Force Production Measurements During a Supine Medicine Ball Throw: a Reliability and Correlation Study.

OBJECTIVES: This study aimed to examine the reliability of supine medicine ball throw peak force and rate of force development (RFD) measurements. A secondary aim was to investigate the correlations between these measurements and vertical jump height. METHODS: Twenty young women (21±3 years) reported for experimental testing on two different occasions. Supine medicine ball throw assessments were performed during each testing session to assess peak force, RFDmax, and RFD at specific percentages of peak force (RFD30% and RFD40-80%). Vertical jumps were performed on a jump mat. The jump mat measured vertical jump height based on flight time. RESULTS: Good intraclass correlation coefficients (≥0.82) and coefficients of variation (≤14.0%) were observed between sessions for peak force, RFDmax, and RFD40-80%, but not for RFD30% (0.55, 27.2%). There were significant correlations between jump height and peak force (r=0.483, P=0.031), RFDmax (r=0.484, P=0.031), and RFD40-80% (r=0.491, P=0.028). There was no significant correlation between jump height and RFD30% (r=0.359, P=0.120). CONCLUSIONS: Our results showed that supine medicine ball throw peak force, RFDmax, and RFD40-80% were reliable measures for assessing upper-body explosive strength in young adults. These measurements were significantly associated with vertical jump height and therefore, may be effective predictors of one's athletic ability.

Can handgrip strength measurements predict postural balance performance in older women?

OBJECTIVES: The link between handgrip strength and postural balance in older adults is not well understood. This study aimed to examine the correlations between postural balance performance and handgrip peak force and rate of force development (RFD) measurements in older women. METHODS: Twenty older women (67 ± 5 years) participated in this study. Handgrip contractions were used to assess peak force, peak RFD, and RFD at 0-100 (RFD100) and 0-200 (RFD200) ms. Postural balance was evaluated using a commercially designed balance testing device, which provides a measurement of static stability based on the sway index. RESULTS: There were significant correlations between sway index and handgrip peak force (r = -0.497, P = 0.026), peak RFD (r = -0.552, P = 0.012), RFD100 (r = -0.539, P = 0.014), and RFD200 (r = -0.499, P = 0.025). Stepwise multiple regression analysis indicated that handgrip peak RFD was the single best predictor of sway index (R2 = 0.305). The other variables, including peak force, did not add any unique variance to the stepwise prediction model. CONCLUSION: These findings suggest that handgrip strength, and in particular peak RFD, may be an effective parameter at predicting postural balance performance in older women.

Short-term Effects of Static Stretching on Hamstring Passive Stiffness in Young and Older Women.

OBJECTIVES: The objectives of this study were to assess the acute effects of static stretching on hamstring passive stiffness in young and older women. A secondary objective was to compare hamstring muscle size and quality measurements (cross-sectional area and echo intensity) between the two groups and to determine if these characteristics are related to passive stiffness at baseline. METHODS: Fifteen young (23±4 years) and 15 older (73±5 years) women underwent two randomized conditions that included a control treatment and an experimental treatment of four, 15-s static stretches of the hamstrings. Passive stiffness was calculated before (pre-test) and after (post-test) each treatment using a passive knee extension test. Ultrasound imaging was used to measure hamstring muscle cross-sectional area and echo intensity. RESULTS: Passive stiffness collapsed across group decreased from pre- to post-test for the stretching treatment (P=0.001) but not for the control (P=0.467). The older women had lower cross-sectional area (P=0.033) and greater baseline (pre-test) passive stiffness (P=0.042-0.049) and echo intensity (P=0.022) than the young women. Moreover, baseline passive stiffness was significantly related to echo intensity (r=0.430, P=0.018) but not cross-sectional area (r=-0.014, P=0.943). CONCLUSION: An acute bout of static stretching decreased passive stiffness in both young and older women.

Tradeoffs of estimating reaction time with absolute and relative thresholds.

Measuring the duration of cognitive processing with reaction time is fundamental to several subfields of psychology. Many methods exist for estimating movement initiation when measuring reaction time, but there is an incomplete understanding of their relative performance. The purpose of the present study was to identify and compare the tradeoffs of 19 estimates of movement initiation across two experiments. We focused our investigation on estimating movement initiation on each trial with filtered kinematic and kinetic data. Nine of the estimates involved absolute thresholds (e.g., acceleration 1000 back to 200 mm/s2, micro push-button switch), and the remaining ten estimates used relative thresholds (e.g., force extrapolation, 5% of maximum velocity). The criteria were the duration of reaction time, immunity to the movement amplitude, responsiveness to visual feedback during movement execution, reliability, and the number of manually corrected trials (efficacy). The three best overall estimates, in descending order, were yank extrapolation, force extrapolation, and acceleration 1000 to 200 mm/s2. The sensitive micro push-button switch, which was the simplest estimate, had a decent overall score, but it was a late estimate of movement initiation. The relative thresholds based on kinematics had the six worst overall scores. An issue with the relative kinematic thresholds was that they were biased by the movement amplitude. In summary, we recommend measuring reaction time on each trial with one of the three best overall estimates of movement initiation. Future research should continue to refine existing estimates while also exploring new ones.

Rate of torque development as a discriminator of playing level in collegiate female soccer players.

OBJECTIVES: This study aimed to examine the efficacy of isometric knee extension and flexion peak torque and rate of torque development (RTD) variables to distinguish starters from non-starters in collegiate female soccer players. METHODS: Eleven starters (20±2 years) and 13 non-starters (19±1 years) performed three isometric maximal voluntary contractions of the knee extensors and flexors. Peak torque, peak RTD, and RTD at 0-100 (RTD100) and 0-200 (RTD200) ms were obtained from each contraction. RESULTS: The starters produced significantly greater (P=0.002-0.015) knee extension and flexion peak RTD, RTD100, and RTD200 values than the non-starters. There were no significant differences (P>0.050) between the starters and non-starters for peak torque. Discriminant analysis revealed thresholds of 9.36, 7.98, and 6.97 Nm⋅s-1⋅kg-1 for knee extension RTD200 and knee flexion peak RTD and RTD100, respectively. These thresholds showed 81.8% sensitivity and 76.9 to 92.3% specificity for identifying playing group membership. CONCLUSIONS: Our findings indicate that RTD may be a better parameter than peak torque at differentiating between playing level in collegiate female soccer players. The discriminant analysis thresholds for the RTD variables demonstrated good sensitivity and specificity, and therefore, may be used as indices to identify players with a high degree of soccer playing ability.

The Acute Effects of Fast-Paced Walking on Isometric Peak Torque and Rate of Torque Development in Regularly Exercising and Inactive Older Women.

This study aimed to examine the acute effects of fast-paced walking on isometric peak torque and rate of torque development (RTD) in regular exercising and inactive older women. Ten regular exercising (67 ± 4 years) and 10 inactive (68 ± 4 years) older women performed three isometric knee extension contractions before and after a control condition (quiet resting) and an experimental condition of fast-paced walking for 6 min. Peak torque and early (RTD100), late (RTD200), and maximum (peak RTD) RTD measurements were obtained from each contraction. Results showed no significant changes in peak torque, peak RTD, or RTD200 after walking for either group (p > .050). A significant decrease in RTD100 was observed after walking for the inactive group (p = .005) but not for the regular exercisers (p = .909). These findings highlight the importance of physical activity and suggest that a task as simple as walking may impair the rapid strength capacities of inactive older women.

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.

Utility of peak torque and rate of torque development characteristics to identify walking performance ability in older women.

OBJECTIVES: It is unclear whether peak torque and rate of torque development (RTD) measurements can characterize functional differences in older adults according to their performance on a six-minute walk test. This study aimed to examine the efficacy of isometric peak torque and RTD characteristics of the knee extensors to differentiate between functional status in older women who are able (higher functioning) versus those who are unable (lower functioning) to walk 550 m in six minutes. METHODS: Ten higher functioning (67±4 years) and 10 lower functioning (68±4 years) older women performed three isometric knee extension maximal voluntary contractions followed by a six-minute walk test. Peak torque and early (RTD100), late (RTD200), and maximum (Peak RTD) RTD measurements were obtained from each contraction. RESULTS: The higher functioning group exhibited greater peak torque, Peak RTD, RTD100, and RTD200 compared to the lower functioning group (P≤0.011), with larger differences occurring for RTD characteristics (39.9-54.9%) than peak torque (20.3%). Multiple regression analysis indicated that RTD200 was the single best predictor of the distance covered during the six-minute walk test (R2=0.437, P=0.002). CONCLUSIONS: These findings suggest that knee extensor muscle strength, and in particular RTD, may be an effective discriminator and predictor of walking performance ability in older women.

Age-Related Differences in Hip Flexion Maximal and Rapid Strength and Rectus Femoris Muscle Size and Composition.

This study aimed to examine the effects of age on hip flexion maximal and rapid strength and rectus femoris (RF) muscle size and composition in men. Fifteen young (25 [3] y) and 15 older (73 [4] y) men performed isometric hip flexion contractions to examine peak torque and absolute and normalized rate of torque development (RTD) at time intervals of 0 to 100 and 100 to 200 milliseconds. Ultrasonography was used to examine RF muscle cross-sectional area and echo intensity. Peak torque, absolute RTD at 0 to 100 milliseconds, and absolute and normalized RTD at 100 to 200 milliseconds were significantly lower (P = .004-.045) in the old compared with the young men. The older men exhibited lower cross-sectional area (P = .015) and higher echo intensity (P = .007) than the young men. Moreover, there were positive relationships between cross-sectional area and absolute RTD at 0 to 100 milliseconds (r = .400) and absolute RTD at 100 to 200 milliseconds (r = .450) and negative relationships between echo intensity and absolute RTD at 100 to 200 milliseconds (r = -.457) and normalized RTD at 100 to 200 milliseconds (r = -.373). These findings indicate that hip flexion maximal and rapid strength and RF muscle size and composition decrease in old age. The relationships observed between ultrasound-derived RF parameters and measurements of RTD suggest that these age-related declines in muscle size and composition may be relevant to hip flexion rapid torque production.

Relationships between hamstring morphological characteristics and postural balance in elderly men.

OBJECTIVES: The link between hamstring morphology and postural balance performance in older adults is not well understood. This study aimed to examine the relationships between hamstring morphological characteristics of muscle size (cross-sectional area [CSA]) and quality (echo intensity [EI]) and postural balance with the eyes open and closed in elderly men. METHODS: Nineteen healthy elderly men (age= 73±4 years) participated in this study. Muscle CSA and EI were determined from ultrasound scans of the hamstrings. Postural balance was assessed with the eyes open and closed using a commercially designed balance testing device, which provides a measurement of static stability based on the sway index. RESULTS: The sway index with eyes closed was significantly related to muscle EI (r=0.474; P=0.040) but not CSA (r=0.021; P=0.932). The sway index with eyes open was not related to muscle CSA (r= -0.036; P=0.883) or EI (r=-0.079; P=0.747). CONCLUSIONS: The significant relationship observed between the sway index with eyes closed and muscle EI suggests that hamstring muscle quality may be a characteristic relevant to postural balance in the absence of visual feedback. These findings may provide important insight regarding the morphological mechanisms involved in maintaining balance and in the development of proper training programs aimed at improving postural stability in older individuals.

Acute Effects of Constant-Angle and Constant-Torque Static Stretching on Passive Stiffness of the Posterior Hip and Thigh Muscles in Healthy, Young and Old Men.

Palmer, TB. Acute effects of constant-angle and constant-torque static stretching on passive stiffness of the posterior hip and thigh muscles in healthy, young and old men. J Strength Cond Res 33(11): 2991-2999, 2019-The purpose of this study was to examine the acute effects of constant-angle (CA) and constant-torque (CT) static stretching on passive stiffness of the posterior hip and thigh muscles in healthy, young and old men. Fifteen young (25 ± 3 years) and 15 old (71 ± 4 years) men underwent 2 passive straight-leg raise (SLR) assessments before and after 8 minutes of CA and CT stretching using an isokinetic dynamometer. Passive stiffness was calculated during each SLR as the slope of the final 10% of the angle-torque curve. The results indicated that passive stiffness decreased from pre- to post-stretching for both treatments (p ≤ 0.001-0.002) and age groups (p ≤ 0.001-0.046); however, greater decreases were observed for the CT than the CA stretching (p = 0.045) and for the old than the young men (p < 0.001). In addition, baseline stiffness was greater for the old than the younger men (p = 0.010) and was also negatively related (r = -0.721; p < 0.001) to the changes in stiffness from pre- to post-stretching. These findings suggest that holding stretches at a constant tension may be a more effective strategy for altering passive stiffness of the posterior hip and thigh muscles. The greater stretch-induced stiffness decreases observed for the older men provide support that acute static stretching may be particularly effective for reducing stiffness in the elderly. As a result, it may be advantageous to prescribe static stretching before exercise for older adults, as this may be used to elicit substantial declines in passive stiffness, which could help reduce the risk of subsequent injury events in this population.

Duration-Dependent Effects of Passive Static Stretching on Musculotendinous Stiffness and Maximal and Rapid Torque and Surface Electromyography Characteristics of the Hamstrings.

Palmer, TB, Pineda, JG, Cruz, MR, and Agu-Udemba, CC. Duration-dependent effects of passive static stretching on musculotendinous stiffness and maximal and rapid torque and surface electromyography characteristics of the hamstrings. J Strength Cond Res 33(3): 717-726, 2019-This study aimed to examine the effects of stretching duration on passive musculotendinous stiffness and maximal and rapid torque and surface electromyography (EMG) characteristics of the hamstrings. Thirteen young females (age = 21 ± 2 years) underwent 2 passive straight-leg raise (SLR) assessments and 2 isometric maximal voluntary contractions (MVCs) of the hamstrings before and after 4 randomized conditions that included a control treatment and 3 experimental treatments of passive static stretching for 30-, 60-, and 120-second durations. Passive stiffness was calculated during each SLR as the slope of the final 10% of the angle-torque curve. Isometric peak torque (PT), rate of torque development (RTD), peak EMG amplitude (PEMG), and rate of EMG rise (RER) were extracted from each MVC. Results indicated that PT and PEMG were not affected (p = 0.993 and 0.422, respectively) by any of the experimental treatments. Rate of torque development and RER decreased from pre- to post-treatment for 120 seconds (p = 0.001 and 0.001) but not for the control (p = 0.616 and 0.466), 30- (p = 0.628 and 0.612), and 60-second (p = 0.396 and 0.815) interventions. The slope coefficient decreased from pre- to post-treatment for the 30- (p = 0.001), 60- (p = 0.002), and 120-second (p = 0.001) stretching interventions but not for the control (p = 0.649). Given the significant stiffness reductions and lack of changes in PT and RTD for the 30- and 60-second interventions, it may be advantageous for practitioners who are using hamstring passive stretching as part of a warm-up routine, to perform such stretching on their clients for short (30-60 seconds) rather than moderate (120-second) stretching durations.

Effect of Energy Drink Consumption on Power and Velocity of Selected Sport Performance Activities.

Jacobson, BH, Hester, GM, Palmer, TB, Williams, K, Pope, ZK, Sellers, JH, Conchola, EC, Woolsey, C, and Estrada, C. Effect of energy drink consumption on power and velocity of selected sport performance activities. J Strength Cond Res 32(6): 1613-1618, 2018-Energy drinks (ED) comprise a multibillion dollar market focused on younger, active, and competitive individuals. Marketing includes claims of improved alertness and performance. The purpose of this study was to assess power (W) and velocity (m·s) of a simulated, isolated forehand stroke (FHS), and a countermovement vertical jump (CVJ) before and after ingestion of a commercially available energy shot (ES) or a placebo (PL). Healthy college-aged men and women (N = 36) volunteers were randomly placed in the ES or PL. Before and 30 minutes after ingesting either the ES or PL, participants performed 3 FHSs and CVJs. Power and velocity of each performance was measured using a linear velocity transducer and the highest value for each measure was used for subsequent analysis. The ES group demonstrated a significant (p = 0.05) increase in velocity and W for the FHS, but not for the CVJ. All measures remained unchanged in the PL group for both, the FHS and CVJ. Females demonstrated a significant increase in velocity over males in FHS, but not in CVJ. It was concluded that while the dose of stimulants in the ES was adequate to improve performance of smaller muscle groups, it may not have been sufficient to affect the larger muscle groups of the lower legs which contribute to the CVJ. While the ES used in the present study contained a caffeine dosage within the NCAA limit and did improve performance for the upper body, it must be noted that there are health risks associated with ED consumption.

Effects of Knee Position on the Reliability and Production of Maximal and Rapid Strength Characteristics During an Isometric Squat Test.

This study aimed to examine the effects of knee position on the reliability and production of peak force (PF) and rate of force development (RFD) characteristics during an isometric squat. Fourteen resistance-trained females performed isometric squats at 90, 120, and 150° knee angles (corresponding to parallel, half, and quarter squat positions, respectively) on 2 different occasions, from which PF, peak RFD, and early (RFD30, RFD50, RFD100) and late (RFD200) phase RFD variables were extracted. PF and RFD200 were highly consistent across trials for all 3 squat positions, with intraclass correlation coefficients (ICCs) ranging between 0.812-0.904 and coefficients of variation (CVs) between 6.6-19.4%. For peak and early RFD characteristics, higher ICCs and lower CV values were observed for the quarter squat (ICCs = 0.818-0.852, CVs = 17.3-19.4%) compared to the parallel (ICCs = 0.591-0.649, CVs = 30.1-55.9%) and half squats (ICCs = 0.547-0.598,CVs = 31.1-34.2%). In addition, isometric PF and RFD200 increased (P ≤ .001-0.04) with squat position (parallel < half < quarter); however, there were no differences for peak RFD (P ≥ .27), RFD30 (P ≥ .99), RFD50 (P ≥ .99), and RFD100 (P ≥ .09). These findings suggest that performing isometric squats at higher (150°) rather than lower knee joint angles (90-120°) may provide for an improved capacity to produce greater PF and RFD200 as well as a more reliable testing position for measuring peak and early RFD characteristics.

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.

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.

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.

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.

Relative differences in strength and power from slow to fast isokinetic velocities may reflect dynapenia.

INTRODUCTION: We compared absolute and normalized values for peak torque (PT), mean power (MP), rate of velocity development, and electromyography (EMG) amplitude during maximal isometric and concentric isokinetic leg extension muscle actions, as well as the %decrease in PT and %increase in MP from 1.05 to 3.14 rad·s(-1) in younger versus older men. METHODS: Measurements were performed twice for reliability. Isokinetic measurements were normalized to the isometric muscle actions. RESULTS: Absolute isometric PT, isokinetic PT and MP, and EMG amplitudes at 1.05 and 3.14 rad·s(-1) were greater in the younger men, although normalizing to isometric PT eliminated the age differences. The older men exhibited greater %decrease in PT (37.2% vs. 31.3%) and lower %increase in MP (87.6% vs. 126.4%) regardless of normalization. CONCLUSIONS: Normalization eliminated absolute differences in isokinetic strength and power, but the relative differences from slow to fast velocities may reflect dynapenia characterized by age-related decreases in fast-twitch fiber function.