One-week quercetin intervention modifies motor unit recruitment patterns before and during resistance exercise in older adults: A randomized controlled trial.

We investigated the effects of one-week quercetin ingestion on motor unit (MU) behavior and muscle contractile properties before, during, and after a single session of resistance exercise in older adults. Twenty-four older adults were divided into two groups: those receiving quercetin glycosides (QUE) or placebo (PLA), and they performed a single session of resistance exercise. MU behavior before and during resistance exercise and electrically elicited contraction before and after resistance exercise were measured (Day 1), and the same measurements were conducted again after 7 days of placebo or quercetin glycoside ingestion (Day 8). The MU recruitment threshold (RT) was decreased (p < 0.001, 25.6 ± 10.1 to 23.6 ± 9.5 %MVC) and the exerted force normalized by the MU firing rate (FR) was increased (p = 0.003, 1.13 ± 0.24 to 1.18 ± 0.22 %MVC/pps) from Days 1 to 8, respectively, in QUE but not PLA (p = 0.263, 22.6 ± 11.9 to 21.9 ± 11.6 %MVC; p = 0.713, 1.09 ± 0.20 to 1.10 ± 0.19 %MVC/pps, respectively). On Day 1, a significant correlation between MURT and%change in MUFR from the first to last contractions during the resistance exercise was observed in both groups (QUE: p = 0.009, rs = 0.308; PLA: p < 0.001, rs = 0.403). On Day 8 %change in MUFR was negatively correlated with MURT in QUE (p = 0.044, rs = -0.251), but there was no significant correlation in PLA (p = 0.844). There was no difference in electrically elicited contraction before and after the resistance exercise between QUE and PLA (p < 0.05). These results suggest that one-week quercetin ingestion in older adults lowered MURT and led to greater fatigue in MU with higher RT than with lower RT during resistance training.

Time-of-day effects on motor unit firing and muscle contractile properties in humans.

Intrinsic factors related to neuromuscular function are time-of-day dependent, but diurnal rhythms in neural and muscular components of the human neuromuscular system remain unclear. The present study aimed to investigate the time-of-day effects on neural excitability and muscle contractile properties by assessing the firing properties of tracked motor units and electrically evoked twitch muscle contraction. In 15 young adults (22.9 ± 4.7 yr), neuromuscular function was measured in the morning (10:00), at noon (13:30), in the evening (17:00), and at night (20:30). Four measurements were completed within 24 h. The measurements consisted of maximal voluntary contraction (MVC) strength of knee extension, recording of high-density surface electromyography (HDsEMG) from the vastus lateralis during ramp-up contraction to 50% of MVC, and evoked twitch torque of knee extensors by electrical stimulation. Recorded HDsEMG signals were decomposed to individual motor unit firing behaviors and the same motor units were tracked among the times of day, and recruitment thresholds and firing rates were calculated. The number of detected and tracked motor units was 127. Motor unit firing rates significantly increased from morning to noon, evening, and night (P < 0.01), but there were no significant differences in recruitment thresholds among the times of day (P > 0.05). Also, there were no significant effects of time of day on evoked twitch torque (P > 0.05). Changes in the motor unit firing rate and evoked twitch torque were not significantly correlated (P > 0.05). These findings suggest that neural excitability may be affected by the time of day, but it did not accompany changes in peripheral contractile properties in a diurnal manner.NEW & NOTEWORTHY We investigated the variations of tracked motor unit firing properties and electrically evoked twitch contraction during the day within 24 h. The variation of motor unit firing rate was observed, and tracked motor unit firing rate increased at noon, in the evening, and at night compared with that in the morning. The variation in motor unit firing rate was independent of changes in twitch contraction. Motor unit firing rate may be affected by diurnal rhythms.

Association between physical fitness tests and neuromuscular properties.

PURPOSE: While various fitness tests have been developed to assess physical performances, it is unclear how these tests are affected by differences, such as, in morphological and neural factors. This study was aimed to investigate associations between individual differences in physical fitness tests and neuromuscular properties. METHODS: One hundred and thirty-three young adults participated in various general physical fitness tests and neuromuscular measurements. The appendicular skeletal muscle mass (ASM) was estimated by bioelectrical impedance analysis. Echo intensity (EI) was evaluated from the vastus lateralis. During submaximal knee extension force, high-density surface electromyography of the vastus lateralis was recorded and individual motor unit firings were detected. Y-intercept (i-MU) and slope (s-MU) from the regression line between the recruitment threshold and motor unit firing rate were calculated. RESULTS: Stepwise multiple regression analyses revealed that knee extension strength could be explained (adjusted R2 = 0.712) by ASM (ß = 0.723), i-MU (0.317), EI (- 0.177), and s-MU (0.210). Five-sec stepping could be explained by ASM (adjusted R2 = 0.212). Grip strength, side-stepping, and standing broad jump could be explained by ASM and echo intensity (adjusted R2 = 0.686, 0.354, and 0.627, respectively). Squat jump could be explained by EI (adjusted R2 = 0.640). Counter-movement jump could be explained by EI and s-MU (adjusted R2 = 0.631). On the other hand, i-MU and s-MU could be explained by five-sec stepping and counter-movement jump, respectively, but the coefficients of determination were low (adjusted R2 = 0.100 and 0.045). CONCLUSION: Generally developed physical fitness tests were mainly explained by morphological factors, but were weakly affected by neural factors involved in performance.

Acute effects of caffeine or quercetin ingestion on motor unit firing pattern before and after resistance exercise.

The aim of the present study was to investigate the acute effect of caffeine or quercetin ingestion on motor unit firing patterns and muscle contractile properties before and after resistance exercise. High-density surface electromyography (HDs-EMG) during submaximal contractions and electrically elicited torque in knee extensor muscles were measured before (PRE) and 60 min after (POST1) ingestion of caffeine, quercetin glycosides, or placebo, and after resistance exercise (POST2) in ten young males. The Convolution Kernel Compensation technique was used to identify individual motor units of the vastus lateralis muscle for the recorded HDs-EMG. Ingestion of caffeine or quercetin induced significantly greater decreases in recruitment thresholds (RTs) from PRE to POST1 compared with placebo (placebo: 94.8 ± 9.7%, caffeine: 84.5 ± 16.2%, quercetin: 91.9 ± 36.7%), and there were significant negative correlations between the change in RTs (POST1-PRE) and RT at PRE for caffeine (rs = - 0.448, p < 0.001) and quercetin (rs = - 0.415, p = 0.003), but not placebo (rs = - 0.109, p = 0.440). Significant positive correlations between the change in firing rates (POST2-POST1) and RT at PRE were noted with placebo (rs = 0.380, p = 0.005) and quercetin (rs = 0.382, p = 0.007), but not caffeine (rs = 0.069, p = 0.606). No significant differences were observed in electrically elicited torque among the three conditions. These results suggest that caffeine or quercetin ingestion alters motor unit firing patterns after resistance exercise in different threshold-dependent manners in males.

Can Neuromuscular Electrical Stimulation Enhance the Effect of Sprint Interval Training?

The aim of this study was to determine the effects of subtetanic neuromuscular electrical stimulation combined with voluntary exercise between repeated Wingate tests on sprint exercise performance and blood lactate accumulation during sprint interval training. Fifteen healthy young males volunteered. After 1-min baseline, participants underwent the Wingate test twice. They performed a 4-min intervention between tests: neuromuscular electrical stimulation with free-weight cycling or voluntary cycling alone [43.6 (8.0) watts], which matched oxygen consumption with neuromuscular electrical stimulation with free-weight cycling. The blood lactate concentration was assessed at the end of the baseline, at 3-min intervention, and on recovery at 1, 3, 5, and 10 min after the second Wingate test. Peak and mean blood lactate concentration during recovery were significantly greater with neuromuscular electrical stimulation with free-weight cycling than voluntary cycling alone (P>0.036 and P=0.011, respectively). Peak power, mean power, and rate of decline (fatigue index) were not significantly different between conditions in both Wingate tests (condition/interaction all P>0.300, partial η2<0.1). Subtetanic neuromuscular electrical stimulation combined with voluntary exercise indicated similar exercise performance and fatigue levels during Wingate tests, but enhanced blood lactate accumulation compared to oxygen consumption-matched voluntary cycling during sprint interval training.

Motor Unit Firing Properties During Force Control Task and Associations With Neurological Tests in Children.

The present study aimed to clarify the development of motor unit (MU) firing properties and the association between those neural properties and force steadiness (FS)/neurological tests in 6- to 12-year-old children. Fifty-eight school-aged children performed maximal voluntary knee extension contraction, a submaximal FS test at 10% of maximal voluntary knee extension contraction, knee extension reaction time to light stimulus test, and single-leg standing test, and data from 38 children who passed the criteria were subject to analysis. During the FS test, high-density surface electromyography was recorded from the vastus lateralis muscle to identify individual MU firing activity. FS was improved with an increase in age (r = -.540, P < .001). The MU firing rate (MUFR) was significantly decreased with an increase in age (r = -.343, P = .035). MUFR variability was not associated with age. Although there was no significant correlation between FS and MUFR, FS was significantly correlated with MUFR variability even after adjustment for the effect of age (r = .551, P = .002). Neither the reaction time nor the single-leg standing test was correlated with any MU firing properties. These findings suggest that MUFR variability makes an important contribution to precise force control in children but does not naturally develop with age.

Subtetanic neuromuscular electrical stimulation can maintain Wingate test performance but augment blood lactate accumulation.

PURPOSE: Concentration- and time-dependent effect of lactate on physiological adaptation (i.e., glycolytic adaptation and mitochondrial biogenesis) have been reported. Subtetanic neuromuscular electrical stimulation (NMES) with voluntary exercise (VOLES) can increase blood lactate accumulation. However, whether this is also true that VOLES can enhance the blood lactate accumulation during sprint exercise is unknown. Thus, we investigated whether VOLES before the Wingate test can enhance blood lactate accumulation without compromising Wingate exercise performance. METHODS: Fifteen healthy young males (mean [SD], age: 23 [4] years, body mass index: 22.0 [2.1] kg/m2) volunteered. After resting measurement, participants performed a 3-min intervention: VOLES (NMES with free-weight cycling) or voluntary cycling alone, which matched exercise intensity with VOLES (VOL, 43.6 [8.0] watt). Then, they performed the Wingate test with 30 min free-weight cycling recovery. The blood lactate concentration ([La]b) was assessed at the end of resting and intervention, and recovery at 1, 3, 5, 10, 20, and 30 min. RESULTS: [La]b during intervention was higher with VOLES than VOL (P = 0.011). The increase in [La]b after the Wingate test was maintained for longer with VOLES than VOL at 10- and 20-min recovery (P = 0.014 and 0.023, respectively). Based on the Wingate test, peak power, mean power, and the rate of decline were not significantly different between VOLES and VOL (P = 0.184, 0.201, and 0.483, respectively). CONCLUSION: The combination of subtetanic NMES with voluntary exercise before the Wingate test has the potential to enhance blood lactate accumulation. Importantly, this combined approach does not compromise Wingate exercise performance compared to voluntary exercise alone.

Effects of 7-day quercetin intervention on motor unit activity and muscle contractile properties before and after resistance exercise in young adults randomized controlled trials.

We investigated whether the alteration of the motor unit recruitment threshold (MURT) caused by quercetin ingestion intervention for 7 days modifies motor unit activation patterns before and after a single session of resistance exercise. Twenty young male and female adults were divided into two groups: ingestion of placebo (PLA) or quercetin glycosides at 200 mg/day (QUE). High-density surface electromyography during submaximal contractions was measured to assess the motor unit firing rate (MUFR) and MURT of the vastus lateralis muscle before (PRE) and after (POST) resistance exercise (DAY1). The same measurements were repeated after 7 days of placebo or quercetin glycoside ingestion (DAY8). In QUE, MURT decreased more from DAY1-PRE to DAY8-PRE (29.1 ± 9.1 to 27.1 ± 9.5% MVC, p < 0.001) but not in PLA (29.8 ± 10.4 to 28.9 ± 9.7% MVC, p < 0.167). For percentage change in MUFR following resistance exercise, there was a significant interaction (day × group, p < 0.001). The degree of changes in MURT from DAY1-PRE to DAY8-PRE was significantly correlated with the percentage change of MUFR from DAY8-PRE to DAY8-POST in QUE (p = 0.014, r = -0.363) but not in PLA (p = 0.518). The study suggests that 7-day quercetin ingestion alters the motor unit recruitment pattern, and this may induce changes in motor unit firing patterns during a single session of resistance training (Trial registration: UMIN000052255, R000059650).

Impact of subtetanic neuromuscular electrical stimulation on cardiac autonomic nervous system in young individuals.

BACKGROUND: Although subtetanic neuromuscular electrical stimulation (NMES) has been proposed as an exercise training and/or rehabilitation tool, the impact of NMES on the autonomic nervous system (ANS) is unclear. Thus, we hypothesized that NMES would alter ANS, i.e., increase sympathetic activity and decrease parasympathetic activity, in young individuals. METHODS: Eighteen healthy young individuals (16 males, mean age: 22 [SD: 4] years, Body Mass Index: 21.7 [2.2] kg/m2) volunteered. Blood pressure (BP), heart rate (HR), and R-R intervals were recorded during 6-minute resting, NMES, and recovery conditions. Short-term heart rate variability analysis of R-R intervals was performed for the frequency and time domains during each condition. Time domain indices included the root mean square of successive R-R interval differences (RMSSD), and the percentage of successive R-R intervals differing by more than 50ms (pRR50%). Frequency domain indices (fast Fourier transform) of R-R intervals included total power (TP), low-frequency (LF) power (0.04-0.15 Hz), and high-frequency (HF) power (0.15-0.4 Hz). RESULTS: BP was not altered but HR was significantly increased during NMES (P<0.001), and it returned to the resting level at recovery. RMSSD and pRR50 decreased from resting to NMES and returned at recovery conditions (P<0.05, respectively). TP and HF decreased from resting to NMES and returned at recovery conditions (P<0.05, respectively). LF increased from NMES to recovery (P<0.05). The LF/HF ratio showed no significant differences between conditions (P=0.210). CONCLUSIONS: Cardiac ANS fluctuated by subtetanic NMES without BP elevation in healthy young individuals. Parasympathetic but not sympathetic activity was affected by NMES stimulation.

Differences in acute neuromuscular response after single session of resistance exercise between young and older adults.

AIMS: The purpose of this study was to investigate differences in the acute response after resistance exercise between young and older adults. METHODS: Seventeen young and 18 older adults performed a single session of resistance exercise, consisting of 3 sets of 10 isometric knee extensions. Maximal voluntary contraction (MVC), motor unit (MU) activity of the vastus lateralis, and electrically elicited torque of the knee extensor were measured before and after the resistance exercise. RESULTS: Although both groups showed the same degree of decline in MVC (young: -15.2 ± 14.3 %, older: -16.4 ± 7.9 %, p = 0.839), electrically elicited torque markedly decreased in the young group (young: -21.5 ± 7.7 %, older: -14.3 ± 9.5 %, p < 0.001), and the decrease in the MU firing rate was greater in the older group (young: -26.1 ± 24.1 %, older: -44.7 ± 24.5 %, p < 0.001). Changes in the MU firing rate following the exercise were correlated with the MU recruitment threshold in the older group (p < 0.001, rs = 0.457), but not young group (p = 0.960). DISCUSSION: These results showed that young adults exhibited a greater acute response in the peripheral component, whereas older adults showed a greater acute response in the central component of the neuromuscular system, and the acute response in MUs with a high recruitment threshold following resistance exercise was smaller than in those with a low recruitment threshold in older adults. These findings may partly explain why there are different chronic adaptations to resistance training between young and older adults.

Longitudinal development of muscle strength and relationship with motor unit activity and muscle morphological characteristics in youth athletes.

Neural and morphological adaptations determine gains of muscle strength. For youth athletes, the importance of morphological adaptation is typically highlighted based on the change in maturity status. However, the long-term development of neural components in youth athletes remains unclear. The present study investigated the longitudinal development of muscle strength, muscle thickness (MT), and motor unit firing activity of the knee extensor and their relationships in youth athletes. Seventy male youth soccer players (mean ± SD age = 16.3 ± 0.6 years) performed neuromuscular, maximal voluntary isometric contraction (MVC), and submaximal ramp contraction (at 30 and 50% MVC) tests with knee extensors, two times with a 10-month measurement interval. High-density surface electromyography was recorded from the vastus lateralis and decomposed to identify each individual motor unit activity. MT was evaluated by the sum of the vastus lateralis and vastus intermedius thicknesses. Finally, sixty-four participants were employed to compare MVC and MT, and 26 participants were employed to analyze motor unit activity. MVC and MT were increased from pre to post (p < 0.05, 6.9 and 1.7% for MVC and MT, respectively). Y-intercept of the regression line between median firing rate vs. recruitment threshold was also increased (p < 0.05, 13.3%). Multiple regression analysis demonstrated that the gains of both MT and Y-intercept were explanatory variables for the gain of strength. These findings suggest that the neural adaptation could also make the important contribution to the strength gain for the youth athletes over a 10-month training period.

Impact of muscle echo intensity on post-exercise blood pressure response in older normotensive and hypertensive females: Pilot study.

Exaggerated post-exercise blood pressure (BP) is considered a risk factor for the development of cardiovascular disease in older females. Muscle echo intensity (EI) using ultrasound can be used to evaluate intramuscular fat, one of the risk factors for cardiovascular disease. This study aimed to determine whether intramuscular fat assessed by muscle echo intensity is associated with the post-exercise BP response in older females. Ten older normotensive (SBP <130 mmHg, 71 ± 4 years), eight systolic BP-controlled (78 ± 4 years), and 17 hypertensive (SBP ≥130 mmHg, 74 ± 6 years) females were studied. After obtaining ultrasound images to assess the EI, participants performed ramp-up exercise until 50% maximal voluntary contraction (MVC: ~30-s; 3% MVC/s gradually increased knee extension force from 0% to 50% MVC followed by sustaining the force at 50% MVC for 10-s) and then five MVCs (~50 s; 10-s rest between each contraction). BP was measured before and immediately after exercise. Mean arterial pressure (MAP) pre- and post-exercise were significantly lower in normotensive and SBP-controlled, than in -uncontrolled hypertensive females (PRE: 85 ± 5 and 87 ± 7 vs. 106 ± 9; POST: 92 ± 8 and 94 ± 9 vs. 103 ± 11 mmHg, respectively, p < 0.05). EI was negatively correlated with ∆diastolic BP (∆DBP) but not ∆SBP and ∆MAP in normotensive females only (∆SBP, r = -0.21, p = 0.56; ∆DBP, R = -0.73, p = 0.02; ∆MAP, R = -0.49, p = 0.15). Greater intramuscular fat as indicated by higher EI is associated with less BP elevation immediately after exercise in older normotensive females. Greater intramuscular fat may lead to lower intramuscular pressure, resulting in less post-exercise BP elevation.

Elite Rugby Players have Unique Morphological Characteristics of the Hamstrings and Quadriceps Femoris Muscles According to their Playing Positions.

Rugby is a popular sport requiring high-intensity and maximal speed actions. Numerous studies have demonstrated that physical performance variables, such as strength, sprinting, and jumping, are different between the forwards and backs. However, there is little information about muscle morphological characteristics specific for each rugby playing position. This study aimed to clarify the morphological characteristics of the thigh muscles in forwards and backs. Ultrasound images were obtained from the proximal, middle, and distal regions of the thigh. Then, the anatomical cross-sectional areas of particular muscles in the hamstrings and quadriceps femoris were calculated for seven forwards, seven backs, and ten non-athletes. The anatomical cross-sectional areas were normalised by the two-third power of lean body mass, and the normalised values of the three regions were averaged as that of the individual muscle. In the hamstrings, the normalised anatomical cross-sectional areas of the biceps femoris long head were significantly greater in forwards than in non-athletes, whereas those of the semitendinosus were significantly greater in backs than in non-athletes. Furthermore, in the quadriceps femoris, the normalised anatomical cross-sectional areas of the rectus femoris and vastus intermedius were significantly greater in forwards than in backs and non-athletes. These results suggest that forwards have great muscularity of the biceps femoris long head and vastus intermedius which can generate large force, whereas backs possess great muscularity of the semitendinosus which can generate high contraction velocity. These findings allow coaches to design more effective training programs according to particular rugby playing positions.

Sub-elite sprinters and rugby players possess different morphological characteristics of the individual hamstrings and quadriceps muscles.

Numerous studies have clarified that sprinters possess unique morphological characteristics of the thigh muscles compared with non-athletes. However, little evidence is available regarding the morphological differences between sprinters and rugby players. This study aimed to examine the morphological differences in the individual hamstrings and quadriceps femoris muscles between sub-elite sprinters and rugby players. Ultrasound images were acquired from the proximal, middle, and distal regions of the thigh. From the images, the anatomical cross-sectional areas were calculated for 14 sub-elite sprinters, 14 rugby players, and 14 non-athletes. The calculated anatomical cross-sectional areas were normalized to two-thirds power of the body mass, and the normalized values of all regions were averaged as those of the individual muscles. In the hamstrings, the sizes of the biceps femoris short head and semitendinosus were greater in the sprinters than in the rugby players and/or non-athletes (all p < 0.05). In contrast, in the quadriceps femoris, the sizes of the rectus femoris, vastus lateralis, and vastus intermedius were the greatest in the rugby players (all p < 0.05). In the middle region of the biceps femoris short head and the proximal-middle regions of the semitendinosus, the muscle sizes were greater in the sprinters than in the rugby players (all p < 0.05), and vice versa in the middle-distal regions of the rectus femoris (all p < 0.05). These results suggest that 1) sub-elite sprinters possess larger sizes of the biceps femoris short head and semitendinosus, whereas rugby players have larger sizes of the rectus femoris, vastus lateralis, and vastus intermedius, and 2) each of the athletes has different size distributions, especially along the lengths of BFsh, ST, and RF. The findings of the present study would be helpful for rugby players in designing training regimens aimed at enhancing sprint performance.

Muscle- and Region-Specific Associations Between Muscle Size and Muscular Strength During Hip Extension and Knee Flexion in the Hamstrings.

CONTEXT: Strength deficits of the hamstrings following sports injuries decrease athletic performance and increase the risk of injury recurrence. Previous studies have shown a high correlation between the muscular strength during hip-extension and knee-flexion and total muscle size of the hamstrings. However, it remains unclear which region of the individual hamstring muscles is closely associated with muscular strength. OBJECTIVE: To investigate the relationship between the size of each region of the individual hamstring muscles and muscular strength during hip extension and knee flexion. DESIGN: Within-subject repeated measures. SETTING: University laboratory. PARTICIPANTS: Twenty healthy young male volunteers who regularly engaged in sports activities. OUTCOME MEASURES: Anatomical cross-sectional areas were acquired from the proximal, middle, and distal regions of the biceps femoris long head, biceps femoris short head, semitendinosus, and semimembranosus. Hip-extension and knee-flexion strength were measured during maximal voluntary isometric and concentric contractions (angular velocities of 60°/s and 180°/s). RESULTS: The anatomical cross-sectional area of the distal regions in biceps femoris long head (r = .525-.642) and semitendinosus (r = .567) were significantly correlated with hip-extension strength under all conditions and only at an angular velocity of 180°/s, respectively. Meanwhile, anatomical cross-sectional areas of the distal regions in biceps femoris short head (r = .587-.684) and semimembranosus (r = .569-.576) were closely associated with knee-flexion strength under all conditions. CONCLUSION: These results suggest that muscle size in the distal regions of biceps femoris long head and semitendinosus greatly contributes to the production of hip-extension strength, whereas that of biceps femoris short head and semimembranosus significantly contributes to the generation of knee-flexion strength. These findings could be useful for designing training and rehabilitation programs to efficiently improve strength deficits following sports injuries such as strain injury and anterior cruciate ligament tears.

Sprinting kinematics and inter-limb coordination patterns at varying slope inclinations.

Uphill training is applied to induce specific overload on the musculoskeletal system to improve sprinting mechanics. This study aimed to identify unique kinematic features of uphill sprinting at different slopes and to suggest practical implications based on comparisons we early stance phase. At take-off, steeper slopes induced significantly more extended joint angles and higher ROMs during the late stance phase. Compared with moderate slopes, more anti-phase coordination patterns were detected at steeper slopes. Thus, uphill sprinting at steeper slopes shares essential kinematic features with the early acceleration phase of level sprinting. Moderate inclinations induce biomechanical adaptations similar to those in the late acceleration phase of level sprinting. Hence, the specific transfer of uphill sprinting to acceleration depends on the slope inclinations.

Effect of Knee Joint Angle on Regional Hamstrings Activation During Isometric Knee-Flexion Exercise.

CONTEXT: Each hamstring muscle is subdivided into several regions by multiple motor nerve branches, which implies each region has different muscle activation properties. However, little is known about the muscle activation of each region with a change in the knee joint angle. Understanding of regional activation of the hamstrings could be helpful for designing rehabilitation and training programs targeted at strengthening a specific region. OBJECTIVE: To investigate the effect of knee joint angle on the activity level of several regions within the individual hamstring muscles during isometric knee-flexion exercise with maximal effort (MVCKF). DESIGN: Within-subjects repeated measures. SETTING: University laboratory. PARTICIPANTS: Sixteen young males with previous participation in sports competition and resistance training experience. INTERVENTION: The participants performed 2 MVCKF trials at each knee joint angle of 30°, 60°, and 90°. OUTCOME MEASURES: Surface electromyography was used to measure muscle activity in the proximal, middle, and distal regions of the biceps femoris long head (BFlh), semitendinosus, and semimembranosus of hamstrings at 30°, 60°, and 90° of knee flexion during MVCKF. RESULTS: Muscle activity levels in the proximal and middle regions of the BFlh were higher at 30° and 60° of knee flexion than at 90° during MVCKF (all: P < .05). Meanwhile, the activity levels in the distal region of the BFlh were not different among all of the evaluated knee joint angles. In semitendinosus and semimembranosus, the activity levels were higher at 30° and 60° than at 90°, regardless of region (all: P < .05). CONCLUSION: These findings suggest that the effect of knee joint angle on muscle activity level differs between regions of the BFlh, whereas that is similar among regions of semitendinosus and semimembranosus during MVCKF.

Three-dimensional CoM energetics, pelvis and lower limbs joint kinematics of uphill treadmill running at high speed.

The purpose of this study was to investigate the effects of slope on three-dimensional running kinematics at high speed. Thirteen male sprinters ran at high speed (7.5 m/s) on a motorised treadmill in each a level and a 5.0% slope condition. Three-dimensional motion analysis was conducted to compare centre of mass (CoM) energetics, pelvis segment and lower limb joints kinematics. We found that contact time was not affected by the slope, whereas flight time and step length were significantly shorter in uphill compared to level running. Uphill running reduced negative CoM work and increased positive CoM work compared to level running. Ankle, knee and hip joints were more flexed at initial ground contact, but only the knee was more extended at the end of stance in uphill compared to level running. Additionally, the hip joint was more abducted, and the free leg side of the pelvis was more elevated at the end of stance in uphill running. Our results demonstrate that joint motion must be developed from a more flexed/adducted position at initial contact through a greater range of motion compared to level running in order to meet the greater positive CoM work requirements in uphill running at high speed.