Interaction of fibre type, potentiation and fatigue in human knee extensor muscles.

AIM: To examine the effect of fibre type on potentiation and fatigue. METHODS: Young men (n = 4 per group) with a predominance of type I [61.4 +/- 6.9% (SD), group I (GI)] or type II [71.8 +/- 9.2%, group II (GII)] fibres in vastus lateralis, performed a fatigue protocol of sixteen 5-s maximal voluntary isometric contractions (MVCs) of the right knee extensors. Maximal twitches and corresponding muscle action potentials (M-waves) were evoked before the first MVC, during the 3-s rest period after each MVC and at intervals during the 5-min recovery period after the last MVC. RESULTS: Group II [49.3 +/- 2.6% (SE)] had a greater decrease in MVC force than GI (22.8 +/- 6.2%) during the fatigue protocol. Group II (126.4 +/- 13.6%) showed greater twitch force potentiation early in the fatigue protocol than GI (38.2 +/- 2.3%), but greater depression at the end (33.7 +/- 13.7% vs.17.4 +/- 3.4%). Twitch time-to-peak torque (TPT) and half relaxation time (HRT) initially decreased but then increased as the fatigue protocol progressed; GII had a greater increase in HRT. During a 5-min recovery period twitch force increased above the prefatigue level and remained so until the end of the recovery period; the pattern was similar in GI and GII. Twitch TPT and HRT remained elevated during recovery. M-wave area increased throughout the fatigue protocol and the first part of recovery before returning to baseline values in GII, whereas there were no significant changes in GI. The interaction between potentiation and fatigue was amplified in GII early in the fatigue protocol with concurrently greater twitch and M-wave potentiation, and greater MVC force decrease and HRT increase. Late in the protocol, GII had a greater decrease in twitch and MVC force combined with greater M-wave potentiation. CONCLUSION: It is concluded that fibre type distribution influences potentiation and fatigue of the twitch, and potentiation of the M-wave during fatiguing exercise.

Effect of temperature on post-tetanic potentiation in human dorsiflexor muscles.

The effect of temperature on post-tetanic potentiation (PTP) has been examined in the muscles of small mammals but not in human skeletal muscle. We examined PTP in the ankle dorsiflexor muscles of 10 young men by evoking twitches before and after a 7-second tetanus at 100 Hz in a control (room air approximately 21 degrees C) condition and after immersion of the lower leg in warm (45 degrees C) and cold (10 degrees C) water baths for 30 min. Exposure to cold decreased tetanus and pre-tetanus twitch peak torque, but increased rise time, half-relaxation time, and muscle action potential (M-wave) amplitude; exposure to warm water had little effect. PTP was smallest in cold exposure 5 s post-tetanus, but persisted throughout the 12 min test period, whereas PTP had subsided by 6 min post-tetanus in control and warm exposures. M-wave amplitude initially decreased after exposure to warm water, recovered, then decreased again by 11 min post-tetanus. In contrast, exposure to cold had no initial effect but did increase the M-wave amplitude during the last half of the 12 min test period, similar to that seen in the control. The greatest immediate decrease in rise time and half-relaxation time was observed in the control; however, by 12 min post-tetanus warm exposure showed the greatest increase in rise time and half-relaxation time above pre-tetanus values. The decrease in the unpotentiated twitch torque with cooling in human dorsiflexors is typical for muscles with a predominance of type I (slow) fibres. The effect of cold on PTP is similar to that seen previously in mammalian muscles with a predominance of type II (fast) fibres, although the underlying mechanism of the cooling effect appears to differ.

The effects of acute passive stretch on muscle protein synthesis in humans.

We examined the effect of an isolated bout of maximal tolerated passive stretch on fractional muscle protein synthetic rate in human soleus muscle. Eight healthy males performed two separate trials with the same leg: one session of passive stretch and one of intermittent active isometric contraction at a force equivalent to that which occurred during the passive stretch trial. This force was approximately 40% of maximum voluntary contraction force and produced volitional fatigue in approximately 27 min. Intermittent passive stretch, for the same duration, elicited a 6.1 degrees increase in joint angle (P<.0005) with silent electromyography. Fractional protein synthetic rate from experimental and control soleus in each trial was assessed from biopsy samples over the period 10-22 hr postexercise by the incorporation rate of L-[1-13C] leucine into muscle. Protein synthesis was elevated in the soleus of the exercised leg following the active contraction trial by 49% (P<.05) but not following the passive stretch trial. Results indicate that a single bout of maximal passive stretch does not significantly elevate fractional muscle protein synthetic rate in humans and thus suggests that muscle stretch per se is not the stimulus for the muscle hypertrophy that occurs with resistance training.

Reduced strength after passive stretch of the human plantarflexors.

The purpose of this study was to assess strength performance after an acute bout of maximally tolerable passive stretch (PS(max)) in human subjects. Ten young adults (6 men and 4 women) underwent 30 min of cyclical PS(max) (13 stretches of 135 s each over 33 min) and a similar control period (Con) of no stretch of the ankle plantarflexors. Measures of isometric strength (maximal voluntary contraction), with twitch interpolation and electromyography, and twitch characteristics were assessed before (Pre), immediately after (Post), and at 5, 15, 30, 45, and 60 min after PS(max) or Con. Compared with Pre, maximal voluntary contraction was decreased at Post (28%) and at 5 (21%), 15 (13%), 30 (12%), 45 (10%), and 60 (9%) min after PS(max) (P < 0.05). Motor unit activation and electromyogram were significantly depressed after PS(max) but had recovered by 15 min. An additional testing trial confirmed that the torque-joint angle relation may have been temporarily altered, but at Post only. These data indicate that prolonged stretching of a single muscle decreases voluntary strength for up to 1 h after the stretch as a result of impaired activation and contractile force in the early phase of deficit and by impaired contractile force throughout the entire period of deficit.

Postactivation potentiation, fiber type, and twitch contraction time in human knee extensor muscles.

In small mammals, muscles with shorter twitch contraction times and a predominance of fast-twitch, type II fibers exhibit greater posttetanic twitch force potentiation than muscles with longer twitch contraction times and a predominance of slow-twitch, type I fibers. In humans, the correlation between potentiation and fiber-type distribution has not been found consistently. In the present study, postactivation potentiation (PAP) was induced in the knee extensors of 20 young men by a 10-s maximum voluntary isometric contraction (MVC). Maximal twitch contractions of the knee extensors were evoked before and after the MVC. A negative correlation (r = -0. 73, P < 0.001) was found between PAP and pre-MVC twitch time to peak torque (TPT). The four men with the highest (HPAP, 104 +/- 11%) and lowest (LPAP, 43 +/- 7%) PAP values (P < 0.0001) underwent needle biopsies of vastus lateralis. HPAP had a greater percentage of type II fibers (72 +/- 9 vs. 39 +/- 7%, P < 0.001) and shorter pre-MVC twitch TPT (61 +/- 12 vs. 86 +/- 7 ms, P < 0.05) than LPAP. These data indicate that, similar to the muscles of small mammals, human muscles with shorter twitch contraction times and a higher percentage of type II fibers exhibit greater PAP.

Postactivation potentiation in endurance-trained male athletes.

PURPOSE: The purpose of the study was to determine whether postactivation potentiation (PAP) was enhanced in the trained muscles of male endurance athletes. METHODS: Triathletes (TRI), distance runners (RUN), active controls (AC), and sedentary control subjects (SED) (N = 10 per group) performed 10-s maximal isometric contractions (MVC) of the elbow extensor and ankle plantarflexor muscles. Maximal twitch contractions were evoked (percutaneous stimulation) before and during a 5-min period after the MVC. PAP was measured as the percentage change in peak twitch torque post-MVC. RESULTS: TRI, who train both upper and lower limb muscles, had enhanced (relative to SED) PAP in both elbow extensor and plantarflexor muscles. In RUN, who train only the lower limbs, enhanced PAP was restricted to the plantarflexors. AC, whose main activity was upper and lower limb weight training, also had enhanced PAP in both muscle groups, although the enhancement in the plantarflexors was not as great as in TRI and RUN. CONCLUSION: PAP is enhanced in endurance athletes. Enhanced PAP may counteract fatigue during endurance exercise. The mechanism(s) responsible for the enhanced PAP remain to be determined.

Effect of postactivation potentiation on dynamic knee extension performance.

Six men and four women performed, in separate trials, maximal dynamic knee extensions with loads of 15%, 30%, 45% and 60% of maximal isometric knee extension peak torque (MVC). The dynamic extensions were done after postactivation potentiation (PAP) had been induced with a 10-s MVC, and in a control trial without PAP. PAP, measured as the increase in evoked twitch torque, was 53 (4)% (SE) and 43 (3)% at the time of the first and second extensions with each load. PAP failed to increase the attained peak velocity with any load; on the contrary, there was a trend for peak velocity to decrease in the first extension, which occurred approximately equal to 15 s after the 10-s MVC. The results suggest that fatigue produced by the 10-s MVC suppressed any benefit that could be derived from the induced PAP. A surface electromyogram (EMG) recorded from one muscle of quadriceps femoris gave no indication of activation failure in the first knee extension; however, activation impairment specific to the rate of force development cannot be ruled out. It is concluded that the strategy employed, namely of having knee extensions performed soon after the 10-s MVC to maximize PAP at the time of performance, was unsuccessful because there had been insufficient time for recovery from fatigue. It is possible that a longer recovery time, even at the cost of a diminished PAP, may have proved beneficial.

Muscle substrate utilization and lactate production.

Biopsies (biceps) were examined in 8 bodybuilders across a typical arm-curl training session (80% 1-RM). [PCr] and [glycogen] decreased 62 and 12% after 1 set (n = 4), and 50 and 24% after 3 sets (n = 4). [Lactate] was 91 and 118 mmol × kg-1, respectively, after 1 and 3 sets. Fatigue was probably partially caused by decreased [PCr] and increased [H+] (first set) and by decreased [H+] in subsequent sets.

Bilateral deficit in plantar flexion: relation to knee joint position, muscle activation, and reflex excitability.

Six male subjects made maximal isometric plantar flexions unilaterally (UL) and bilaterally (BL), with the knee joint angle positioned at 90 degrees and 0 degrees (full extension) and the ankle joint kept at 90 degrees. Plantar flexion torque and electromyogram (EMG) of the lateral gastrocnemius (LG) and the soleus (Sol) muscles were recorded. There was a deficit in torque in BL compared to UL (P<0.05), and the deficit was greater when the knee was extended than when bent to 90 degrees (13.9% vs 6.6%). The integrated EMG (iEMG) of UL and BL did not differ when the knee was at 90 degrees. On the other hand, when the knee was extended iEMG of LG was smaller for BL than for UL, suggesting that the larger bilateral deficit when the knee was extended was due to a reduced activity of the LG motor units. In addition, the H-reflex recorded from Sol when the contralateral leg was performing a maximal unilateral plantarflexion was reduced. This would indicate that the force deficit was associated with a reduction of motoneuron excitability.

A comparison of strength and muscle mass increases during resistance training in young women.

Strength gains with resistance training are due to muscle hypertrophy and nervous system adaptations. The contribution of either factor may be related to the complexity of the exercise task used during training. The purpose of this investigation was to measure the degree to which muscle hypertrophy contributes to gains in strength during exercises of varying complexity. Nineteen young women resistance trained twice a week for 20 weeks, performing exercises designed to provide whole-body training. The lean mass of the trunk, legs and arms was measured by dual energy x-ray absorptiometry and compared to strength gains (measured as the 1-repetition maximum) in bench press, leg press and arm curl exercises, pre-, mid- (10 weeks) and post-training. No changes were found in a control group of ten women. For the exercise group, increases in bench press, leg press and arm curl strength were significant from pre- to mid-, and from mid- to post-training (P < 0.05). In contrast, increases in the lean mass of the body segments used in these exercises followed a different pattern. Increases in the lean mass of the arms were significant from pre- to mid-training, while increases in the lean mass of the trunk and legs were delayed and significant from mid- to post-training only (P < 0.05). It is concluded that a more prolonged neural adaptation related to the more complex bench and leg press movements may have delayed hypertrophy in the trunk and legs. With the simpler arm curl exercise, early gains in strength were accompanied by muscle hypertrophy and, presumably, a faster neural adaptation.

Influence of gender on post-tetanic potentiation in human dorsiflexors.

Twitch contractions of the ankle dorsiflexors were evoked before and after 7 s of tetanic stimulation at 100 Hz in young women and men. Torque decreased more in men (18%) than in women (12%) during the tetanus. There was no gender difference in twitch peak torque potentiation over the 5-min post-tetanus. Potentiation was 42% (women) and 45% (men) at 5 s post-tetanus, and still present at 5 min (women 24%, men 25%). The immediate (5 s) shortening of twitch rise time was similar in women (14%) and men (13%), but during the 5-min men's rise time came to exceed whereas women's only approached pretetanus values (e.g., +9% vs. -1% at 5 min). The immediate decrease in half-relaxation time was also similar in women (24%) and men (22%); however, women's but not men's values remained less than pretetanus values for most of the 5-min period. Twitch rate of torque development increased similarly (75%) in women and men at 5 s, with no gender difference over 5 min. In contrast, rate of torque relaxation increased significantly only in men. Rate of torque development normalized to peak torque was similar in women and men pretetanus and increased similarly 5 s post-tetanus, but women had greater values through most of the 5-min post-tetanus. Normalized rate of torque relaxation was similar in women and men and not affected by tetanus. In the dorsiflexor muscles, young women and men show a similar amount and pattern of twitch force potentiation, but there are gender differences in time-related twitch contractile properties in the first 5 min after tetanus.

Ballistic movement performance in karate athletes.

Nine male karate athletes and 13 untrained men did maximal voluntary isometric (MVC) and ballistic elbow extension actions, the latter unloaded (L0) and against a load equal to 10% MVC (L10). The karate group achieved greater (P < 0.05) isometric (32%) and ballistic action peak torque with L0 (30%) and L10 (40%). With L10 the ratio of ballistic action to isometric action, peak torque was 13% greater in the karate group, indicating a load specific training adaptation. With L0 the corresponding ratio did not differ significantly between groups. Ballistic action peak rate of torque development (51%, 51%) and peak acceleration (15%, 9%) with L0 and L10, respectively, were greater in the karate group. In contrast, peak velocity and movement time did not differ significantly between groups. Electromyographic recordings of agonist triceps and antagonist biceps were made during the isometric and ballistic actions. Since ballistic actions (L10) were initiated from a preloaded condition, the occurrence and duration of premovement agonist depression were monitored. In ballistic actions there were no group differences in agonist activation, the ratio of ballistic to isometric action agonist activation, or antagonist coactivation. Premovement agonist depression occurred infrequently in both groups, with no group differences. It is concluded that karate athletes have enhanced elbow extension ballistic performance, but it could not be related to amplified agonist activation, altered antagonist activation, or more frequent occurrence of agonist premovement depression.

Reproducibility of ballistic movement.

The reproducibility of ballistic actions performed on a custom-made apparatus was assessed in six untrained men who reported to the laboratory two times over a span of 10 d, on each occasion performing three isometric maximal voluntary contractions (MVC, elbow extension) and 10 ballistic elbow extension movements in each of two conditions: unloaded (L0) and with a load equal to 10% MVC (L10). For both the average of the trials and the best trial, method errors and Pearson correlation coefficients (r) were calculated for isometric peak torque and 10 (5 at each of L0 and L10) selected ballistic movement parameters (peak torque, peak rate of torque development, movement time, peak velocity, and peak acceleration). Based on method errors obtained and given an alpha level of 0.05 and power of 0.9, needed sample sizes were calculated for treatment effects ranging from 5 to 20%. Method errors for isometric peak torque were 5.1 and 3.1% for best and average of trials, respectively. For the 10 ballistic parameters, the corresponding method errors averaged 7.1 +/- 5.4% (SD) and 6.8 +/- 3.6%. The largest method errors were found in peak torque and peak rate of torque development with L0. The expected high negative correlation between method errors and r values was not found. For 5, 10, and 20% treatment effects, 4, 6, and 9 of 10 best trial measures met the sample size criterion of N < or = 20. For the average of trials, 3, 8, and all 10 met the criterion. With the exception of rate of torque development, the reproducibility of the described ballistic measures is acceptable for longitudinal training studies and cross-sectional group comparisons.

Posttetanic potentiation of human dorsiflexors.

Twitch contractions of the ankle dorsiflexors were evoked before and after applied 7-s tetanic stimulation at 100 Hz in 20 young adults. Torque decreased 15% during the tetanus. At 5 s after tetanus, twitch peak torque had potentiated 45%. Potentiation declined to 28% after 1 min, rose slightly to 33% at 2 min, and declined slowly with potentiation still 25% after 5 min. There was large intersubject variation in the amount of potentiation (5-140%) and its persistence (5 to >/=20 min). The muscle compound action potential (M wave) did not change significantly (from pretetanic value) at 5 s after tetanus but increased sharply (26%) at 2 min and then subsided. Twitch half relaxation time (23%) decreased significantly more than twitch rise time (13%) 5 s after tetanus and recovered more slowly. Twitch rates of torque development (75%) and relaxation (71%) increased similarly 5 s after tetanus and were still elevated (approximately 25%) at 5 min. The extent of twitch torque potentiation was significantly inversely correlated with pretetanic twitch rise time (r = -0.69), half relaxation time (r = -0.61), and twitch-to-tetanus ratio (r = -0.66). The data indicate that posttetanic potentiation has a greater effect on twitch half relaxation time than on time to peak torque and is more prominent in muscles with a short twitch time course and small twitch-to-tetanus ratio.

Influence of velocity on agonist and antagonist activation in concentric dorsiflexion muscle actions.

Five women and 5 men performed maximal isometric and concentric dorsiflexion actions on an isokinetic dynamometer. The concentric actions were done at 10 present velocities ranging from 0.26 to 5.23 rad.s-1. Electromyographic (EMG) recordings were made from one agonist (tibialis anterior (TAI) and two antagonists (soleus [S], lateral gastrocnemius [LG]). The men produced greater absolute torque than the women, but there was no gender difference in the torque/body mass ratio. The shape of the torque-velocity relation was similar in men and women and approximated, but did not match, that obtained in animal preparations or in human studies using electrical stimulation. Agonist TA activation (integrated EMG/movement time) decreased with increasing velocity over the same range of velocities as torque. Antagonist S, but not LG, activation also decreased. The S/TA activation ratio was greater in men than women. In men the S/TA ratio tended to be greater than the LG/TA ratio, whereas the converse was true for women. These data indicate that velocity influences the relative activation of two antagonists in maximal dorsiflexion muscle actions, and that there is an apparent gender difference in the relative activation of two antagonists.

Twenty weeks of weight training increases lean tissue mass but not bone mineral mass or density in healthy, active young women.

Twenty young women (20.3 +/- 1.0 years) participated in a weight training program in which upper and lower body exercises were done twice per week for 20 weeks. Ten other women (20.2 +/- 0.4 years) served as a control group. Training resulted in significant (p < 0.05) increases in arm curl (73%), bench press (33%), and leg press (23%) lifting performance. Whole body (3.7%), trunk (3.0%), arm (9.7%), and leg (3.3%) lean tissue mass also increased significantly, based on measurements made by dual energy x-ray absorptiometry (DEXA). Changes in the control group were small and nonsignificant. In contrast, training did not increase DEXA-measured bone mineral content (BMC) and density (BMD) in a whole body measure nor in arm, leg, ribs, thoracic and lumbar spine, and pelvis segments. Similarly, hip BMC and BMD at femoral neck, trochanter, intertrochanter, and Ward's triangle sites, and total hip did not increase with training. The data indicate that a resistance training program that effectively increases strength and lean tissue mass in young women may fail to increase BMC or BMD over a 20-week training period.

A 12-year follow-up study of ankle muscle function in older adults.

The purpose of this study was to examine changes in strength over time in a cohort of healthy elderly people who underwent assessments of ankle muscle function 12 years earlier. The isometric strength and contractile characteristics of the dorsiflexors and plantarflexors were studied in 11 male and 11 female subjects, ranging from 73-97 yrs (mean age 84 +/- 7.1 yrs). The same footplate apparatus was used as during the original testing. From 1982 to 1994, plantarflexor strength decreased 2.1% per year in females, and 2.5% per year in males (p < .01). The loss was relatively less in the dorsiflexor muscles; strength decreased 0.3% per year in females, and 0.8% per year in male (p > .05). There were no significant changes in evoked twitch torque in either muscle group, which may be due to the fact that passive tension of the connective tissue increased (p < .01) over the 12-year period. We conclude from this longitudinal assessment of ankle muscle function that the rate of loss of voluntary strength can vary considerably between antagonistic muscle groups. Factors influencing this variable loss warrant further investigation.

Comparative effectiveness of accommodating and weight resistance training modes.

Six men and six women trained the elbow flexors of both arms 3 d.wk-1 for 20 wk. In each training session, one arm did 3-5 sets of 10 maximal concentric actions on an accommodating resistance device (ARD), the other arm 3-5 sets of 8-12 coupled eccentric/concentric actions (repetitions) to volitional failure (8-12 RM) on a weight resistance device (WRD). The average "intensity" (force of concentric actions) was approximately 1.25 times greater in ARD training, the average "volume" (number of actions x force of actions) 1.6 times greater in WRD training, and the time required to complete a training session the same for each. Both types of training produced significant increases in a single maximum weight lift (1 RM on the WRD), in the peak force of a single maximal concentric action measured on the ARD and an isovelocity dynamometer, and in biceps, brachialis, and total elbow flexor cross-sectional area (CSA). Biceps Type I and II fiber area did not change significantly. WRD training produced greater increases than ARD training in the 1 RM test on the WRD and in brachialis CSA. The data indicate that both of these common training regimens effectively increase strength and muscle mass, but the weight training regimen may be more effective for increasing muscle mass.

Response to resistance training in young women and men.

Six women and 6 men trained the elbow flexors 3 days per week for 20 wks, one arm performing in each session 3-5 sets of 10 maximal concentric actions on an accommodating resistance device, the other arm 3-5 sets of 8-12 coupled eccentric/concentric actions on a weight training device. With results collapsed across the two training modes, the women made significantly (p < 0.05) greater relative increases than men in strength measured on the weight (116 vs. 46%) and accommodating (99 vs. 46%) resistance devices, and greater absolute (3.5 vs. -1.3 N.m) and relative (13.7 vs. -3.2%) increases in strength measured on an isokinetic dynamometer. Absolute (cm2) and relative (%) biceps, brachialis, and total elbow flexor cross-sectional area (from CT scans) increased significantly; however, the women's vs. men's respective relative and absolute increases did not differ significantly: biceps (13 vs. 7%, 0.9 vs. 1.0 cm2), brachialis (53 vs. 31%, 2.1 vs. 2.3 cm2), and total (26 vs. 15%, 3.1 vs. 3.3 cm2) flexor area. Biceps type I and II fiber area, and the II/I area ratio did not increase significantly. The data indicate that in response to the same short-term training program, muscle size increases similarly in women and men but women make greater relative increases in strength.

Exercise and bone mineral density.

A decrease in physical activity may lead to an increased loss of bone and an increase in the incidence of osteoporotic fractures. Studies have demonstrated increases in bone formation in animals and increases in bone mineral density in humans. Studies of animals show that bone has enhanced physical and mechanical properties following periods of increased stress. Strains which are high in rate and magnitude, and of abnormal distribution, but not necessarily long in duration, are best for inducing new bone formation, resulting in the strengthening of bone by increased density. Cross-sectional studies show that athletes, especially those who are strength-trained, have greater bone mineral densities than nonathletes, and that strength, muscle mass and maximal oxygen uptake correlate with bone density. Longitudinal training studies indicate that strength training and high impact endurance training increase bone density. Strain induction, the deformation that occurs in bone under loading, may cause a greater level of formation and an inhibition of resorption within the normal remodelling cycle of bone, or it may cause direct activation of osteoblastic bone formation from the quiescent state. Various mechanisms have been proposed for the transformation of mechanical strain into biochemical stimuli to enhance bone formation. These include prostaglandin release, piezoelectric and streaming potentials, increased bone blood flow, microdamage and hormonally mediated mechanisms. These mechanisms may act on their own or in concert, depending on the loading situation and the characteristics of the bone.