Acute effects of static stretching on maximal eccentric torque production in women

The purpose of this study was to examine the acute effects of static stretching on peak torque (PT) and the joint angle at PT during maximal, voluntary, eccentric isokinetic muscle actions of the leg extensors at 60 and 180 degrees x s(-1) for the stretched and unstretched limbs in women. Thirteen women (mean age +/- SD = 20.8 +/- 0.8 yr; weight +/- SD = 63.3 +/- 9.5 kg; height +/- SD = 165.9 +/- 7.9 cm) volunteered to perform separate maximal, voluntary, eccentric isokinetic muscle actions of the leg extensors with the dominant and nondominant limbs on a Cybex 6000 dynamometer at 60 and 180 degrees x s(-1). PT (Nm) and the joint angle at PT (degrees) were recorded by the dynamometer software. Following the initial isokinetic assessments, the dominant leg extensors were stretched (mean stretching time +/- SD = 21.2 +/- 2.0 minutes) using 1 unassisted and 3 assisted static stretching exercises. After the stretching (4.3 +/- 1.4 minutes), the isokinetic assessments were repeated. The statistical analyses indicated no changes (p > 0.05) from pre- to poststretching for PT or the joint angle at PT. These results indicated that static stretching did not affect PT or the joint angle at PT of the leg extensors during maximal, voluntary, eccentric isokinetic muscle actions at 60 and 180 degrees x s(-1) in the stretched or unstretched limbs in women. In conjunction with previous studies, these findings suggested that static stretching may affect torque production during concentric, but not eccentric, muscle actions.     

The rate of force development obtained at early contraction phase is not influenced by active static stretching

The objective of this study was to investigate the influence of active static stretching on the maximal isometric muscle strength (maximal voluntary contraction [MVC]) and rate of force development (RFD) determined within time intervals of 30, 50, 100, and 200 milliseconds relative to the onset of muscle contraction. Fifteen men (aged 21.3 ± 2.4 years) were submitted on different days to the following tests: (a) familiarization session to the isokinetic dynamometer; (b) 2 maximal isometric contractions for knee extensors in the isokinetic dynamometer to determine MVC and RFD (control); and (c) 2 active static stretching exercises for the dominant leg extensors (10 × 30 seconds for each exercise with a 20-second rest interval between bouts). After stretching, the isokinetic test was repeated (poststretching). Conditions 2 and 3 were performed in random order. The RFD was considered as the mean slope of the moment-time curve at time intervals of 0-30, 0-50, 0-100; 0-150; and 0200 milliseconds relative to the onset of muscle contraction. The MVC was reduced after stretching (285 ± 59 vs. 271 ± 56 N · m, p < 0.01). The RFD at intervals of 0-30, 0-50, and 0-100 milliseconds was unchanged after stretching (p > 0.05). However, the RFD measured at intervals of 0-150 and 0-200 milliseconds was significantly lower after stretching (p < 0.01). It can be concluded that explosive muscular actions of a very short duration (<100 milliseconds) seem less affected by active static stretching when compared with actions using maximal muscle strength.     

Acute effects of static versus dynamic stretching on isometric peak torque, electromyography, and mechanomyography of the biceps femoris muscle

The purpose of this study was to examine the acute effects of static versus dynamic stretching on peak torque (PT) and electromyographic (EMG), and mechanomyographic (MMG) amplitude of the biceps femoris muscle (BF) during isometric maximal voluntary contractions of the leg flexors at four different knee joint angles. Fourteen men ((mean +/- SD) age, 25 +/- 4 years) performed two isometric leg flexion maximal voluntary contractions at knee joint angles of 41 degrees , 61 degrees , 81 degrees , and 101 degrees below full leg extension. EMG (muV) and MMG (m x s(-2)) signals were recorded from the BF muscle while PT values (Nm) were sampled from an isokinetic dynamometer. The right hamstrings were stretched with either static (stretching time, 9.2 +/- 0.4 minutes) or dynamic (9.1 +/- 0.3 minutes) stretching exercises. Four repetitions of three static stretching exercises were held for 30 seconds each, whereas four sets of three dynamic stretching exercises were performed (12-15 repetitions) with each set lasting 30 seconds. PT decreased after the static stretching at 81 degrees (p = 0.019) and 101 degrees (p = 0.001) but not at other angles. PT did not change (p > 0.05) after the dynamic stretching. EMG amplitude remained unchanged after the static stretching (p > 0.05) but increased after the dynamic stretching at 101 degrees (p < 0.001) and 81 degrees (p < 0.001). MMG amplitude increased in response to the static stretching at 101 degrees (p = 0.003), whereas the dynamic stretching increased MMG amplitude at all joint angles (p 相似文献   

Acute effects of static stretching on peak torque in women

The purpose of this study was to examine the effects of static stretching on concentric, isokinetic leg extension peak torque (PT) at 60 and 240 degrees.s(-1) in the stretched and unstretched limbs. The PT of the dominant (stretched) and nondominant (unstretched) leg extensors were measured on a calibrated Cybex 6000 dynamometer. Following the prestretching PT assessments, the dominant leg extensors were stretched using 1 active and 3 passive stretching exercises. After the stretching, PT was reassessed. The results of the statistical analyses indicated that PT decreased following the static stretching in both limbs and at both velocities (60 and 240 degrees.s(-1)). The present findings suggested that the stretching-induced decreases in PT may be related to changes in the mechanical properties of the muscle, such as an altered length-tension relationship, or a central nervous system inhibitory mechanism. Overall, these findings, in conjunction with previous studies, indicated that static stretching impairs maximal force production. Strength and conditioning professionals should consider this before incorporating static stretching in preperformance activities. Future studies are needed to identify the underlying mechanisms that influence the time course of stretching-induced decreases in maximal force production for athletes and nonathletes across the age span.     

Effects of creatine supplementation and three days of resistance training on muscle strength, power output, and neuromuscular function

Previous studies have demonstrated increases in peak torque (PT) and decreases in acceleration time (ACC) after only 2 days of resistance training, and other studies have reported improvements in isokinetic performance after 5 days of creatine supplementation. Consequently, there may be a combined benefit of creatine supplementation and short-term resistance training for eliciting rapid increases in muscle strength, which may be important for short-term rehabilitation and return-to-play for previously injured athletes. The purpose of this study, therefore, was to examine the effects of 3 days of isokinetic resistance training combined with 8 days of creatine monohydrate supplementation on PT, mean power output (MP), ACC, surface electromyography (EMG), and mechanomyography (MMG) of the vastus lateralis muscle during maximal concentric isokinetic leg extension muscle actions. Twenty-five men (mean age +/- SD = 21 +/- 3 years, stature = 177 +/- 6 cm, and body mass = 80 +/- 12 kg) volunteered to participate in this 9-day, double-blind, placebo-controlled study and were randomly assigned to either the creatine (CRE; n = 13) or placebo (PLA; n = 12) group. The CRE group ingested the treatment drink (280 kcal; 68 g carbohydrate; 10.5 g creatine), whereas the PLA group received an isocaloric placebo (70 g carbohydrate). Two servings per day (morning and afternoon) were administered in the laboratory on days 1-6, with only 1 serving on days 7-8. Before (pre; day 1) and after (post; day 9) the resistance training, maximal voluntary concentric isokinetic leg extensions at 30, 150, and 270 degrees x s(-1) were performed on a calibrated Biodex System 3 dynamometer. Three sets of 10 repetitions at 150 degrees x s(-1) were performed on days 3, 5, and 7. Peak torque increased (p = 0.005; eta(2) = 0.296), whereas ACC decreased (p < 0.001; eta(2) = 0.620), from pretraining to posttraining for both the CRE and PLA groups at each velocity (30, 150, and 270 degrees x s(-1)). Peak torque increased by 13% and 6%, whereas ACC decreased by 42% and 34% for the CRE and PLA groups, respectively, but these differences were not statistically significant (p > 0.05). There were no changes in MP, EMG, or MMG amplitude; however, EMG median frequency (MDF) increased, and MMG MDF increased at 30 degrees x s(-1), from pretraining to posttraining for both the CRE and PLA groups. These results indicated that 3 days of isokinetic resistance training was sufficient to elicit small, but significant, improvements in peak strength (PT) and ACC for both the CRE and PLA groups. Although the greater relative improvements in PT and ACC for the CRE group were not statistically significant, these findings may be useful for rehabilitation or strength and conditioning professionals who may need to rapidly increase the strength of a patient or athlete within 9 days.     

Reliability of concentric and eccentric strength of hip abductor and adductor muscles in young soccer players

The concentric and eccentric strength profile and muscular balance of the hip joint are important parameters for success in soccer. This study evaluated the reliability for the assessment of hip abduction and adduction isokinetic strength over a range of angular velocities (30 and 90°/s) and types of muscular actions (concentric and eccentric) in young soccer players. The reliability for the assessment of reciprocal (conventional and functional) and bilateral torque ratios was also examined. Fifteen male soccer players (15±1 years) performed two sessions, separated by three days. The testing protocol consisted of five maximal concentric and eccentric hip abductions and adductions of both legs at angular velocities of 30°/s and 90°/s. The peak torque was evaluated in young soccer players using an isokinetic dynamometer (Cybex Norm), and the reciprocal strength ratios (conventional and functional) and bilateral ratios (non-preferred to preferred leg ratios) were calculated. The test-retest reliability for the assessment of peak torque (ICC = 0.71-0.92) and of reciprocal muscle group ratios (ICC = 0.44-0.87) was found to be moderate to high. Bilateral torque ratios exhibited low to moderate reliability (ICC = 0.11-0.64). In conclusion, isokinetic strength of hip abductor and adductor muscles and the conventional and functional strength ratios can be reliably assessed in young soccer players, especially at low angular velocities. The assessment, however, of bilateral strength ratios for hip abductor/adductor muscles should be interpreted with more caution.     

In vivo changes in the human patellar tendon moment arm length with different modes and intensities of muscle contraction

The purpose of this study was to examine the effect of different muscle contraction modes and intensities on patellar tendon moment arm length (d(PT)). Five men performed isokinetic concentric, eccentric and passive knee extensions at an angular velocity of 60 deg/s and six men performed gradually increasing to maximum effort isometric muscle contractions at 90( composite function) and 20( composite function) of knee flexion. During the tests, lateral X-ray fluoroscopy imaging was used to scan the knee joint. The d(PT) differences between the passive state and the isokinetic concentric and extension were quantified at 15( composite function) intervals of knee joint flexion angle. Furthermore, the changes of the d(PT) as a function of the isometric muscle contraction intensities were determined during the isometric knee extension at 90( composite function) and 20( composite function) of knee joint flexion. Muscle contraction-induced changes in knee joint flexion angle during the isometric muscle contraction were also taken into account for the d(PT) measurements. During the two isometric knee extensions, d(PT) increased from rest to maximum voluntary muscle contraction (MVC) by 14-15%. However, when changes in knee joint flexion angle induced by the muscle contraction were taken into account, d(PT) during MVC increased by 6-26% compared with rest. Moreover, d(PT) increased during concentric and eccentric knee extension by 3-15%, depending on knee flexion angle, compared with passive knee extension. These findings have important implications for estimating musculoskeletal loads using modelling under static and dynamic conditions.     

Power output,mechanomyographic, and electromyographic responses to maximal,concentric, isokinetic muscle actions in men and women

The purpose of this study was to examine the responses of peak torque (PT), mean power output (MP), mechanomyographic (MMG) and electromyographic (EMG) amplitudes, and mean power frequencies (MPFs) of the vastus lateralis (VL), rectus femoris (RF), and vastus medialis (VM) in men and women during dynamic muscle actions. Twelve women (mean +/- SD age = 22 +/- 3 years) and 11 men (22 +/- 3 years) performed maximal, concentric, isokinetic leg extensions at velocities of 60, 120, 180, 240, and 300 degrees x s(-1) on a Cybex 6000 dynamometer. Piezoelectric MMG-recording sensors and bipolar surface EMG electrodes were placed over the VL, RF, and VM muscles. No sex-related differences were found among the velocity-related patterns for PT, MP, MMG amplitude, MMG MPF, or EMG MPF. There were, however, sex-related differences in the patterns of EMG amplitude across velocity. The results indicated similar velocity-related patterns of increase of MP and MMG amplitude for all 3 muscles and of EMG amplitude for the VL and VM in the women. Velocity-related decreases (p 0.05) across velocity. MMG MPF increased (p < or = 0.05) only between 240 and 300 degrees x s(-1). Overall, these findings suggested that there were sex- and muscle-specific, velocity-related differences in the associations among motor unit activation strategies (EMG amplitude and MPF) and the mechanical aspects of muscular activity (MMG amplitude and MPF). With additional examination and validation, however, MMG may prove useful to practitioners for monitoring training-induced changes in muscle power output.     

Are there mode-specific and fatigue-related electromechanical delay responses for maximal isokinetic and isometric muscle actions?

This study used a combined electromyographic, mechanomyographic, and force approach to identify electromechanical delay (EMD) from the onsets of the electromyographic to force signals (EMD_E-F), onsets of the electromyographic to mechanomyogrpahic signals (EMD_E-M), and onsets of mechanomyographic to force signals (EMD_M−F). The purposes of the current study were to examine: (1) differences in EMD_E-M, EMD_M−F, and EMD_E-F from the vastus lateralis between maximal isokinetic and maximal concentric isometric leg extensions; and (2) the effects of fatigue and recovery on EMD_E-M, EMD_M−F, and EMD_E-F. These EMD measures were obtained from twelve men during maximal concentric isokinetic and isometric leg extensions pretest, posttest, and after 3-min and 5-min of recovery from 25 maximal isokinetic leg extensions at 60° s⁻¹. The results indicated no differences between maximal isokinetic and isometric muscle actions for EMD_E-M, EMD_M-F, or EMD_E-F during the pretest, posttest, 3-min recovery, and 5-min recovery measurements. These findings support the comparison of voluntary EMD measures between studies with different modes of exercise as long as the methodology for the determination of EMD are consistent. There were, however, fatigue-induced pretest to posttest increases in EMD_E-M, EMD_M-F, and EMD_E-F which remained elongated after 3-min of recovery, but returned to pretest values after 5-min of recovery.     

Mechanomyographic and electromyographic responses of the vastus medialis muscle during isometric and concentric muscle actions

The purpose of this study was to examine the patterns for the mechanomyographic (MMG) and electromyographic (EMG) amplitude and mean power frequency (MPF) vs. torque relationships during submaximal to maximal isometric and isokinetic muscle actions. Seven men (mean +/- SD age, 22.4 +/- 1.3 years) volunteered to perform isometric and concentric isokinetic leg extension muscle actions at 20, 40, 60, 80, and 100% of maximal voluntary contraction (MVC) and peak torque (PT) on a Cybex II dynamometer. A piezoelectric MMG recording sensor was placed between bipolar surface EMG electrodes on the vastus medialis. Polynomial regression and separate 1-way repeated-measures analysis of variance were used to analyze the EMG amplitude, MMG amplitude, EMG MPF, and MMG MPF data for the isometric and isokinetic muscle actions. For the isometric muscle actions, EMG amplitude (R(2) = 0.999) and MMG MPF (R(2) = 0.946) increased to MVC, mean MMG amplitude increased to 60% MVC and then plateaued, and mean EMG MPF did not change (p > 0.05) across torque levels. For the isokinetic muscle actions, EMG amplitude (R(2) = 0.988) and MMG amplitude (R(2) = 0.933) increased to PT, but there were no significant mean changes with torque for EMG MPF or MMG MPF. The different torque-related responses for EMG and MMG amplitude and MPF may reflect differences in the motor control strategies that modulate torque production for isometric vs. dynamic muscle actions. These results support the findings of others and suggest that isometric torque production was modulated by a combination of recruitment and firing rate, whereas dynamic torque production was modulated primarily through recruitment.     

Agonist muscle activity and antagonist muscle co-activity levels during standardized isotonic and isokinetic knee extensions

This study aimed to analyze the effects of the contraction mode (isotonic vs. isokinetic concentric conditions), the joint angle and the investigated muscle on agonist muscle activity and antagonist muscle co-activity during standardized knee extensions. Twelve healthy adult subjects performed three sets of isotonic knee extensions at 40% of their maximal voluntary isometric torque followed by three sets of maximal isokinetic knee extensions on an isokinetic dynamometer. For each set, the mean angular velocity and the total external amount of work performed were standardized during the two contraction modes. Surface electromyographic activity of vastus lateralis (VL), vastus medialis (VM), rectus femoris (RF), semitendinosus (ST) and biceps femoris (BF) muscles was recorded. Root mean square values were then calculated for each 10° between 85° and 45° of knee extension (0° = horizontal position). Results show that agonist muscle activity and antagonist muscle co-activity levels are significantly greater in isotonic mode compared to isokinetic mode. Quadriceps activity and hamstrings co-activity are significantly lower at knee extended position in both contraction modes. Considering agonist muscles, VL reveals a specific pattern of activity compared to VM and RF; whereas considering hamstring muscles, BF shows a significantly higher co-activity than ST in both contraction modes. Results of this study confirmed our hypothesis that higher quadriceps activity is required during isotonic movements compared to isokinetic movements leading to a higher hamstrings co-activity.     

Effect of active recovery on acute strength deficits induced by passive stretching

We herein examined whether immediate muscular activity (active recovery) after stretching decreased stretch-induced strength deficits in human muscles. Our within-subject study included 8 subjects who were used as their own controls. For each subject, both legs were subjected to the same warm-up and stretching treatments, and then one leg was exposed to active recovery (experimental treatment) while the other was allowed to recover passively (control). Unilateral maximal voluntary contraction (MVC) of knee extensors was measured at baseline, poststretching, and postrecovery to monitor strength evolution. Our results revealed that the MVC strength at the baseline time point for control (590.8 +/- 104.2) and treated (602.2 +/- 112.7) legs decreased poststretching by 8.0 and 8.9%, respectively, and further decreased postrecovery by 1.3 and 1.2%, respectively. Maximal voluntary contraction strength tests demonstrated very good reliability, having intraclass coefficients of correlation ranging from 0.92-0.98. Mixed analysis of variance showed that the stretching program yielded significantly increased flexibility (p < 0.01) and significantly decreased MVC (p < 0.001) in both legs. The over-time variability between legs was marginal (1%), and no significant between-leg differences were observed. Indeed, the improvement in strength restoration due to active vs. passive recovery was -0.5 +/- 15 N, which was significantly lower (p < 0.01; 1-tailed t-test) than the amount of strength inhibition (32.6 N), estimated as 60% of the overall strength deficit (54.3 +/- 29.7 N). These results confirm that significant strength is lost poststretching but fail to show greater improvement in strength following active vs. passive recovery. Collectively, the present findings indicate that, contrary to the belief of many coaches, muscular exercises during the poststretching period are unlikely to minimize stretch-induced strength deficits.     

Effects of eccentrically and concentrically induced unilateral fatigue on the involved and uninvolved limbs.

This study extended findings of others related to the fatigue resistance of maximum voluntary knee extension contractions performed eccentrically on an isokinetic dynamometer. Twelve subjects performed either 75 unilateral isokinetic concentric MVCs or 75 unilateral isokinetic eccentric MVCs at 30 degrees s(-1). A uniquely-configured dynamometer provided the opportunity to describe the effect of the concentric or eccentric fatigue protocol on the concentric or eccentric MVC force of the contralateral limb, which was not involved in the fatigue protocol, immediately following the conclusion of the fatigue protocol. Eccentric MVC of the eccentrically fatigued group decreased significantly (13%, p = 0.001) although the decrease was significantly smaller than that of the concentric MVC of the concentrically fatigued group (39%). Concentric MVC of the contralateral limb was unaffected following the concentric fatigue protocol but the eccentric MVC of the contralateral limb increased 11% (p = 0.028) following the eccentric fatigue protocol. These results suggest that eccentric MVCs are not resistant to fatigue but do follow a different time course than fatigue induced with concentric contractions. The extent to which eccentrically performed MVCs fatigue may reflect the influence of protocol parameters such as the isokinetic speed, the number of repetitions, the criteria by which the protocol is terminated and the subject selection. However, the explanations for why eccentric MVCs fatigue to such a smaller extent necessitate further systematic investigation including electrophysiologic methods, as do the results relative to the contralateral leg. Both results can be considered within the framework of current thought about the disparate nature of nervous system control of eccentric contractions.     

Neuromuscular fatigue development during maximal concentric and isometric knee extensions.

This study aimed to investigate mechanisms of neuromuscular fatigue during maximal concentric and isometric leg extensions inducing similar torque decrements. Nine physically active men performed two separate fatiguing sessions maintained until similar torque decreases were obtained. The first session, only conducted under isokinetic concentric conditions (CON), consisted of three series of 30 maximal voluntary concentric knee extensions (60 degrees/s). The second session, exclusively isometric (ISO), mimicked the torque decreases registered during the CON session while performing three long-lasting ISO contractions. Maximal voluntary torque, activation level (twitch interpolation technique), electromyographic activity (root mean square and median frequency) of the vastus lateralis muscle, and electrically evoked doublet-twitch mechanical properties were measured before and at the end of each of the three series. After the three series, similar torque decrements were obtained for both fatiguing procedures. The total fatiguing contraction durations were not different among procedures. With equivalent voluntary torque decrements, the doublet-twitch amplitude reduction was significantly greater (P<0.01) during the two first series of the CON procedure compared with ISO. No difference was observed for the third series. Although no difference was recorded with fatigue for median frequency changes between CON and ISO, activation levels and root mean square values demonstrated greater reductions (P<0.05) for all three series during the ISO procedure compared with CON. Performing CON or ISO fatiguing exercises demonstrated different fatigue origins. With CON exercises, peripheral fatigue developed first, followed by central fatigue, whereas with ISO exercises the fatigue pattern was inverted.     

Effect of muscle fatigue on the compliance of the gastrocnemius medialis tendon and aponeurosis

The aim of the present study was to examine whether or not the compliance of the gastrocnemius medialis (GM) tendon and aponeurosis is influenced by submaximal fatiguing efforts. Fourteen elderly male subjects performed isometric maximal voluntary plantarflexion contractions (MVC) on a dynamometer before and after two fatiguing protocols. The protocols consisted of: (1) submaximal concentric isokinetic contractions (70% isokinetic MVC) at 60 degrees /s and (2) a sustained isometric contraction (40% isometric MVC) until failure to hold the defined moment. Ultrasonography was used to determine the elongation and strain of the GM tendon and aponeurosis. To account for the axis misalignment between ankle and dynamometer, the kinematics of the leg were captured at 120 Hz. The maximum moment decreased from 85.9+/-17.9 Nm prior fatigue to 79.2+/-19 Nm after isokinetic fatigue and to 69.9+/-16.4 Nm after isometric fatigue. The maximal strain of the GM tendon and aponeurosis before fatigue, after isokinetic and after isometric fatigue were 4.9+/-1.1%, 4.4+/-1.1% and 4.3+/-1.1% respectively. Neither the strain nor the elongation showed significant differences before and after each fatiguing task at any 100 N step of the calculated tendon force. This implies that the compliance was not altered after either the isokinetic or the isometric fatiguing task. Therefore it was concluded that the strains during the performed submaximal fatiguing tasks, were too small to provoke any structural changes in tendon and aponeurosis.     

Acute effect of static stretching on power output during concentric dynamic constant external resistance leg extension

The purpose of the present study was to clarify the effect of static stretching on muscular performance during concentric isotonic (dynamic constant external resistance [DCER]) muscle actions under various loads. Concentric DCER leg extension power outputs were assessed in 12 healthy male subjects after 2 types of pretreatment. The pretreatments included (a) static stretching treatment performing 6 types of static stretching on leg extensors (4 sets of 30 seconds each with 20-second rest periods; total duration 20 minutes) and (b) nonstretching treatment by resting for 20 minutes in a sitting position. Loads during assessment of the power output were set to 5, 30, and 60% of the maximum voluntary contractile (MVC) torque with isometric leg extension in each subject. The peak power output following the static stretching treatment was significantly (p < 0.05) lower than that following the nonstretching treatment under each load (5% MVC, 418.0 +/- 82.2 W vs. 466.2 +/- 89.5 W; 30% MVC, 506.4 +/- 82.8 W vs. 536.4 +/- 97.0 W; 60% MVC, 478.6 +/- 77.5 W vs. 523.8 +/- 97.8 W). The present study demonstrated that relatively extensive static stretching significantly reduces power output with concentric DCER muscle actions under various loads. Common power activities are carried out by DCER muscle actions under various loads. Therefore, the result of the present study suggests that relatively extensive static stretching decreases power performance.     

Age-related effect of static and cyclic loadings on the strain-force curve of the vastus lateralis tendon and aponeurosis

The objective of the present study was to investigate the age-related effects of submaximal static and cyclic loading on the mechanical properties of the vastus lateralis (VL) tendon and aponeurosis in vivo. Fourteen old and 12 young male subjects performed maximal voluntary isometric knee extensions (MVC) on a dynamometer before and after (a) a sustained isometric contraction at 25% MVC and (b) isokinetic contractions at 50% isokinetic MVC, both until task failure. The elongation of the VL tendon and aponeurosis was examined using ultrasonography. To calculate the resultant knee joint moment, the kinematics of the leg were recorded with eight cameras (120 Hz). The old adults displayed significantly lower maximal moments but higher strain values at any given tendon force from 400 N and up in all tested conditions. Neither of the loading protocols influenced the strain-force relationship of the VL tendon and aponeurosis in either the old or young adults. Consequently, the capacity of the tendon and aponeurosis to resist force remained unaffected in both groups. It can be concluded that in vivo tendons are capable of resisting long-lasting static (~4.6 min) or cyclic (~18.5 min) mechanical loading at the attained strain levels (4-5%) without significantly altering their mechanical properties regardless of age. This implies that as the muscle becomes unable to generate the required force due to fatigue, the loading of the tendon is terminated prior to provoking any significant changes in tendon mechanical properties.     

Training can improve muscle strength and endurance in 78- to 84-yr-old men.

Nine men, 78-84 yr of age, participated in a dynamometer training program 2-3 times/wk, totaling 25 sessions, using voluntary maximal isometric, concentric, and eccentric right knee-extension actions (30 and 180 degrees/s). Measurements of muscle strength with a Kin-Com dynamometer and simultaneous electromyograms (EMG) were performed of both sides before and after the training period. Muscle biopsies were taken from the right vastus lateralis muscle. The total quadriceps cross-sectional area was measured with computerized tomography. Training led to an increase in maximal torque for concentric (10% at 30 degrees/s) and eccentric (13-19%) actions in the trained leg. The EMG activity increased at maximal eccentric activities. The total cross-sectional quadriceps area of the trained leg increased by 3%, but no changes were recorded in muscle fiber areas in these subjects, who already had large mean fiber areas (5.15 microns 2 x 10(3)). The fatigue index measured from 50 consecutive concentric contractions at 180 degrees/s decreased and the citrate synthase activity increased in all but one subject. The results demonstrate that increased neural activation accompanies an increase in muscle strength at least during eccentric action in already rather active elderly men and that muscle endurance may also be improved with training.     

The duration of the inhibitory effects with static stretching on quadriceps peak torque production

Although several studies have investigated the acute effect of static stretching exercises, the duration of exercises that negatively affects performance has not been ascertained. This study was conducted to determine the acute effect of different static stretching durations on quadriceps isometric and isokinetic peak torque production. The 50 participants were randomly allocated into five equivalent sized groups and were asked to perform a stretching exercise of different duration (no stretch, 10-second stretch, 20-second stretch, 30-second stretch, and 60-second stretch). The knee flexion range of motion and the isometric and concentric isokinetic peak torques of the quadriceps were measured before and after a static stretching exercise in the four experimental groups. The same parameters were examined in the control group (no stretch) without stretching, before and after a 5-minute passive rest. There were no significant differences among groups before the experimentation regarding their physical characteristics and performances (P > 0.05). These results reflect the different groups' homogeneity. Significant knee joint flexibility increases (P < 0.001) and significant isometric and isokinetic peak torque reductions (P < 0.05-0.001) have been shown to occur only after 30 and 60 seconds of quadriceps static stretching. Stretching reduced isometric peak torque by 8.5% and 16.0%, respectively. Concerning isokinetic peak torque after 30 and 60 seconds of stretching, it was reduced by 5.5% vs. 11.6% at 60 degrees/s and by 5.8% vs. 10.0% at 180 degrees/s. We suggest that torque decrements are related to changes of muscle neuromechanical properties. It is recommended that static stretching exercises of a muscle group for more than 30 seconds of duration be avoided before performances requiring maximal strength.     

The effects of normalization method on antagonistic activity patterns during eccentric and concentric isokinetic knee extension and flexion

The purpose of this study was to compare different normalization methods of electromyographic (EMG) activity of antagonists during isokinetic eccentric and concentric knee movements. Twelve women performed three maximum knee extensions and flexions isometrically and at isokinetic concentric and eccentric angular velocities of 30 °·s⁻¹, 90 °·s⁻¹, 120 °·s⁻¹ and 150 °·s⁻¹. The EMG activity of the vastus lateralis, rectus femoris, vastus medialis and hamstrings was recorded. The antagonist integrated IEMG values were normalized relative to the EMG of the same muscle during an isometric maximal action (static method). The values were also expressed as a percentage of the EMG activity of the same muscle, at the same angle, angular velocity and muscle action (dynamic method) when the muscle was acting as an agonist. Three-way analysis of variance (ANOVA) designs indicated significantly greater IEMG normalized with the dynamic method compared to the EMG derived using the static method (P < 0.05). These differences were more evident at concentric angular velocities and at the first and last 20 ° of the movement. The present findings demonstrate that the method of normalization significantly influences the conclusions on antagonistic activity during isokinetic maximum voluntary efforts. The dynamic method of normalization is more appropriate because it considers the effects of muscle action, muscle length and angular velocity on antagonist IEMG.