%maxRFD/sec) produced a faster drop in the impulse response through the fatigue protocol (p<0.05). However, central activation ratio and the EMG amplitude did not mimic the faster drop in the impulse response.

CONCLUSION: The slowest RFD condition produced a faster fatigue onset and it seemed to be produced by peripheral factors since EMG amplitude increased and voluntary activation was not affected by changes in the RFD.

2406 Board #I38 9:30 AM - 10:30 AM Transient Recruitment of Motor Units is Altered in Weightlifters During Sustained Submaximal Contractions Thorsten RudroF, Jens Bojsen-Mnlld, Brach Poston', Roger M. Enokal. 'Universi~ of Colorado, Bouldes CO. 21nstitute of Sports Medicine, Bi.~pebjerg Hospital, Cijpenhagen, Denmark Email: Thorsten.Rudroff~colorado.edu

The duration that non-strength trained subjects can sustain a submaximal isometric contraction with the elbow flexor muscles when the forearm is supinated primarily depends on the frequency of average electromyographic (aEMG) bursts in the brachioradialis muscle.

PURPOSE: To identify those factors that could explain the tlme to failure for a submaximal isometric contraction performed by weightlifters.

METHODS: Five subjects (5 men, 25 i 5 yrs) maintalned a constant elbow-joint angle with a contraction sustained at 20% MVC force for as long as possible. The elbow was flexed to 1.57 rad and the forearm was vertical and supinated. A stepwise linear regression model was used to predict the time to task failure from the (aEMG) activity, the frequency of bursts in the aEMG, target force, fluctuations in limb acceleration, mean arterial pressure (MAP), and heart rate (HR). The bursts of activity in the EMG slgnal were used as an index of the transient recn~itment of motor units.

RESULTS: The averagc target force was 83.4 * 10.6 N (range: 66 to 95 N) and the time to task failure was 267 1 90 s (range: 190 to 422 s). In contrast with non-strength trained subjects, the task was terminated when the weightlifters were unable to maintain the elbow joint at a right angle. A stepwise linear regression model indicated that the time to failure could be predicted by the frequency of aEMG bursts in the short and long heads of biceps brachii (R2 = 0.806, P < 0.001). Subjects who exhibited a greater frequency of transient recruitment of motor units in the short and long heads of biceps brachii were able to maintain the arm position while supporting the inertial load for a longer time.

CONCLUSIONS: The duration that the task could be sustained by weightlifters and non- strenbqh trained subjects was lim~ted by the activity of different muscles, which underscores the influence of the training stahls of the subjects when performing such tasks.

Supported by: NINDS NS43275 --

2407 Board #139 8 5 0 AM - 9:30 AM The Effects of Time of Day on Static and Dynamic Postural Control Phillip Gribble, W. Steven Tucker, Paul A. White. University of Toledo, Ath/etic Training Research Laboratory, Toledo, OH. Email: phillip.gibble@utoledo.edu

Instrumented and clinical tests exist to quantify static and dynamic poshlral control. Physiological and neurological functions in humans are influenced by diurnal patterns that follow a proposed circadian rhythm. There has been little investigation into an influence of the time of day on postural control.

PURPOSE: To determine the influence of time of day on static and dynamic postural control.

METHODS: Thirty subjects (I3 males, 17 females; 21.8i3.74 yr.; 173.57A7.90cm; 70.12i13.67kg) reported to the laboratory for six separate testing sessions over a forty-eight hour period during which static and dynamic postural control was assessed at three times (10:00, 15:00, and 20:00) on both days. The order of the static and dynamic tests was randomized. Static postural control was assessed for 30 seconds in a single limh stance on the dominant limh using a forceplate with eyes open and closed conditions. For each condition, three trials were averaged. The order of condition was randomized. Center of pressure velocity in the AlP(C0PVX) and MIL (COPVY) was calculated. Dynamic postural control was assessed with the anterior reach of the Star Excursion Balance Test (SEBT) utilizing the dominant limb. Three reaches were averaged and the reach distances were normalized to leg length (%MAXD). For both COPVX and COPVY, separate Day (one, two) by Time (10:00, 15:00,20:00) by Eye (EO, EC) by Gender (male, female) repeated measures ANOVA's were performed. For %MAXD, a Day (one, two) by Time (10:00, 15:00, 20:OO) by Gender (male, female) repeated measures ANOVA was performed.

RESULTS: For COPVX, there was a statistically significant Day by Time interaction (F,,,,=3.849; p=.027). On Day 1, COPVX was significantly less at 10:OO (0.120A0.008 rnis) compared to 15:OO (0.125-0.008 m's). For COPVY, there was a statistically sigmficant Day by Time interaction (F,,,,=5.804; p=.005). On Day I, C O P W was significantly less at 10:00 (0.13010.007 d s ) compared to 15:00 (0.141i0.008 rnls).

For %MAXD, there was a significant Time by Gender interaction (F,,,=3.42, p=.04). Among the females, %MAXD was significantly greater at 10:OO (0.824i0.015) compared to 20:OO (0.81 1*0.016). A significant main effect for Time (F,,,=3.205, pz.048) was also present for %MAXD, indicating that 10:OO produced the greatest %MAXD (0.821*0.011) compared to 15:OO (0.814i0.012) and 20:OO (0.814i0.012).

CONCLUSIONS: There seems to be an influence of the time of day on static and dynamic postural control that suggests postural control may be better in the mornings compared to the afternoons. This may influence serial postural control testing as well as between group comparisons of postural control.

2408 Board #I40 9:30 AM - 10:30 AM Muscle Fatigue and Activation Pattern of Upper Back and Shoulder Girdle Muscles in Archers Huang Yiming, Liao Chien Nan. Institude of coachingscience, National College of Pbsical Education & Sport, Taouan county, Taiwan Republic of China. Email: iminl27@ms55.hinet.net

Control strategy of movement, especially in multiple joint, may alter as one of muscles fatigue. In archery, consistent and repeated movement is performed during whole practice session or in competition game. Therefore, an archer may easily change activation pattern if one of the major contributive muscles is fatigue. It has been wildly using median frequency (MF) of EMG signal as a examine way to muscle fatigue.

PURPOSE: To investigate the strategy and fatigue state of upper back and shoulder girdle muscles during a whole practice session in elite and sub-elite archers.

METHODS: Two archers with one won a gold medal of personal archery game in 2005 World University Games were recruited in this study. They performed on practice session with 144 arrows and rules simulated to formal competition with the duration was 2-3 hours approximately. Electromyographic (EMG) signals were recorded from upper trapezius (TRA), Infra-spinatus (INFRA), posterior teltoid (TEL), Rhomboid (RHOM), Latissimus dorsi (LAT) muscles on pull side of the body. Maximal voluntary isometric contraction (MVIC) before practice session was used to normalize EMG signal. Normalized root mean square (RMS) EMG and median frequency (MF) were calculated within one second by releasing an m o w which defined by accelerometers attached on wrist. The first and the last 3 sets with 18 trials (n=18) individually (6 trials per set) were compared. T-test for repeated measures was used.

RESULTS: Significant decrease in MF in INFRA muscle (79+5 Hz vs. 59+6 Hz, p<0.05) was demonstrated on sub-elite archer during the session. In elite one, it revealed slightly shift in MF but not significant in WFRA muscle. Besides, TEL, TRA, INTRA, RHOM and LAT muscles activities detected from sub-elite archer were significantly increased (21 .(K4.6% vs. 25.8*5.0%; 33.IM.1% vs. 34.5+5.7%; 26.3i5.0°h vs. 36.1*5.O0h; 21.6*3.0% vs. 27.7*4.6%; 17.512.4%vs.19.2f3.0% respectively, p<0.05), but only INFRA muscle activity was increased significantly in elite one (17.6*2.2% vs. 19.6*2.7%, p<0.05).

CONCLUSION: Muscle fatigue did occur in sub-elite archer's INFRA muscle but not in elite one during a practice session. Muscle activation pattern would alter following one of the mainly muscle fatigue. The scientific information may offer the coach as a reference resource to avoid overtraining due to muscle fatigue.

2409 Board #I41 8:30 AM - 9:30 AM Proprioceptive Abilities Retween Different Age Groups with and without a Knee Brace Riidiger Reerl, Marc Zieglerl, Serge P. von Duvillard, FACSW, Klaus-Michael Braumannl. 'Institute for Sports and Exercise Medicine, University of Hamburg, Hamburg, Gennany. 2Human Peformance Laboraioiy, Department of Health, Kinesiology & Sports Studies, Terar A&M University, Commerce, TX (Sponsor: Serge P. von Duvillard, FACSM, FACSM) Email: reeauni-hamburg.de

External stabilization devices play an important role in the prevention, rehabilitation and therapy of knee injuries via proprioceptive stimulation.

PURPOSE: To determine the influence of an external stabilizing device (Soffec knee brace, Bauerfeind) on proprioceptive response among different age groups.

METHODS: 20 very young (12 yo), 20 young (16 yo) and a group of 20 adult subjects (27 yo) participated in proprioceptive tests with (+b) and without brace (-b) in a randomized order. An angle reproduction test determined via a goniometer and a dynamic balance test on the KAT 2000 system were used as testing modalities.

RESULTS: In the angle reproduction test (loo, 25" and 40" starting from fully extended position defined as 0") an improvement in +b with the range of 20-30% was detected for all three age groups. However, the results were less favorable with increasing flexion of the knee joint. During the dynamic balance test, when applying the KAT 2000 system, an improvement in the performance of the single leg stance test for the dominant and non-dominant leg were evident for the +b tests only. In the two- legged stance test, however, the results for +b were reduced between both groups of 12 and 16 year old subjects. The impairment was more pronounced among the youngest group of individuals tested.

CONCLUSION: Our results suggest that brace application leads to a predominantly

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