Age-related effects of neuromuscular fatigue and acute recovery responses on maximal and rapid torque measures of the leg extensors and flexors.

PURPOSE: To examine the effects of neuromuscular fatigue and recovery on maximal and rapid torque characteristics in young and old men for the leg extensors and flexors. METHODS: Twenty-one young (age = 24.8 years) and 19 old (72.1 years) men performed maximal voluntary contractions (MVCs) before and at 0, 7, 15, and 30 min following an intermittent submaximal fatigue task. Outcome measures included endurance time, maximal (peak torque; PT) and rapid (absolute and normalized rate of torque development; RTD and nRTD) torque characteristics. RESULTS: The old men had greater endurance times than the young men. Differential recovery patterns were observed for PT, and early and late RTD phases between the leg extensor and flexor muscle groups such that the early rapid torque variables and the flexors demonstrated slower recovery compared to later rapid torque variables and the extensors. The normalized RTD variables were reduced less after the fatigue task and differential muscle and age effects were observed where the flexors were reduced more at the early phase (nRTD1/6) compared to the extensors, however, for the later phase (nRTD2/3) the young men exhibited a greater reduction compared to the old men. CONCLUSIONS: Dissimilar fatigue recovery patterns across different phases of RTD, lower limb muscles, and age groups may have important fatigue-related performance and injury risk implications across the adult lifespan.

Effects of Multi-joint Eccentric Training on Muscle Function When Combined With Aquatic Plyometric Training: A Minimal Dose, Mixed Training Study.

OBJECTIVES: To examine the effects of a combined eccentric overload and aquatic-based plyometric training program on muscle function/performance measures and soreness versus an eccentric-only training protocol using a minimal dose training paradigm. METHODS: Twenty-five participants were randomized into either an eccentric-only training group (ECC) or a combined eccentric and aquatic plyometric group (ECC + AQP). The ECC group performed eccentric training once per week for 6-weeks while the ECC + AQP group performed the same eccentric training but with an additional aquatic plyometric training session. RESULTS: There was no group × trial interactions for any of the variables. However, the training elicited large improvements in eccentric strength in both ECC (27%; ES = 1.33) and ECC+AQP (17%; ES = .86) groups. Isometric strength improved moderately for ECC and ECC+AQP groups (17.2%, ES = .53;9%, ES = .45). A moderate increase was observed for depth jump height for both ECC and ECC+AQP groups (13.1%, ES = .48;8.8%, ES = .36). No changes were observed for countermovement jump or sprint time and muscle soreness did not differ between groups. CONCLUSIONS: Minimal dose multi-joint eccentric overload training improved strength and depth jump outcomes after 6-weeks regardless of the training condition but adding a minimal dose aquatic plyometric protocol does not improve muscle function-based outcomes.

Effects of Strength and Conditioning on Maximal Isometric Strength, Motor Unit Behavior, and Concentric Isokinetic Peak Torque in Middle-School Boys'.

ABSTRACT: MacLennan, RJ, Mota, JA, Thompson, BJ, and Stock, MS. Effects of strength and conditioning on maximal isometric strength, motor unit behavior, and concentric isokinetic peak torque in middle-school boys. J Strength Cond Res 36(5): 1318-1326, 2022-It has long been theorized that improvements in muscle strength in young athletes are mediated by motor unit adaptations. The ability to decompose surface electromyographic signals obtained during isometric contractions now allow for such research questions to be answered. We examined changes in isometric and concentric isokinetic strength, as well as vastus lateralis motor unit behavior, after 16 weeks of strength training and conditioning in middle-school aged boys. Nine boys (mean ± SD age = 12 ± 1 years) participated in training. Five boys (age = 13 ± 1 years) served as control subjects. The training subjects performed 90 minutes of high-intensity, multi-joint exercise twice per week. Assessments of unilateral maximal voluntary isometric contraction (MVIC) force of the knee extensors, concentric peak torque at velocities of 60, 180, and 300°·s-1, and vastus lateralis motor unit data during 50 and 80% MVIC tests were performed. Strength training and conditioning did not improve MVIC force. Greater training-induced strength increases were observed at faster isokinetic velocities, with a large effect size at 300°·s-1 (d = 0.813). The slopes and y-intercepts of the mean firing rate vs. recruitment threshold relationship and the action potential amplitude vs. recruitment threshold relationship were unaffected by training. Sixteen weeks of middle-school strength and conditioning did not enhance maximal isometric strength or vastus lateralis motor unit control, but improvements were observed during rapid isokinetic muscle actions. Given the lack of training (multi-joint) vs. testing (single-joint) specificity, we propose that motor unit adaptations in youth are task specific.

Comparison of High Versus Low Eccentric-Based Resistance Training Frequencies on Short-Term Muscle Function Adaptations.

ABSTRACT: Crane, JS, Thompson, BJ, Harrell, DC, Bressel, E, and Heath, EM. Comparison of high versus low eccentric-based resistance training frequencies on short-term muscle function adaptations. J Strength Cond Res 36(2): 332-339, 2022-Eccentric resistance training is beneficial for improving a number of performance and health metrics. However, the recommendations on eccentric training frequency have not been established. This study investigates the effects of volume-matched resistance training frequency comparing 1 vs. 3 training days per week of isokinetic multiple-joint eccentric training on strength and lower-body function adaptations during a 4-week training period. Thirty subjects were assigned to either 3 days per week (high-frequency [HF]) or 1 day per week (low-frequency [LF]) training conditions for 4 weeks. An eccentric dynamometer was used for the training and testing. Eccentric strength and vertical jump (VJ) measures were taken at Pre, Mid (2 weeks), and Post (4 weeks) intervention. Soreness (visual analog scale [VAS]) and rate of perceived exertion (RPE) were taken throughout the training period. There was no group × trial interaction for eccentric strength (p = 0.06) or VJ (p = 0.87). For eccentric strength, all trials were significantly different (p < 0.001) from each other. For VJ, there was a main effect for trial such that VJ increased from Pre to Post (p < 0.001) and Mid to Post (p < 0.01). High frequency reported lower RPE (p < 0.01) and soreness (p = 0.04) compared with LF. Both HF and LF protocols elicited large (36.8 and 27.4% strength increases, respectively) and rapid neuromuscular adaptations for improved strength. Eccentric-based workload may be dispersed across a given period to allow for reduced soreness and perceived exertion levels without compromising neuromuscular adaptations. Some eccentric training transfer to functional (VJ) task may also be observed, independent of training frequency.

Changes in Knee Extension and Flexion Maximal and Rapid Torque Characteristics During a Collegiate Women's Soccer Season.

ABSTRACT: Akehi, K, Palmer, TB, Conchola, EC, Thompson, BJ, Kasl, A, Bice, M, and Unruh, S. Changes in knee extension and flexion maximal and rapid torque characteristics during a collegiate women's soccer season. J Strength Cond Res 36(5): 1389-1395, 2022-The purpose of this study was to assess the changes in maximal and rapid torque capacities of the knee extensor and flexor muscles over the course of a competitive season in NCAA Division II women's soccer players. Eighteen female soccer athletes performed 2 maximal voluntary isometric contractions (MVICs) of the knee extensor and flexor muscles before, during, and at the end of the competitive season. Peak torque (PT) and rate of torque development (RTD) at 50 (RTD50), 200 (RTD200), and 100-200 (RTD100-200) milliseconds were extracted from each MVIC for both legs. The rapid (RTD50) to maximal force ratio (RTD:PT), hamstring-to-quadriceps (H:Q) strength ratio, and bilateral strength differences were also calculated. Results indicated that PT, RTD50, and RTD200 decreased 11-21% from the preseason to the midseason for the knee extensors (p < 0.02) and RTD50 increased approximately 11% from the midseason to the end of season for the knee flexors (p < 0.01). Rate of torque development-to-PT ratios for the knee extensors and flexors increased 12-25% at the end of the season (p < 0.05). Also, H:Q strength ratios using PT, RTD50, and RTD200 increased 12.5-24% after the season started (p = 0.001-0.04). There were no bilateral strength differences (dominant vs. non-dominant limbs) across the season (p > 0.05). These findings suggest that participation in a soccer season can change maximal and rapid torque production of the knee extensors and flexors. Coaches and clinicians should consider incorporating a season-long strength training and maintenance plan for soccer players with the aim to improve athletic performance and minimize the risk of musculoskeletal injuries to the lower extremities.

Rapid muscle activation changes across a competitive collegiate female soccer season.

OBJECTIVE: The purpose of this study was to examine the effects of a competitive soccer season on rapid activation properties of the knee extensors and flexors in Division II female soccer players. METHODS: Eighteen collegiate female soccer players participated in the present study, however, due to injuries during the season a final sample of 16 players were included for study analysis. Participants performed two maximal voluntary isometric contractions (MVICs) of the knee extensors and flexors before, during, and at the end of a competitive college soccer season. Electromyography root mean square (EMG RMS; µV), rate of EMG rise (RER; %Peak EMG•s-1), and electromechanical delay (EMD; ms) were examined on both legs for the knee extensors and flexors. RESULTS: EMG RMS at early time intervals (0-50, 0-100, and 50-100 ms) and RER at 0-75 ms for the knee extensors and flexors significantly increased from the pre-season to the end of the season (P≤0.010-0.026, η2=0.36-0.81). EMD of the knee flexors significantly decreased at the mid-season and the end of the season compared to the pre-season (P<0.001, η2=0.95). CONCLUSIONS: These findings may have important implications for monitoring improvements on thigh neuromuscular activation and developing lower extremity injury prevention strategies during a competitive collegiate female soccer season.

Echo intensity as an indicator of skeletal muscle quality: applications, methodology, and future directions.

PURPOSE: This narrative review provides an overview of the current knowledge of B-mode ultrasound-derived echo intensity (EI) as an indicator of skeletal muscle quality. METHOD: PubMed and Google Scholar were used to search the literature. Advanced search functions were used to find original studies with the terms 'echo intensity' and/or 'muscle quality' in the title and/or abstract. Publications that conceptually described muscle quality but did not include measurement of EI were not a focus of the review. RESULT: Importantly, the foundational premise of EI remains unclear. While it is likely that EI reflects intramuscular adiposity, data suggesting that these measurements are influenced by fibrous tissue is limited to diseased muscle and animal models. EI appears to show particular promise in studying muscular aging. Studies have consistently reported an association between EI and muscle function, though not all chronic interventions have demonstrated improvements. Based on the existing literature, it is unclear if EI can be used as a marker of muscle glycogen following exercise and nutritional interventions, or if EI is influenced by hydration status. Inconsistent methodological approaches used across laboratories have made comparing EI studies challenging. Image depth, rest duration, participant positioning, probe tilt, and the decision to correct for subcutaneous adipose tissue thickness are all critical considerations when interpreting the literature and planning studies. CONCLUSION: While some areas show conflicting evidence, EI shows promise as a novel tool for studying muscle quality. Collaborative efforts focused on methodology are necessary to enhance the consistency and quality of the EI literature.

Development and examination of a functional reactive agility test for older adults.

BACKGROUND: Reactive agility tests have become popular in sport for talent identification; however, the ability of these tests to evaluate physical function and falls risk in clinical populations warrants future study. AIM: To examine the reliability and construct validity of a novel functional reactive agility test (FRAT) across the lifespan. METHODS: Forty-three young (24 ± 2 years), 32 middle-aged (50 ± 2 years), and 19 old (66 ± 4 years) men performed a FRAT that included a rapid lateral movement (1.5 m) in response to a randomly delayed visual stimulus. Test-retest reliability and minimum difference (MD) scores were determined from a subset of participants. RESULTS: There was no systematic error (P > 0.087) between testing days, and the intraclass correlation coefficients, standard error of measurement (% of the mean), and MD values for decision time, movement time, and total time ranged from 0.876 to 0.949, 4.16-9.24%, and 0.128-0.138 s, respectively. The young men had faster decision times (P = 0.027) when compared to the older men and faster total times when compared to the middle-aged and older men (P < 0.001). DISCUSSION: The FRAT demonstrated acceptable reliability and construct validity between different age groups. CONCLUSION: Due to its limited space requirements, the FRAT may serve as a useful tool in future studies examining clinical populations.

Effects of Age, Joint Angle, and Test Modality on Strength Production and Functional Outcomes.

This study investigated the impact of age, knee joint angle, and strength testing modality on lower body maximal and rapid strength production and assessed the transferability of these characteristics to mobility-related function. Twenty young (age=21.9 yrs) and eighteen elderly (71.1 yrs) adults performed single-joint and multiple-joint isometric maximal voluntary contractions at three knee angles. Outcome measures included peak torque (PT), rate of torque development (RTD), jump height, power, 10 and 400 m walk, and timed chair stand. Older adults exhibited greater reductions in RTD for the multiple-joint (45%) than for the single-joint mode (18%). The 10 m walk was best predicted by multiple-joint RTD at 90°, the 400 m walk by jump height, and the chair stand by single-joint PT at 20°. Single-joint strength tests may underestimate age-related rapid strength impairments, likely due to requirements to exert muscular force in excess of one's body mass in an upright position.

A lower extremity strength-based profile of NCAA Division I women's basketball and gymnastics athletes: implications for knee joint injury risk assessment.

This study aimed to provide a comprehensive strength-based physiological profile of women's NCAA Division I basketball and gymnastic athletes; and to make sport-specific comparisons for various strength characteristics of the knee flexor and extensor muscles. A focus on antagonist muscle balance (hamstrings-to-quadriceps ratios, H:Q) was used to elucidate vulnerabilities in these at-risk female athletes. Fourteen NCAA Division I women's basketball and 13 gymnastics athletes performed strength testing of the knee extensors and flexors. Outcome measures included absolute and relative (body mass normalised) peak torque (PT), rate of torque development at 50, 100, 200 ms (RTD50 etc.) and H:Q ratios of all variables. The basketball athletes had greater absolute strength for all variables except for isokinetic PT at 240°s-1 and isometric RTD50 for the knee extensors. Gymnasts showed ~20% weaker body mass relative concentric PT for the knee flexors at 60 and 120°·s-1, and decreased conventional H:Q ratios at 60 and 240°·s-1 (~15%). These findings suggest that collegiate level gymnastics athletes may be prone to increased ACL injury risk due to deficient knee flexor strength and H:Q strength imbalance. Coaches may use these findings when implementing injury prevention screening and/or for individualised strength training programming centered around an athletes strength-related deficits.

Influence of age on passive stiffness and size, quality, and strength characteristics.

INTRODUCTION: We examined the effects of aging on passive stiffness, size, quality, and strength characteristics of the posterior hip and thigh muscles. METHODS: Fifteen young (25 ± 3 years) and 15 old (72 ± 5 years) men participated in this study. Echo intensity (EI) and cross-sectional area (CSA) were determined from ultrasound scans of the hamstrings. Straight-leg raises were used to assess passive stiffness, which was calculated from the slopes of the initial (phase 1) and final (phase 2) portions of the angle-torque curve. Peak torque (PT) and rate of torque development (RTD) were assessed through maximal voluntary contractions. RESULTS: Phase 2 slope and EI were higher (P ≤ 0.024), and CSA, PT, and RTD were lower (P ≤ 0.011) in the old compared with the young men; however, no difference (P = 0.145) was observed for phase 1 slope. CONCLUSIONS: Decreases in muscle quality, as indicated by increases in EI, may contribute to the greater passive stiffness observed in older adults. Muscle Nerve 55: 305-315, 2017.

Echo intensity and muscle thickness as predictors Of athleticism and isometric strength in middle-school boys.

INTRODUCTION: We examined correlations among echo intensity and muscle thickness versus measures of athleticism and isometric strength in healthy children. METHODS: B-mode ultrasonography was used to examine the vastus lateralis (VL) and rectus femoris (RF) in 28 boys (mean age = 12 years). Tests of athleticism included jump height and peak velocity, sprint speed, and agility. Peak torque and the rate of torque development (RTD) at 100 and 200 ms from torque onset were assessed. RESULTS: Several significant correlations existed after controlling for age and mass. RF thickness showed the strongest correlations with the dependent variables. Both RTD time intervals, but not peak torque, correlated with VL thickness. Peak torque was greatly influenced by age and mass. Echo intensity and muscle thickness correlated highly with agility performance. CONCLUSIONS: VL and RF echo intensity and muscle thickness may help predict athleticism, and to a lesser extent, RTD, in boys. Muscle Nerve 55: 685-692, 2017.

Age-related effects on maximal and rapid hamstrings/quadriceps strength capacities and vertical jump power in young and older females.

BACKGROUND AND AIMS: Declines in hamstrings and quadriceps maximal and rapid strength are commonly reported as a consequence of aging; however, few studies have investigated the influence of age on maximal and rapid hamstrings to quadriceps (H/Q) strength ratios. This study aimed to examine the effects of age on leg extension and flexion maximal and rapid torque production, H/Q strength ratios, and muscle power in healthy, young and old females. METHODS: Fifteen young (21 ± 2 years) and 15 old (69 ± 7 years) females performed 3 isometric leg extension and flexion maximal voluntary contractions (MVCs), from which peak torque (PT), early (0-30 ms) and late (0-200 ms) phase rate of torque development (RTD), and PT and RTD H/Q strength ratios were extracted. Lower-body peak power output (P max) was assessed through a vertical jump test. RESULTS: P max, PT, and early (RTD30) and late (RTD200) RTD variables of the leg extensors and flexors were lower (P ≤ 0.048) and PT and RTD200 H/Q strength ratios were higher (P ≤ 0.030) in the old compared to the younger females; however, no difference was observed for RTD30 H/Q (P = 0.300). There were also significant negative relationships in the older (r = -0.616 and -0.522; P = 0.014 and 0.046) but not the younger (r = 0.295 and -0.109; P = 0.286 and 0.698) females between P max and RTD30 and RTD200 H/Q strength ratios. CONCLUSION: The higher RTD200 H/Q ratio for the older females may contribute to the muscle power deficits often observed in elderly populations. Practitioners may use these findings to help identify older adults with low muscle power capacities and possibly overall functional decline.

Effects of Accumulating Work Shifts on Performance-Based Fatigue Using Multiple Strength Measurements in Day and Night Shift Nurses and Aides.

Objective This study aimed to examine the effects of accumulating nursing work on maximal and rapid strength characteristics in female nurses and compare these effects in day versus night shift workers. Background Nurses exhibit among the highest nonfatal injury rates of all occupations, which may be a consequence of long, cumulative work shift schedules. Fatigue may accumulate across multiple shifts and lead to performance impairments, which in turn may be linked to injury risks. Method Thirty-seven nurses and aides performed isometric strength-based performance testing of three muscle groups, including the knee extensors, knee flexors, and wrist flexors (hand grip), as well as countermovement jumps, at baseline and following exposure to three 12-hour work shifts in a four-day period. Variables included peak torque (PT) and rate of torque development (RTD) from isometric strength testing and jump height and power output. Results The rigorous work period resulted in significant decreases (-7.2% to -19.2%) in a large majority (8/9) of the isometric strength-based measurements. No differences were noted for the day versus night shift workers except for the RTD at 200 millisecond variable, for which the night shift had greater work-induced decreases than the day shift workers. No changes were observed for jump height or power output. Conclusions A compressed nursing work schedule resulted in decreases in strength-based performance abilities, being indicative of performance fatigue. Application Compressed work schedules involving long shifts lead to functional declines in nurse performance capacities that may pose risks for both the nurse and patient quality of care. Fatigue management plans are needed to monitor and regulate increased levels of fatigue.

Age-Related Differences in Maximal and Rapid Torque Characteristics of the Hip Extensors and Dynamic Postural Balance in Healthy, Young and Old Females.

Palmer, TB, Thiele, RM, and Thompson, BJ. Age-related differences in maximal and rapid torque characteristics of the hip extensors and dynamic postural balance in healthy, young and old females. J Strength Cond Res 31(2): 480-488, 2017-The purpose of this study was to examine age-related differences in maximal and rapid torque characteristics of the hip extensor muscles and dynamic postural balance in healthy, young and older females. Eleven younger (age, 26 ± 8 years) and 11 older (age, 67 ± 8 years) females performed 2 isometric maximal voluntary contractions (MVCs) of the hip extensor muscles. Absolute and relative peak torque (PT) and rate of torque development (RTD) at early (0-50 ms) and late (0-200 ms) phases of muscle contraction were examined during each MVC. Dynamic postural balance was assessed using a commercially designed balance testing device, which provides a measurement of dynamic stability based on the overall stability index (OSI). Results indicated that absolute PT and early (RTD50) and late (RTD200) RTD variables were lower (p = 0.009-0.050), and postural OSI was higher (p = 0.011) in the old compared with the younger females; however, no differences were observed for relative PT or RTD variables (p = 0.113-0.895). A significant relationship was also observed in the older (r = -0.601; p = 0.050) but not the younger (r = -0.132; p = 0.698) females between RTD50 and OSI. The lower absolute PT and RTD and higher OSI values for the old females may contribute to the increased functional limitations often observed in older adults. The significant relationship observed in the older females between OSI and RTD50 perhaps suggests that these age-related declines in explosive strength may be an important characteristic relevant to dynamic balance scores, especially in older populations.

Adaptations Associated With an After-School Strength and Conditioning Program in Middle-School-Aged Boys: A Quasi-Experimental Design.

High-intensity strength and conditioning programs aimed at improving youth performance are becoming increasingly prevalent. The purpose of this study was to investigate the effects of a 16-week after-school strength and conditioning program on performance and body composition in middle-school-aged boys. Subjects in the training group (n = 16, mean age = 11.8 years) performed 90 minutes of supervised plyometric and resistance training twice weekly for 16 weeks. A group of control subjects (n = 9, age = 12.1 years) maintained their current activity levels. Sprint speed, 5-10-5 proagility, jump height, isometric peak torque of the leg extensors and flexors, and dual energy x-ray absorptiometry-derived body composition were examined during pretesting and posttesting. Data were analyzed by performing independent samples t-tests on the absolute change scores between groups. The primary findings were that the training intervention elicited significant improvements in 20-m sprint times (p = 0.03; mean change for training group = -0.17 seconds) and body-fat percentage (p = 0.03; 2.5% absolute improvement), the latter of which was a function of reduced fat mass (p = 0.06; -0.84 kg). Between-group differences were not noted for agility, jump height, lean mass, or strength measures; however, effect sizes generally showed greater improvements for the training group. In contrast to findings in longitudinal studies performed in collegiate athletes, sprint speed may be particularly adaptable during adolescence. In addition to potentially improving sport performance, high-intensity plyometric and resistance training programs offer the added benefit of improved body composition. These programs appear less effective for agility and jump performance and do not elicit substantial improvements in muscle mass above maturation.

Effects of neuromuscular fatigue on electromechanical delay of the leg extensors and flexors in young men and women.

INTRODUCTION: We examined the effects of neuromuscular fatigue on volitional electromechanical delay (EMD) of leg extensors and flexors between genders. METHODS: Twenty-one men and 20 women performed 2 maximal voluntary contractions (MVCs), followed by intermittent isometric contractions of leg extensors and flexors using a 0.6 duty cycle (6-s contraction, 4-s relaxation) at 50% of MVC until volitional fatigue was achieved. MVCs were again performed at 0, 7, 15, and 30 min post-fatigue. RESULTS: EMD was greater compared with baseline at all post-fatigue time phases for the leg flexors (P = 0.001-0.007), while EMD was greater at Post0, Post15 and Post30 (P = 0.001-0.023) for the leg extensors. EMD was also greater for leg extensors compared with leg flexors only at Post0. CONCLUSION: No differential gender-related fatigue effects on EMD were shown. There were different fatigue-induced responses between leg extensors and flexors, with leg extensors exhibiting higher EMD immediately post-fatigue.

The identification of fall history using maximal and rapid isometric torque characteristics of the hip extensors in healthy, recreationally active elderly females: a preliminary investigation.

BACKGROUND AND AIMS: Maximal and rapid torque characteristics of the hip extensor muscles play an important role in fall prevention and other balance-related performances; however, few studies have investigated the ability of these variables at identifying fall-history status in healthy, recreationally active elderly adults. This study aimed to examine the effectiveness of maximal and rapid isometric torque characteristics of the hip extensor muscles to differentiate between healthy, recreationally active elderly females with (fallers) and without (non-fallers) a history a falls. METHODS: Six elderly female fallers (mean ± SD: age = 73 ± 7 year; mass = 68 ± 16 kg; height = 160 ± 5 cm) and nine elderly female non-fallers (age = 71 ± 7 year; mass = 66 ± 16 kg; height = 157 ± 6 cm) performed two isometric maximal voluntary contractions (MVCs) of the hip extensor muscles. Peak torque (PT) and absolute and relative rate of torque development (RTD) at the early (0-50 ms) and late (100-200 ms) phases of muscle contraction were examined during each MVC. RESULTS: Absolute and relative RTD at 0-50 ms were greater (P = 0.039 and 0.011, respectively) in the non-fallers compared to the fallers. However, no group-related differences (P = 0.160-0.573) were observed for PT nor absolute and relative RTD at 100-200 ms. CONCLUSIONS: Early rapid strength production of the hip extensor muscles may be a sensitive and effective measure for discriminating between elderly females of different fall histories. These findings may provide important insight regarding implications for the assessment of fall risk and in the development of proper training programs aimed at minimizing the occurrence of falls and other balance-related injuries in the elderly.

Influence of Hamstring Fatigue on the Estimated Percentage of Fast-Twitch Muscle Fibers for the Vastus Lateralis.

A previous study has demonstrated the ability to roughly estimate the percentage of fast-twitch muscle fibers for the vastus lateralis through the analysis of peak torque values during fatiguing isokinetic testing. We examined whether use of the hamstrings influenced peak torque and electromyographic (EMG) responses for the quadriceps during fatiguing isokinetic muscle actions. On 2 separate occasions, 21 men (mean age = 23 years) performed 50 repeated, maximal concentric isokinetic muscle actions of the left leg extensors at a velocity of 180°·s. For 1 trial, the subjects maximally flexed the knee joint after each full extension to bring the dynamometer's lever arm back to the starting position. For the other trial, the subjects relaxed after each maximal extension and an investigator assisted in returning the lever arm. Surface EMG signals were detected from the vastus lateralis and biceps femoris throughout testing. Dependent variables that assessed the decline in peak torque and EMG mean frequency for the vastus lateralis were examined using dependent samples t-tests, effect size statistics, and the number of subjects who exceeded the minimal difference needed to be considered real. Our results showed small mean differences between the trials (Cohen's d ≤0.136). For the estimated percentage of fast-twitch fibers, none of the subjects showed a difference between trials that we considered meaningful. The mean estimated percentages of fast-twitch fibers were 61.6 and 60.1. Collectively, use of the hamstrings during fatiguing isokinetic testing of the quadriceps had little influence on peak torque and EMG.

Acute Postexercise Time Course Responses of Hypertrophic vs. Power-Endurance Squat Exercise Protocols on Maximal and Rapid Torque of the Knee Extensors.

The aim of this study was to examine the effects of a medium-intensity high-volume vs. explosive squat protocol on the postexercise time course responses of maximal and rapid strength of the knee extensors. Seventeen resistance-trained men (mean ± SD: age = 22.0 ± 2.6 years) performed maximal voluntary contractions (MVCs) of the knee extensors before and after performing a squat workout using either a low-intensity fast velocity (LIFV) (5 × 16 at 40% 1 repetition maximum) or a traditional high-intensity slow velocity (TISV) (5 × 8 at 80% 1RM) exercise protocol. For each MVC, peak torque (PT), peak rate of torque development (RTDpeak), absolute (RTDabs), and relative RTD (RTDnorm) at early (0-50 milliseconds) and late (100-200 milliseconds) phases of muscle contraction were examined at pre- (Pre) and post-exercise at 0, 7, 15, and 30 (Post0...30) minutes. There were no intensity × time interactions for any variables (p = 0.098-0.832). Peak torque was greater at Pre than Post0 and Post7 (p = 0.001-0.016) but was not greater than Post15 and Post30 (p = 0.010-0.189). RTDpeak and early absolute RTD (RTD50abs) were greater at Pre than all postexercise time phases (p = 0.001-0.050); however, later absolute RTD (RTD100-200abs) was only greater at Pre than Post0 and Post30 (p = 0.013-0.048). Early relative RTD (RTD50norm) was only higher at Pre compared with Post0 (p = 0.023), whereas no differences were observed for later relative RTD (RTD100-200norm) (p = 0.920-0.990). Low-intensity fast velocity and TISV squat protocols both yielded acute decreases in maximal and rapid strength capacities following free-weight squats, with rapid strength showing slower recovery characteristics than maximal strength.