Maximal Voluntary Contraction Values Research Articles

Reliability of repeated isometric neck strength and endurance testing in a simulated contact posture.

The purpose of this study was to examine the reliability of repeated isometric measurements of neck strength and endurance in a simulated rugby contact posture. Data were collected from healthy active university students (n = 20) over 3 sessions. Each session consisted of a single maximal voluntary contraction (MVC) and endurance trial for extension and flexion. The endurance trials were analyzed to determine the area under the force curve (%AUC) and the time to fatigue (TTF). Reliability was calculated using the SEM and minimal detectable change (MDC). The results revealed that the MVC values for extension were nonsignificant for session (p = 0.21), whereas some session differences were seen for flexion (p = 0.01), where session 1, 115.3 N was less than session 3, 126.4 N (p = 0.03). For %AUC, flexion values (%AUC 86.0) were greater than extension (%AUC 59.4) (p = 0.02). A similar effect was observed for TTF with flexion (125.0 seconds) having significantly greater TTF than extension (86.1 seconds). For peak force, the greatest SEM and MDC were achieved for extension (15.3 and 42.3 N), while flexion produced lower values (11.3 and 31.2 N). In contrast, for the endurance trials, SEM and MDC were greater in flexion than in extension. The findings from this study indicate that the neck musculature can be reliably assessed in a simulated contact posture. For peak force, extension provided more consistent measures across time as there seems to be a learning effect for flexion. Additionally, the SEM and MDC scores provide a useful tool for future assessment of neck strength and endurance.

Effect of PNF stretching training on the properties of human muscle and tendon structures

The purpose of this study was to investigate the influence of a 6-week proprioceptive neuromuscular facilitation (PNF) stretching training program on the various parameters of the human gastrocnemius medialis muscle and the Achilles tendon. Therefore, 49 volunteers were randomly assigned into PNF stretching and control groups. Before and after the stretching intervention, we determined the maximum dorsiflexion range of motion (RoM) with the corresponding fascicle length and pennation angle. Passive resistive torque (PRT) and maximum voluntary contraction (MVC) of the musculo-articular complex were measured with a dynamometer. Muscle-tendon junction (MTJ) displacement allowed us to determine the length changes in tendon and muscle, and hence to calculate stiffness. Mean RoM increased from 31.1 ± 7.2° to 33.1 ± 7.2° (P = 0.02), stiffness of the tendon decreased significantly in both active (from 21.1 ± 8.0 to 18.1 ± 5.5 N/mm) and passive (from 12.1 ± 4.9 to 9.6 ± 3.2 N/mm) conditions, and the pennation angle increased from 18.5 ± 1.8° to 19.5 ± 2.1° (P = 0.01) at the neutral ankle position (90°), only in the intervention group, whereas MVC and PRT values remained unchanged. We conclude that a 6-week PNF stretching training program increases RoM and decreases tendon stiffness, despite no change in PRT.

FLS tasks can be used as an ergonomic discriminator between laparoscopic and robotic surgery

Robotic surgery may result in ergonomic benefits to surgeons. In this pilot study, we utilize surface electromyography (sEMG) to describe a method for identifying ergonomic differences between laparoscopic and robotic platforms using validated Fundamentals of Laparoscopic Surgery (FLS) tasks. We hypothesize that FLS task performance on laparoscopic and robotic surgical platforms will produce significant differences in mean muscle activation, as quantified by sEMG. Six right-hand-dominant subjects with varying experience performed FLS peg transfer (PT), pattern cutting (PC), and intracorporeal suturing (IS) tasks on laparoscopic and robotic platforms. sEMG measurements were obtained from each subject's bilateral bicep, tricep, deltoid, and trapezius muscles. EMG measurements were normalized to the maximum voluntary contraction (MVC) of each muscle of each subject. Subjects repeated each task three times per platform, and mean values used for pooled analysis. Average normalized muscle activation (%MVC) was calculated for each muscle group in all subjects for each FLS task. We compared mean %MVC values with paired t tests and considered differences with a p value less than 0.05 to be statistically significant. Mean activation of right bicep (2.7 %MVC lap, 1.3 %MVC robotic, p = 0.019) and right deltoid muscles (2.4 %MVC lap, 1.0 %MVC robotic, p = 0.019) were significantly elevated during the laparoscopic compared to the robotic IS task. The mean activation of the right trapezius muscle was significantly elevated during robotic compared to the laparoscopic PT (1.6 %MVC lap, 3.5 %MVC robotic, p = 0.040) and PC (1.3 %MVC lap, 3.6 %MVC robotic, p = 0.0018) tasks. FLS tasks are validated, readily available instruments that are feasible for use in demonstrating ergonomic differences between surgical platforms. In this study, we used FLS tasks to compare mean muscle activation of four muscle groups during laparoscopic and robotic task performance. FLS tasks can serve as the basis for larger studies to further describe ergonomic differences between laparoscopic and robotic surgery.

Are Repeated Single-Limb Heel Raises and Manual Muscle Testing Associated With Peak Plantar-Flexor Force in People With Inclusion Body Myositis?

Repeated heel raises have been proposed as a method of ankle plantar-flexor strength testing that circumvents the limitations of manual muscle testing (MMT). The study objective was to examine the relationships among ankle plantar-flexion isometric maximum voluntary contraction (MVC), repeated single-limb heel raises (SLHRs), and MMT in people with myositis. This was a cross-sectional study with a between-group design. The ability to complete 1 SLHR determined group assignment (SLHR group, n=24; no-SLHR group, n=19). Forty-three participants with myositis (13 women; median age=64.9 years) participated. Outcome measures included MVC, predicted MVC, Kendall MMT, and Daniels-Worthingham MMT. The Kendall MMT was unable to detect significant ankle plantar-flexor weakness established by quantitative methods and was unable to discriminate between participants who could and those who could not perform the SLHR task. Ankle plantar-flexion MVC was not associated with the number of heel-raise repetitions in the SLHR group (pseudo R(2)=.13). No significant relationship was observed between MVC values and MMT grades in the SLHR and no-SLHR groups. However, a moderate relationship between MVC values and MMT grades was evident in a combined-group analysis (ρ=.50-.67). The lower half of both MMT grading scales was not represented in the study despite the profound weakness of the participants. Both Kendall MMT and Daniels-Worthingham MMT had limited utility in the assessment of ankle plantar-flexor strength. Repeated SLHRs should not be used as a proxy measure of ankle plantar-flexion MVC in people with myositis.

Neuromuscular changes following simulated high-intensity cycling performance in moderate hypoxia

The purpose of this study was to determine whether central activation (CA) is reduced following a simulated 20-km cycling time trial (20TT) under normoxic and hypoxic conditions. It was hypothesized that CA, maximal voluntary contraction (MVC), and peripheral variables would become reduced during the 20TT exercise under both normoxic and hypoxic conditions, but to a greater extent under the hypoxic condition. Eight experienced male cyclists performed two simulated 20TTs in random order in a hypoxic chamber at either 15% or 21% fraction of inspired oxygen (FIO2). Using the interpolated twitch technique during MVC of the quadriceps, measurements were collected before the exercise, and at 1, 2, 3, and 4 minutes postexercise. The CA values at 1, 2, 3, and 4 minutes postexercise were all significantly reduced from the pre-exercise value. Significant decreases were also detected in all four postexercise MVC measurements and in the third and fourth peak twitch force (PTF) measurements. All four postexercise MVC measurements were significantly decreased. These findings suggest that CA, MVC, and PTF values were significantly reduced and remain reduced at 4 minutes following a self-paced, simulated endurance cycling performance. However, the hypoxic condition had no effect on CA, MVC, or peripheral variables when compared with the normoxic condition.

Trait emotional intelligence, the Big Five personality traits and isometric maximal voluntary contraction level under stress in athletes

ObjectivesThe purpose of the study was to investigate the effects of trait emotional intelligence (EI) and the Big Five personality traits on athletes’ ability to tolerate stress during isometric maximal voluntary contraction (MVC). MethodsFifty-two elite athletes with a mean age of 21.5 year (17 female) volunteered to participate. The Short Form Big Five Inventory and the Schutte Self-Report Emotional Intelligence Test measured personality and trait EI, respectively. Electromyography (EMG) determined the MVC level of the triceps surae muscle group in the absence and presence of a stressor. The mental stressor was to count downwards, repeatedly subtracting 13 from a starting value of 1000. ResultsAthletes’ MVC values were positively associated with conscientiousness and trait EI regardless of the presence or absence of a mental stressor. Two hierarchical regressions were conducted to determine whether personality traits and trait EI could predict MVC values measured during the absence and presence of mental stressor. Results show that personality traits explain variability in MVC values measured in both the absence and the presence of a mental stressor. Further, trait EI increased the model's ability to predict MVC. Personality traits combined with trait EI had stronger predictive ability for MVC values during exposure to a mental stressor. ConclusionAthletes with higher levels of conscientiousness and trait EI may be able to tolerate more stress without a decrease in performance.

Influence of Squatting Depth on Jumping Performance

It is unclear if increases in 1 repetition maximum (1RM) in quarter squats result in higher gains compared with full depth squats in isometric force production and vertical jump performance. The aim of the research projects was to compare the effects of different squat variants on the development of 1RM and their transfer effects to Countermovement jump (CMJ) and squat jump (SJ) height, maximal voluntary contraction (MVC), and maximal rate of force development (MRFD). Twenty-three women and 36 men (mean age: 24.11 ± 2.88 years) were parallelized into 3 groups based on their CMJ height: deep front squats (FSQ, n = 20), deep back squats (BSQ, n = 20), and quarter back squats (BSQ¼, n = 19). In addition, a control group (C, n = 16) existed (mean age: 24.38 ± 0.50 years). Experimental groups trained 2 d·wk for 10 weeks with a strength-power block periodization, which produced significant (p ≤ 0.05) gains of the specific squat 1RM. The FSQ and BSQ attained significant (p ≤ 0.05) elevations in SJ and CMJ without any interaction effects between both groups (p ≥ 0.05). The BSQ¼ and C did not reveal any significant changes of SJ and CMJ. The FSQ and BSQ had significantly higher SJ scores over C (p ≤ 0.05). The BSQ did not feature any significant group difference to BSQ¼ (p = 0.116) in SJ, whereas FSQ showed a trend toward higher SJ heights over BSQ¼ (p = 0.052). The FSQ and BSQ presented significantly (p ≤ 0.05) higher CMJ heights over BSQ¼ and C. Posttest in MVC and MRFD demonstrated no significant changes for BSQ. Significant declines in MRFD for FSQ in the right leg (p ≤ 0.05) without any interaction effects for MVC and MRFD between both FSQ and BSQ were found. Training of BSQ¼ resulted in significantly (p ≤ 0.05) lower MRFD and MVC values in contrast to FSQ and BSQ. Quarter squat training elicited significant (p ≤ 0.05) transfer losses into the isometric maximal and explosive strength behavior. These findings therefore contest the concept of superior angle-specific transfer effects. Deep front and back squats guarantee performance-enhancing transfer effects of dynamic maximal strength to dynamic speed-strength capacity of hip and knee extensors compared with quarter squats.

Effect of Lead Use on Back and Shoulder Postural Muscle Activity in Healthy Young Adults

The primary goal of this study was to test the hypothesis that wearing the 3.7 kg vest portion of a radiological shielding garment (a "lead") significantly increases lower back and shoulder muscle activity in quasistatic erect and forward-flexed postures. Secondarily, the authors examined the effects of gender and forward-flexed posture as well as their interactions with lead use. The use of a lead is mandatory for interventionalists during surgical procedures. Because the vest portion of a lead weighs considerably more than normal clothing, there is concern that its use increases the risk of developing back and shoulder pain. In a repeated-measures study design, 19 young healthy male and female adults assumed standardized erect or forward-flexed postures, both with and without wearing the vest portion of a lead. Shoulder and lower back muscle activity was measured via surface electromyography, normalized by maximum voluntary contraction values. Data were analyzed using general linear models and repeated-measures ANOVA (significant for p < .05). Use of the lead did not result in a significant increase in muscle activity in the lower back or shoulders, despite perceived increases in effort and discomfort. Posture proved to be the most significant secondary factor affecting activity in the lower back, and participant gender proved insignificant. Short-term use of the lead does not appear to contribute to the incidence of back pain or injury in interventionalists. Avoiding flexed postures could more directly reduce the likelihood of pain or injury. Potential applications include assessing and improving operating room ergonomics for physicians.

Loss of capacity to recover from acidosis on repeat exercise in chronic fatigue syndrome: a case–control study

Chronic fatigue syndrome (CFS) patients frequently describe difficulties with repeat exercise. Here, we explore muscle bioenergetic function in response to three bouts of exercise. A total of 18 CFS (CDC 1994) patients and 12 sedentary controls underwent assessment of maximal voluntary contraction (MVC), repeat exercise with magnetic resonance spectroscopy and cardio-respiratory fitness test to determine anaerobic threshold. Chronic fatigue syndrome patients undertaking MVC fell into two distinct groups: 8 (45%) showed normal PCr depletion in response to exercise at 35% of MVC (PCr depletion >33%; lower 95% CI for controls); 10 CFS patients had low PCr depletion (generating abnormally low MVC values). The CFS whole group exhibited significantly reduced anaerobic threshold, heart rate, VO(2) , VO(2) peak and peak work compared to controls. Resting muscle pH was similar in controls and both CFS patient groups. However, the CFS group achieving normal PCr depletion values showed increased intramuscular acidosis compared to controls after similar work after each of the three exercise periods with no apparent reduction in acidosis with repeat exercise of the type reported in normal subjects. This CFS group also exhibited significant prolongation (almost 4-fold) of the time taken for pH to recover to baseline. When exercising to comparable levels to normal controls, CFS patients exhibit profound abnormality in bioenergetic function and response to it. Although exercise intervention is the logical treatment for patients showing acidosis, any trial must exclude subjects who do not initiate exercise as they will not benefit. This potentially explains previous mixed results in CFS exercise trials.

Monitoring hand flexor fatigue in a 24-h motorcycle endurance race

Motorcycle riders must endure high levels of muscle tension for long periods of time, especially in their arms and forearms, when steering and using handlebar controls. Because the right hand operates the gas handle and front brakes, the present research focuses on fatigue in the right hand flexors. Ten adult riders, aged 32.5 ± 5.5 years, volunteered to participate in this study. During the 24 h race each rider, on completion of a relay stage, visited the assessment box to do the following handgrip test sequence: (1) 10 s of EMG recording at rest, (2) one 3-s maximal voluntary contraction (MVC), (3) 1 min rest interval and (4) 50% MVC maintained during 10 s. EMG amplitude (MP: μV) and median and mean frequency (MF and MPF: Hz) over the superficial finger flexors were recorded during the whole handgrip test sequence with adhesive surface electrodes. MVC values were maintained during the first two relays (50–60 min duration in total) and dropped gradually thereafter ( p < 0.01). During the monitoring of the 50% MVC, mean amplitude increased ( p = 0.024) while median and mean frequency tended to decrease. These results suggest fatigue is produced in motorcycle riders in a 24 h race. However, the expected reduction of EMG frequency was not confirmed given to a potentially large variability.

Influence of Sportswear Shape on Dressing Motion and Muscle Activity in Dummy Elderly Persons

The long-term goal of this study is to design sportswear for the elderly. Previously, we reported dressing motion and muscle activity on young and dummy elderly persons while they are putting on four different styles of sportswear: long-sleeved, sleeveless, zippered, and zipper-less. In the part of the study reported in the present paper, we photographed their movements while they were putting on the four styles of sportswear and measured the wearing time, maximum working area, and muscle activity. Further, we investigated how the sportswear style influenced the psychological burden. From motion analysis, we found that, when the dummy elderly persons pass an arm through the sportswear at process (2), the wearing times for sleeveless sportswear were lower than those for long-sleeved sportswear. In addition, the maximum working area and %MVC (percentage maximum voluntary contraction) values for zippered sportswear were higher than those for zipper -less sportswear. When wearing the sportswear at process (3), the maximum working area and %MVC values for zipper-less sportswear were higher than those for zippered sportswear. From these results, it was concluded that the sportswear style influences wearing time, maximum working area, and muscle activity.

Effects of rituximab in two patients with dysferlin-deficient muscular dystrophy

BackgroundThe administration of rituximab (RTX) in vivo results in B-cell depletion, but evidence for multiple mechanisms of action have been reported. Surprisingly, B cell depletion produced a response in patients with polymyositis, which is characterized as a T cell-mediated autoimmune disorder with biopsy findings similar to Miyoshi myopathy (MM). Indeed, in dysferlinopathies, there is evidence of immune system involvement including the presence of muscle inflammation and a down regulation of the complement inhibitory factor, CD55.MethodsTwo patients were treated with four weekly infusions of RTX 375 mg/m2. To measure the improvement in muscle strength after treatment, the isometric hand grip maximal voluntary contraction (MVC) was measured by load cell four times during treatment, and again after one year. In order to assess the reproducibility of our grip assessment, we determined the hand MVC analysis in 16 healthy subjects. Moreover, we measured the number of B cells present in patients by flow cytometric analysis during the course of treatment.ResultsThe analysis of B cell number during the course of treatment showed that CD20- and CD19-positive cells were depleted to 0-0.01%. The decrease in B cells was followed by an improvement in the mobility of the pelvic and shoulder girdles as shown by the MRC%. The MVC values of both patients began at values lower than normal whereas during treatment patients had improved percentage of muscle strength. The strength peak in both patients coincided with the minimum B cell values. There were no severe adverse events associated with an infusion of RTX.ConclusionWe consider the increase in muscle strength observed in both treated patients to be a consequence of their treatment with RTX. To our knowledge, these are the first cases of increased muscle strength in patients with MM. Furthermore, the results of this study indicate that B cell depletion with RTX may be useful in the treatment of patients affected by MM, suggesting a possible role for B cells in the pathophysiology of this muscle disorder.

Describing The Percent Inactivation Versus Force Relationships And Predicting True Force With Polynomial Regression

PURPOSE: The purpose of this study was to examine the percent voluntary inactivation versus submaximal force relationships with polynomial regression to predict each subjects' true maximal voluntary contraction (MVC) capabilities. METHODS: Nine participants (mean±SD age=21±3 yrs) completed nine randomly-ordered isometric plantar flexions from 10 to 90% of the MVC. Transcutaneous electrical stimuli were delivered to the tibial nerve using a high-voltage constant-current stimulator (Digitimer DS7AH, Herthfordshire, UK). Doublets were administered at a supramaximal stimulus intensity during the submaximal MVC plateau and then again 3-5 s after the submaximal MVC trial at rest. Percent inactivation was calculated for each submaximal MVC. The nine subjects selected for this study demonstrated < 5% voluntary inactivation at the 90% MVC, which suggested that these subjects were highly activated. Percent voluntary inactivation versus force relationships were analyzed on a subject-by-subject basis using polynomial regression (linear, quadratic, and cubic models). The predicted true MVC for each subject was extrapolated with their best fit polynomial model. A paired-samples t test was used to compare the actual MVC to the predicted true MVC. RESULTS: Polynomial regression indicated that 3 of 9 subjects (33%) were best fitted with a linear model, 2 of 9 subjects (22%) were best fitted with a quadratic model, and 4 of 9 subjects (44%) were best fitted with a cubic model. The t test indicated that the actual MVC (mean±SD 446±114 N) was greater than the predicted MVC (mean±SD 405±124 N) values (P = 0.003). Percent difference for each subject between actual MVC versus predicted MVC values ranged from 0.7% to 20.5%. CONCLUSIONS: Fitting the percent inactivation versus force relationships of highly activated subjects resulted in 33% linear and 66% nonlinear relationships. These results suggested that these patterns should be analyzed on a subject-by-subject basis. However, the predicted true MVC values from the best fit polynomial models were significantly less than the actual MVC values.

Muscular Function Response to Walking Exercise in Combination with Vascular Restriction

Recent studies concluded that once-daily vascular restricted walk training produced physiologically significant increases in thigh muscle volume and knee joint strength, however the effects of vascular restriction (VR) on motor unit activation from electromyographic (EMG) activity has not been determined. Investigating the muscular function response to walking exercise with VR may help researchers to define the neuromuscular changes, if any, and to explain the adaptations occurring in skeletal muscle strength and size following training. PURPOSE: The purpose of this study was to examine the changes in EMG amplitude (RMS) and median frequency (MDF) patterns during maximum voluntary contraction (MVC) before and after walking combined with or without reduced blood flow. METHODS: Nine young healthy males (Mean ± SD age = 24.3 ± 6.5 years; height = 174.4 ±7.3 cm; weight = 81.3 ± 7.7 kg) had their pre-walk MVC determined by using Biodex. Surface EMG electrodes were placed along the longitudinal axis of the vastus lateralis (VL) and rectus femoris (RF) of the right thigh. The electrode placements on the VL were placed to a mark that was made at 33.3%, and RF 50%, of the distance from the lateral femoral epicondyle to the greater trochanter. On two separate testing sessions, each subject walked for 20 minutes (two bouts of 10 min walking) on a treadmill at a speed of 80.4 m.min-1 with and without blood flow restriction, with a 2 min rest interval between the bouts. After completing the walking exercise, each subject repeated the exact same MVC procedure using the Biodex to measure post-walk MVC. RESULTS: No significant condition (with or without VR) effect was detected (p>0.05) for the pre- and post-walk MVCs, but there was significant increases in EMG MDF from the pre- to post-walk MVC for both the VL and RF (p<0.01). There were no significant changes in EMG RMS from pre to post-walk MVC and torque values for either condition. CONCLUSIONS: Twenty minutes of walking with or without VR resulted in increases in EMG MDF values indicating increases in action potential conduction velocity of the active muscle fibers for both the VL and RF of the right leg during MVC.

Age And Sex Differences In Muscle Fatigability Maintaining The Same Absolute Force

Old adults are able to sustain a submaximal isometric contraction for a longer duration than young adults when maintained at the same relative intensity. Many daily tasks, however, require maintenance of an absolute force or load that does not vary between young and old adults. PURPOSE: The purpose was to compare the time to failure, maximal voluntary contraction (MVC), and supraspinal fatigue of young and old men and women for an isometric fatiguing contraction performed at the same absolute force with the elbow flexor muscles. METHODS: Ten young adults (5 men and 5 women; mean ± ∑D; 21± 1 years) and 10 old adults (5 men and 5 women; 71 ± 4 years) sustained an isometric fatiguing contraction at a force equivalent to a 6 kg load until the target force could no longer be achieved. Supraspinal fatigue was quantified as the reduction in voluntary activation using cortical stimulation. RESULTS: Young men were stronger than old men (99.6 ± 20.7 Nm vs 69.6 ± 6.3 Nm) but there was no significant difference between young and old women (41.8 ± 8.1 Nm vs 35.8 ± 18.6 Nm). Men were stronger than women for both age groups. Young adults sustained a relatively lower contraction force than old adults (24.8 ± 10.7 vs 31.2 ± 10.8 %MVC) and men sustained relatively less than women (18.8 ± 4.3 vs 37.2 ± 6.7 %MVC). There was no difference in time to task failure between young and old adults but there was a sex difference such that men had a longer time to failure than the women. The time to task failure of the young men and women were 20.7 ± 11.8 and min 5.2 ± 2.5 min respectively, and the old men and women were 16.4 ± 6.8 min and 5.5 ± 2.2 min respectively. Voluntary activation was similar for men and women before and after the fatiguing contraction (P < 0.05) but the age difference was P = 0.06 indicating old adults showed less voluntary activation than the young. The age and sex difference for time to failure was minimized when co-varied for the initial MVC values. Furthermore, MVC torque was the main predictor of time to task failure (r2 = 0.54, P < 0.001). CONCLUSION: The age difference in muscle fatigability often observed when the relative force is similar was minimized when sustaining a similar absolute force. In contrast, the sex difference in muscle fatigability when sustaining relative forces was reversed for a similar absolute force.

Prior Muscular Exercise Affects Cycling Pattern

The aim of this study was to investigate the effect of concentric or eccentric fatiguing exercise on cycling pattern. Eleven well trained cyclists completed three sessions of cycling (control cycling test [CTRL], cycling following concentric [CC] or eccentric [ECC] knee contractions) at a mean power of 276.8 +/- 26.6 Watts. Concentric and eccentric knee contractions were performed at a load corresponding to 80 % of one repetition maximum with both legs. Before and after CTRL, CC or ECC knee contractions and after cycling, a maximal voluntary contraction (MVC) test was performed. Cardiorespiratory, mechanical and electromyographic activity (EMG) of the rectus femoris, vastus lateralis and biceps femoris muscles were recorded during cycling. A significant decrease in MVC values was observed after CC and ECC exercises and after the cycling. ECC exercise induced a significant decrease in EMG root mean square during MVC and a decrease in pedal rate during cycling. EMG values of the three muscles were significantly higher during cycling exercise following CC exercise when compared to CTRL. The main finding of this study was that a prior ECC exercise induces a greater neuromuscular fatigue than a CC exercise, and changes in cycling pattern.

Percentile values for determining maximum endurance times for static muscular work

The maximum endurance time (MET) is a key parameter for the ergonomic evaluation of static work. The aim of this study is to offer practitioners MET values in relation to the %MVC (percentage of maximum voluntary contraction) that take into account the results of several studies published on MET. Twenty four published MET models were combined to extract MET percentile values that reflect the interindividual variability and differences among the experimental methods used to develop the models. The MET differences by muscle group are also verified. The results of this study show that, (1) there is a significant difference between the MET values for the back/hip and the MET values for the upper limbs, for a same %MVC value; (2) the model-generated MET values follow a lognormal distribution. A summary table is presented showing the most frequently used percentile values (5th, 10th and 15th percentiles) of the MET corresponding to a range of %MVC values. The data from this paper allow engineers and ergonomics practitioners to design workstations that can accommodate a desired proportion of the worker population, i.e., designs that are not restricted only to the 50th percentile. Relevance to industry Static and intermittent static muscular work is a known risk factor for musculoskeletal disorders (MSD). Estimation of MET to accommodate a percentage of the worker population is a necessary step in determining appropriate work-rest regimens for static muscular work in industrial settings.

Simultaneous measurement of intramuscular pressure and surface electromyography of the multifidus muscle.

The anatomic proof of a spinal compartment and the clinical symptoms of compartment syndrome in patients with chronic back pain are inconsistent with the rarely met measuring criteria of intramuscular pressure (IMP). Previous studies assume a dependence of the IMP on spinal alignment (degree of lumbar spine flexion) and the degree of muscle activation. The significance of these disturbance variables in the interpretation of IMP could explain the above discrepancy. This study therefore investigates the influence of both a 30% increase in trunk flexion and alterations in muscle contraction from 100% to 60%. Sixteen healthy subjects participated in the study. The IMP and mean rectified amplitude of the multifidus surface EMG signal were determined at rest and 0 degrees and approximately 30 degrees of lumbar spine flexion, and they were compared. Subsequently, both parameters were measured during both 100% and 60% maximal voluntary contraction (MVC) of the muscle and then correlated. During rest and 0 degrees flexion, the median IMP was 9.3 mmHg (range 0.0-22.5) while the median mean rectified amplitude (MRA) of the EMG signal was 1.98 microV (range 1.32-7.38). In 30 degrees flexion, the median IMP went up to 24.3 mmHg (range 1.4-97.3) with hardly any increase in the median MRA of 2.32 microV (range 1.20-9.72). Under 60% MVC, the median IMP rose to 186.6 mmHg (range 15.4-375.4) and the median MRA to 21.02 microV (range 4.63-43.63). During 100% MVC, the median MRA increased to 34.38 microV (range 12.99-102.54) while the median IMP rose to 273.4 mmHg (range 90.4-395.1). Spearman's rank correlation coefficient for the IMP and MRA quotients of the 100/60% MVC values was r= -0.21. To sum up, it can be said that IMP was subject to great interindividual variation in all the experiments. This parameter is highly dependent on spinal alignment and muscular activity. Further studies are needed so that the IMP can be interpreted properly when diagnosing a chronic compartment of the erector spinae muscles.

Task differences with the same load torque alter the endurance time of submaximal fatiguing contractions in humans.

Endurance time, muscle activation, and mean arterial pressure were measured during two types of submaximal fatiguing contractions that required each subject to exert the same net muscle torque in the two tasks. Sixteen men and women performed isometric contractions at 15% of the maximum voluntary contraction (MVC) force with the elbow flexor muscles, either by maintaining a constant force while pushing against a force transducer (force task) or by supporting an equivalent inertial load while maintaining a constant elbow angle (position task). The endurance time for the force task (1402 +/- 728 s) was twice as long as that for the position task (702 +/- 582 s, P < 0.05), despite a similar reduction in the load torque at exhaustion for each contraction. The rate of increase in average electromyographic activity (EMG, % peak MVC value) for the elbow flexor muscles was similar for the two tasks. However, the average EMG was greater at exhaustion for the force task (22.4 +/- 1.2%) compared with the position task (14.9 +/- 1.0%, P < 0.05). In contrast, the rates of increase in the mean arterial pressure, the rating of perceived exertion, anterior deltoid EMG, and fluctuations in motor output (force or acceleration) were greater for the position task compared with the force task (P < 0.05). Furthermore, the rate of bursts in EMG activity, which corresponded to the transient recruitment of motor units, was greater for the brachialis muscle during the position task. These results indicate that the briefer endurance time for the position task was associated with greater levels of excitatory and inhibitory input to the motor neurons compared with the force task.

Effects of dehydration and rehydration on EMG changes during fatiguing contractions.

This study measured the effects of sauna-induced dehydration (Dhy) and the effectiveness of rapid rehydration on muscle performance and EMG frequency spectrum changes associated with fatigue during isometric contractions. Knee extensor muscle strength during isometric maximal voluntary contraction (MVC) and endurance time at 25% and 70% of MVC (ET25 and ET70, respectively) were measured three times in 11 healthy male subjects, under euhydration conditions (Eu), after Dhy, and after rehydration following Dhy (Rhy). Dhy led to a decrease in body weight by 2.95 +/- 0.05%. No significant effect of the hydration status was shown on MVC values. A 23% decrease in ET25 was recorded during Dhy (P < 0.01), whereas ET70 only tended to decrease (-13%, P = 0.06). ET25 was higher during Rhy than Dhy (8%, P < 0.05) but remained lower than during Eu (-17%, P < 0.05). The EMG root mean square (RMS) increased earlier during Dhy than Eu. Opposite changes were shown for the mean power frequency (MPF) of EMG, and Dhy resulted in an accelerated fall in MPF. However, because ET25 decreased with dehydration, RMS and MPF changes were similar during Eu and Dhy when reported to normalized contraction time, exhaustion was thus associated with similar values of RMS and MPF for all conditions. RMS and MPF changes during Rhy showed an intermediate pattern between Eu and Dhy. Dhy induced an increase in muscle fatigue, associated with early changes in EMG spectral parameters. It is not clear whether these alterations could be attributed to biochemical modifications, and the role of increased perception of effort when subjects were dehydrated should be clarified.