Sex differences in the functional hamstring to quadriceps ratio and neuromuscular performance

Increased relative risk of non-contact anterior cruciate ligament (ACL) injury has been attributed to numerous biomechanical, anatomical and neuromuscular factors. Females are at greater relative risk of non-contact ACL injury compared with males. Dynamic knee stability is an important component required to reduce relative risk of injury, especially to the knee joint. It is difficult to directly measure knee stability; however the eccentric ability of the hamstrings to co-contract to counter the torque produced by concentric quadriceps actions during knee extension is important in stabilising the knee (determined as the functional H/Q ratio [FH/Q]). One of the proposed reasons for a greater incidence of non-contact ACL injury in females is a reduced capacity for neuromuscular functioning to stabilise the knee joint effectively. Most injuries occur in the second half of an athletic event when fatigue is commonly present, therefore identifying fatigue as a potential risk factor for ACL injury and this may allow for the development of improved prevention strategies. The three experimental studies included within this thesis (chapter 4-6) have generated novel data on sex differences in the FH/Q ratio and neuromuscular performance prior to and following a downhill running fatigue task. One hundred and ten healthy males (n=55; mean ± SD 29 ± 5 yrs) and females (n=55; mean ± SD 27 ± 6 yrs) were recruited from the university population. Isokinetic torque of the hamstrings and quadriceps was determined at 60, 120 and 240˚·s–1with the hip flexed at 0°. Range of movement of the knee joint was 90° with 0° determined as full volitional extension. Concentric (CON) torque was determined first followed by eccentric (ECC) torque, with the slowest velocity tested first. Torque was gravity corrected and filtered to only include constant velocity periods. For functional relevance FH/Q ratio was determined at 15, 30, 45º (as these are the joint angles where injury is most likely to occur) and where peak torque (PT) was achieved (to compare with the extant literature) for each movement velocity. Surface electromyography was recorded from the semitendinosus (ST), semimembranosus (SM) and biceps femoris (BF) of the dominant limb using an 8-channel DelSys EMG telemetry system. The biodex square wave synchronization pulse was configured with the EMG software via a trigger system so that EMG and torque data were completely time aligned. Raw EMG data were collected at a sampling frequency of 1024 Hz and included a common mode rejection ratio of <80 dB and an amplifier gain of 1000. Raw EMG data was band pass filtered at 20 – 450 Hz. The electromechanical delay (EMD) was determined as the time delay between the onset of muscle activation (change in activation of +15 μV) and onset of torque production (9.6 N·m) according to the procedures described by Zhou et al (1995). The aim of the first study (chapter 4) was to explore sex differences in the FH/Q ratio whilst taking into account joint angle and movement velocity. A 2 (sex) x 3 (movement velocity) x 4 (joint angle) ANOVA was performed to determine interactions and main effects. FH/Q ratio ranged from 59 to 98% in females and 66 to 109% in males across joint angles and movement velocities. No significant differences between males and females in age but males were significantly taller and had greater body mass. Irrespective of sex the FH/Q ratio increased with joint angle and movement velocity to improve knee stability during high velocity movement and near full extension. However, the FH/Q ratio is significantly lower in females compared with males and importantly this sex difference increases as movement velocity increases. Females have a lower FH/Q ratio than males close to full knee extension and during high velocity movements, both of which are predisposing factors for increased injury risk. This reduced FH/Q ratio may impair dynamic knee stability in females during fast velocity movements and may predispose them to a greater relative risk of knee injury. The aim of the second study (chapter 5) was to examine sex differences in the EMD of the hamstring muscles during eccentric muscle actions at 60, 120 and 240˚·s–1. A 2 (sex) x 3 (muscle group) x 3 (movement velocity) ANOVA was performed to determine interactions and main effects. During eccentric hamstring muscle actions there were no differences in the EMD of the 3 muscles examined. Irrespective of sex, significant main effects for angular velocity was demonstrated, indicating an increase in the delay time with increasing angular velocity. This increased delay during fast velocity movements may account for the increased risk of injury during fast movements. No significant sex differences in EMD was found irrespective of movement velocity of muscle examined, suggesting that females do not have impaired neuromuscular performance of the hamstring compared with males during eccentric hamstring muscle actions in the rested state. The aim of the third study (chapter 6) was to examine the effects of a fatigue task on sex differences of the FH/Q ratio and EMD. The procedures used in study 1 and 2 were repeated pre and post a downhill running fatigue task to explore fatigue related effects on neuromuscular functioning. Each participant performed a 40 min intermittent downhill running protocol consisting of 5 × 8 min bouts on a -10% decline, with 2 min standing rest between each bout. Irrespective of sex, joint angle or angular velocity, the FH/Q ratio was lower and EMD of hamstrings muscle was longer post-fatigue compared to pre-fatigue. Significant interactions between sex and time (pre-post) for the FH/Q ratio and EMD of hamstring muscles were found. The interactions showed a significantly lower FH/Q ratio and significantly longer EMD post fatigue in females compared to males. These data suggest that functional stability of the knee is reduced when fatigue is present and the impact of fatigue is greater in females. The findings of this thesis indicate the importance of determining the FH/Q ratio using angle specific torque as well as taking into account movement velocity, rather than simply using PT values to monitor muscle function of the knee. The findings of the present thesis support the notion that fatigue compromises the stability of the knee by reducing the FH/Q ratio and lengthening EMD. These effects are greater in females compared to males and may predispose them to greater relative risk of injury. Therefore, movement velocity, joint angle and fatigue resistance all need to be considered when designing training programmes to reduce the relative risk of injury. The focus of such training should be aimed at eccentric conditioning of the hamstring muscles to improve both muscular and neuromuscular functioning to limit the fatigue related effects, especially in females

Effect of Palm Temperature on Fatigue During High Intensity Bench Press Exercise

Local cooling, or cryotherapy can induce an ergogenic effect during short term intense exercise. One proposed method of personal cooling involves heat extraction from the palm using a device called the Rapid Thermal Exchanger (RTX). PURPOSE: In this study we hypothesized that local palm cooling during rest intervals between progressive weight training sets will increase total repetitions and exercise volume in resistance trained subjects in a thermoneutral environment and that local palm heating will have no effect. METHODS: Sixteen male subjects (mean±SD, age = 26±6 yr, height = 178±7 cm, body mass = 81.5±11.3kg, 1RM Bench press = 123.5±12.6 kg, % body fat = 10.3±5.4%, weight training experience = 10±6 yr) performed 4 sets of 85% 1RM bench press exercise to fatigue, with 3min rest intervals. Exercise trials were performed in counterbalanced order on three days, separated by at least 3 days; Thermoneutral (TN), Palm Heating (PH), and Palm Cooling (PC). Heating and cooling were applied using the RTX set to 45\uf0b0C and 10\uf0b0C, respectively. Data were analyzed using 2-way repeated measures ANOVA and Tukeys post hoc tests. RESULTS: Total exercise volume (kg) during the 4 PC sets (2480±636) was significantly higher than during TN (1972±632) and PH (2156±668) sets, (p \u3c0.01). The RMS of the surface EMG with PC exercise was higher (p \u3c0.01), and heart rate and RPE were lower (p \u3c0.01 and p \u3c0.05, respectively) during PC compared to TN and PH. CONCLUSION: Palm cooling was associated with increased exercise repetitions and exercise volume, possibly related to a delayed central fatigue or to a peripheral counter-irritation effect. These results suggest that palm cooling may enhance training during progressive resistive exercise.\u2

Neuromuscular Function Following Lengthening Contractions

Unaccustomed lengthening contractions have been shown to impair muscle function - however little is known regarding this impairment on muscle power - specifically, the velocity component of power during voluntary contractions in humans. The four studies presented in my thesis investigated power-loss following lengthening contractions in healthy young and old women and young men. The purpose of Study 1 was to determine reliability of velocity-dependent power of the dorsiflexors using the isotonic mode of the Biodex Dynamometer. I determined the isotonic mode is reliable and can be used to track changes in velocity and power following fatigue and lengthening contractions. The purpose of Study 2 was to investigate changes in neuromuscular properties of the ankle dorsiflexors during and following repetitive lengthening contractions and throughout recovery in 21 (10 men, and, 11 women) recreationally active young adults (25.8 ± 2.3 y). The protocol for the following 3 studies involves subjects performing 5 sets of 30 lengthening contractions, with neuromuscular measures (i.e., electrically evoked twitch, tetanus, voluntary activation, voluntary contractions) recorded at baseline, during the task, and throughout recovery. Exercise induced muscle damage ultimately led to velocity-dependent (i.e., isotonic) power loss at a moderate load (i.e., 20% maximum voluntary strength). Compared with isometric and isokinetic tasks, less is known regarding velocity-dependent muscle power and recovery in older adults following repeated lengthening contractions. In Study 3 we tested 9 old (68.3 ± 6.1 y) and 9 young women (25.1 ± 1.3 y). Old were more impaired following the task than young as shown by greater low-frequency torque depression at task termination leading to a more pronounced initial loss of power than young. However, power remained reduced in both groups during the 30 min recovery period. Older women were more susceptible to power loss than young following lengthening contractions likely owing to a greater fatigue response. In Study 4, power curves were constructed [8 men (27 ± 3 y), 8 women (26 ± 4 y)] using various isotonic loads before and following task termination. There was a preferential loss of power at higher loads, with a relative maintenance of maximal shortening velocity shifting the power curve down and leftward. When stressed with heavier loads during dynamic contractions, force modulators arranged in parallel seem to be affected more by damage than those organized in series (velocity), which was highlighted by the attenuation of power at higher versus lower resistances. The main findings of my thesis are that repetitive lengthening contractions fatigued and temporarily weakened the dorsiflexors, thus impairing their power producing ability immediately (i.e., fatigue + weakness) and longer term (i.e., weakness) owing to an inability to generate torque rapidly

THE EFFECT OF INCREASING LEVELS OF EXERTION ON KNEE JOINT PROPRIOCEPTION

THE EFFECT OF INCREASING LEVELS OF EXERTION ON KNEE JOINT PROPRIOCEPTIONWilliam S. Gear, PhDUNIVERSITY OF PITTSBURGH, 2004The purpose of this study was to examine the effect of incremental levels of isokinetic concentric muscle exertion on passive reproduction of passive positioning (PRPP) and active reproduction of passive positioning (ARPP) at the knee joint in male and female collegiate soccer and basketball players.Subjects for this study included 20 (10 males and 10 females) volunteers. Subjects performed knee extension and flexion concentric isokinetic exercise until torque output fell below the 10%, 30%, or 50% of maximum hamstring torque for three consecutive repetitions. Subjects were then tested on either PRPP or ARPP following the isokinetic exercise session. Following testing of the first independent measure, subjects were given a 20 minute rest period. Following the rest period, the procedure was repeated for two more exercise sessions. Testing of PRPP and ARRP was counterbalanced between trials and sessions in order to decrease the chance of a learning effect on the results of each testing session.The major findings of this study indicate that increasing levels of exertion do not have a significant effect on either active reproduction ability [ARPP-45 degrees (F2,38 = 0.88, p = 0.42), ARPP-30 degrees (F2,38 = 0.69, p = .51), and ARPP-15 (F2,38 = .23, p = 0.80) or passive reproduction ability [PRPP-60 degrees*s-1 (F2,38 = 0.25, p = .78) , PRPP-90 degrees*s-1 (F2,38 = 0.31, p = 0.73), and PRPP120 degress*s-1 (F2,38 = 1.58, p = 0.22)]. However, the reliability of all PRPP and ARPP measures at 15 degrees demonstrated poor reliability.Fatigue has long been theorized to be a contributing factor in decreased proprioceptive acuity, and therefore a contributing factor to joint injury. The lack of significant findings may be explained by the idea that as the level of muscle fatigue increases muscle spindle discharge increases. Poor reliability for all PRPP and ARPP at 15 degrees draws into question the meaningfulness of the results for these measures

An Optimal Interset Rest Period For Strength Recovery During A Common Isokinetic Test

Introduction: Isokinetic testing is used in rehabilitation settings on a regular basis, yet there is a lack of consistency in rest period usage among protocols. Purpose: The purpose of this study was to establish an optimal rest period that would allow reproducibility of strength during a common isokinetic strength-test. Methods: Twentyseven healthy college-aged males underwent isokinetic strength testing to determine peak torque at 60, 180 and 300 deg/sec, respectively. Work:rest ratios of 1:3, 1:8 and 1:12 were counterbalanced between sets. A 3 X 3 repeated measures ANOVA was used to analyze the data. The p \u3c .05 level of significance was used for all tests. Results: There was no significant difference in either knee extension or knee flexion peak torque when comparing work:rest ratios. Conclusion: These findings suggest that a 1:3 work:rest ratio is sufficient during a common isokinetic strength test

Muscle function after exercise-induced muscle damage.

Muscle function after exercise-induced muscle damage has traditionally been evaluated by measures of isometric strength at a single joint angle or muscle length. The thesis investigates the effect of muscle damage on other muscle function parameters such as, isometric strength as a function of muscle length, concentric strength as a function of angular velocity, strength across muscle actions, the stretch-shortening cycle, power output, and fatigability. Study 1 The first part of this study aimed to determine how the muscle length at which strength is measured affects reductions in isometric strength following eccentric exercise-induced muscle damage. The damaging exercise protocol consisted of 100 maximal voluntary eccentric actions of the knee extensors, performed in the prone position through a range of motion from 40° to 140° (0° = full extension) at an angular velocity of 90 deg's-1. Isometric strength of the knee extensors was measured at short muscle length (10° knee flexion) and optimal length (80°). A significantly greater relative loss of strength was observed at short versus optimal muscle length (76.3 ± 2.5% vs. 82.1 ± 2.7% of pre-exercise values, P<0.05) over the seven day testing period following eccentric exercise. The second part of the study investigated isometric strength at optimal length and concentric strength at slow (30 deg's 1) and fast (180 deg's 1) angular velocities of movement. No differences were apparent in the magnitude and rate of recovery of strength across isometric (82.1 ± 2.7%) and slow (86.6 ± 2.0%) and fast (84.3 ± 1.5%) concentric muscle actions. Both the popping sarcomere hypothesis of 2 muscle damage and a failure in excitation-contraction coupling are possible explanations for the reduction in strength being affected by the muscle length at which it is measured. Both would be expected to affect strength to a greater extent at short versus optimal muscle lengths. Study 2 The second study investigated knee extensor muscle strength during isometric, concentric and eccentric muscle actions and vertical jump performance under conditions of squat jump (SJ), countermovement jump (CMJ) and drop jump (DJ). These measures were taken before, 1 hour after, and on days 1,2,3,4 and 7 following a damaging exercise protocol consisting of 100 barbell squats (10 sets x 10 reps @ 70% body mass load). Strength was significantly reduced for four days, however, no differences were observed in the magnitude or rate of recovery of isometric strength at 80° knee flexion and concentric and eccentric strength at 90 deg's'. Vertical jump performance was significantly reduced for three days and was dependent on the type of jump being performed. The relative decline in SJ performance was significantly greater than that in CMJ performance (91.6 ± 1.1% vs. 95.2 ± 1.3% of pre-exercise values, P<0.05) and the relative decline in SJ was significantly greater than that in DJ performance (91.6 ± 1.1% vs. 95.2 ± 1.4%, P<0.05). No differences were observed in the relative decline in CMJ and DJ performance (95.2 ± 1.3% vs. 95.2 ± 1.4%, P> 0.05). The stretch-shortening cycle (SSC) of muscle function is utilised in CMJ and DJ but not in SJ. The SSC has a clear purpose: to allow the final phase (concentric action) to take place with greater force or power output, as compared to the condition where the movement is initiated by a concentric action alone. 3 Utilisation of the SSC in performance seems to attenuate the detrimental performance effects of exercise-induced muscle damage. Study 3 The third and final study investigated the effects of exercise-induced muscle damage on maximal power output and knee extensor fatigability under isometric and dynamic conditions. Under isometric conditions, strength was assessed at 40° and 80° knee flexion and fatigability was assessed by a sustained 60s maximum voluntary contraction (MVC) at each joint angle. For dynamic conditions, maximum power output and fatigue were assessed during a maximal 30s cycle ergometer test. These measures were taken before, 1 hour after, and on days 1,2,3, and 7 following a damaging exercise protocol consisting of 100 eccentric squats (10 sets x 10 reps @ 80% concentric 1 RM). Isometric strength was significantly reduced (P < 0.05) for seven days but no significant differences were observed in the magnitude of strength loss and the pattern of recovery between the two joint angles. Fatigability was quantified as the slope (b) of a linear regression line fitted to the torque and power decay during the 60s MVC and the 30s cycle test, respectively. Prior to muscle damage, subjects were significantly less fatigable (P < 0.05) at 40° (b = -2.39 ± 0.26) versus 80° (b = -5.50 ± 0.72). After muscle damage, subjects became significantly less fatigable at both 40° and 80° with recovery taking three days at 401 and seven days at 80°. Before damaging exercise, a greater rate of fatigue was observed under dynamic (b = -12.75 ± 2.3) versus isometric (80°) conditions (b = -5.50 ± 0.72). Isometric and dynamic fatigue 4 followed a similar temporal pattern after damaging exercise. When the effects of muscle damage on strength at 801 and maximal power output were compared, differences in the extent of performance loss and the time course of recovery were observed. At 1 hour post-exercise, strength was affected to a greater extent (30% reduction) than power (13% reduction) and whereas strength followed a linear recovery pattern, power suffered further decrements at day 1 (18%) and day 2 (16%) before starting to recover. The results indicate that under conditions of voluntary activation muscle becomes weaker but less fatigable under isometric and dynamic conditions following exercise-induced muscle damage. The lower starting torque / power output and the slower rate of decline in torque / power output observed in post-damage fatigue curves may be a phenomenon of selective type II fibre damage. Evidence suggests that type II fibres are selectively damaged during eccentric exercise and therefore post-damage fatigue curves may be missing their contribution to performance. The different recovery patterns observed for isometric and dynamic performance may indicate an inability to maintain central motor drive during complex dynamic tasks when damage is present

Gait Changes During Exhaustive Running

Runners adopt altered gait patterns as they fatigue which may increase energy expenditure and susceptibility to certain overuse injuries. Previous investigations have described changes in muscle performance and kinematic gait variables resulting from running fatigue. The purpose of this investigation was to characterize changes in joint moment patterns that develop as runners fatigue in order to better understand the kinetic bases for kinematic fatigue effects. It was hypothesized that when fatigued, runners would demonstrate increases in stance time, peak stance phase knee flexion angle, peak knee extension moment, peak swing phase hip flexion moment, and hip extension and plantarflexion angular impulse generated during stance. Eight recreational rearfoot-striking runners (height = 170.0 ± 9.8 cm; mass = 69.6 ± 15.1 kg) ran on a force-instrumented treadmill at a velocity 5% slower than that of lactate threshold until volitional exhaustion. Force and motion data were collected each minute and analyzed at six evenly-spaced time points over the run duration. Of the six hypotheses, only stance time changed significantly (p = 0.004), increasing in a linear manner over the course of the run (r2 = 0.946). Of the remaining hypotheses, peak knee flexion (r2 = 0.899) and hip extension angular impulse (r2 = 0.608) followed increasing trends over the course of the run. Individual participants often demonstrated joint moment trends consistent with fatigue effects reported in the literature, despite inconclusive group results. High variability in individual responses underscores the complexity of gait adaptations used by distance runners as they fatigue

Quantification of muscle fatigue in cerebal palsy and its relationship to impairments and function

Three experiments were designed to explore the measurement of muscle fatigue in people with cerebral palsy (CP). The four aims were to 1) develop a feasible and reliable isokinetic protocol to assess muscle fatigue of the knee flexors and extensors in this population, 2) determine if muscle fatigue of the knee flexors and extensors in people with CP differs from subjects without a motor disorder, 3) determine whether muscle fatigue is related to functional measures of activity and participation, and 4) investigate possible contributing factors of muscle fatigue. Results show that muscle fatigue can be reliably assessed through an isokinetic protocol consisting of 35 consecutive knee extension and flexion repetitions at 60 degrees/second by calculation of a fatigue index (FI) and the slope of the decline in peak torque. When compared to a control group of age-matched peers without motor disorder, the knee flexors and extensors in subjects with CP were observed to be less fatigable. Furthermore, muscle fatigue of the knee extensors and flexors in the group with CP was positively correlated with transfers and basic mobility. Muscle fatigue of the knee extensors was also positively correlated with overall global functioning, participation in sports and physical function, and fast walking velocity. Lower Gross Motor Function Classification System Levels (GMFCS) (i.e. less involved subjects) were also associated with higher levels of muscle fatigability. Strength was directly related to muscle fatigability, where weaker subjects had lower levels of fatigue, regardless of muscle. Cocontraction and quadriceps stiffness, on the other hand, were inversely related to muscle fatigability. The strongest predictors of hamstring fatigability were hamstrings strength and quadriceps stiffness, whereas the strongest predictor of quadriceps fatigability was hamstring cocontraction. The presence of spasticity, regardless of muscle group, was associated with lower fatigability compared to control subjects. In summary, the results indicate that the knee flexors and extensors of people with CP are less fatigable than age-matched peers without motor disability. In addition, lower levels of muscle fatigability are associated with lower levels of function and participation. Furthermore, weakness, spasticity, stiffness, and cocontraction are possible contributing factors to the observed fatigue resistance