Use of a Kin-Com dynamometer to study the stretch-shortening cycle during plantar flexion

The aim of this study was to evaluate the Kin-Com II dynamometer in the study of the stretch-shortening cycle (a concentric muscle action preceded by an eccentric muscle action). Measurements were made of plantar flexion at different angular velocities (120 degrees.s-1 and 240 degrees.s-1) with the knee at two different angles (0 degree and 90 degrees). Ten healthy women ranging in age from 22 to 41 years were studied. Torque values were recorded simultaneously with surface electromyograms (EMG): maximal voluntary concentric torque values were recorded and, after a short rest, the torque values of the concentric action which followed immediately after an eccentric action of the same velocity, both with maximal effort. Mean values were taken at different ankle positions and also averaged over different ranges. A concentric action preceded by an eccentric action generated a torque value on an average about 100% larger than a concentric action alone. The EMG activity was lower or unchanged. It was concluded that the present method could be useful in the study of the stretch-shortening cycle in plantar flexion and in the testing of the behaviour of the elastic components in people with disabilities in the lower limbs.     

Internal work and physiological responses during concentric and eccentric cycle ergometry

Internal mechanical work during cycling, required to raise and lower the legs and change their velocities, is shown to be an important factor when interpreting physiological responses to cycle ergometer exercise. The internal work required to move the legs during concentric and eccentric cycle ergometry at different speeds and workloads was calculated from segmental energy changes determined using cinematography and directly using an eccentric ergometer. The mean internal work rates obtained at pedal frequencies of 30, 60 and 90 min-1 were 11.5, 20 and 62 W respectively. When these estimates were added to the external work rates, they increased concentric and decreased eccentric work rates. The largest differences were seen at low work rates and high pedal frequencies during which concentric work rates increased by 51% and eccentric decreased 60% by the inclusion of internal work. When comparisons of concentric and eccentric cycling at equal uncorrected work rates were made, neglecting to include internal work introduced errors ranging from 12 to 97%. The calculated estimates of internal work agreed well with the power supplied by the eccentric ergometer to move the legs passively. The investigations show that the inclusion of internal work is important when comparing physiological responses during concentric and eccentric ergometry, especially when pedal frequencies exceed 60 min-1 and when work rates are small.     

Myonuclear apoptosis occurs during early posthatch starvation

Apoptosis is a naturally occurring process; it is important for the final shape and size of developing tissues, and it is characterized by some morphological features such as plasma membrane blebbing, nuclear breakdown, chromosomal fragmentation and apoptotic bodies followed by phagocytosis. The objective of the study was to evaluate the occurrence of apoptosis in chickens immediately posthatch under fed and starved conditions. Male broiler chickens were or were not provided feed for the first 3 days posthatch. Chickens were killed immediately after hatch, at 1 day of age, at 2 days of age and at 3 days of age. The Pectoralis thoracicus was removed, fixed, dehydrated, cleared and embedded in paraffin. Muscle sections were labeled using terminal deoxynucleotidyl transferase (TdT)-mediated dUTP Nick-End Labeling (TUNEL) for detection of apoptotic nuclei. Body weights were lower (P<0.05) in the starved compared to the fed group at 2 and 3 days posthatch. Myofiber cross-sectional area was only smaller (P<0.05) in the starved compared to the fed birds at 3 days posthatch. TUNEL-positive nuclei were present at all days for the fed and starved groups. The proportion of TUNEL-positive nuclei was higher (P<0.05) for the starved group at day 2 and day 3 posthatch compared to the fed group at 3 days posthatch. Apoptosis is a mechanism that contributes to the smaller myofiber size observed at 3 days posthatch.     

Relationships between stretch-shortening cycle performance and maximum muscle strength

This study aimed to examine the relationships between muscle power output using the stretch-shortening cycle (SSC) and maximum strength, as measured by the 1 RM (1 repetition maximum) test and the isokinetic dynamometer under elbow flexion. Sixteen trained, young adult males pulled a constant load of 40% MVC (maximum voluntary elbow flexion contraction) by ballistic elbow flexion under the following two preliminary conditions: 1) the static relaxed muscle state (SR condition) and 2) using the SSC (SSC condition). Muscle power was determined from the product of the pulling velocity and load mass by a power measurement instrument with a rotary encoder. The 1 RM bench press (1RM BP) and isokinetic maximum strength under elbow flexion with the Cybex-325 were measured as indicators of dynamic maximum strength. 1) The early power output exerted under the SSC condition showed a significant and high correlation with the 1 RM BP (r = 0.83), but only moderate correlation with the isokinetic muscle strength (r = 0.50-0.67). 2) The contribution of the 1 RM BP to the early muscle contraction velocity exerted under the SSC condition was large. These results suggested that muscle power exerted using the SSC shows a stronger relationship with maximum muscle strength measured by a 1 RM test rather than isokinetic maximum strength.     

Age-related differences in the neural regulation of stretch-shortening cycle activities in male youths during maximal and sub-maximal hopping

The aim of the current study was to investigate potential age-related differences in neural regulation strategies during maximal and sub-maximal hopping. Thirty-two boys from three different age groups (9-, 12- and 15-years), completed trials of both maximal and sub maximal hopping, and based on contact and flight times, measures of reactive strength index (RSI = jump height/contact time) and leg stiffness (peak ground reaction force/peak displacement of centre of mass) were collected respectively. During all trials, surface electromyograms (EMG) were recorded from four different muscle sites of the dominant lower limb, during 100 ms pre-ground contact, and then four subsequent stretch reflex phases: background muscle activity (0-30 ms), short-latency stretch reflex (31-60 ms), intermediate15 latency stretch reflex 61-90 ms and long-latency stretch reflex (91-120 ms). Reactive strength index and leg stiffness were measured during the hopping trials. During maximal hopping, both 12- and 15-year olds produced significantly greater RSI (P < 0.02) than 9-year olds, with 15-year olds utilising significantly greater soleus muscle activity during the 100 ms prior to ground contact than the younger age groups (P < 0.01). During sub-maximal hopping, 15-year olds produced significantly greater absolute leg stiffness than both 12- and 9-year olds (P < 0.01), with 9-year olds producing significantly less soleus muscle activity during the 31-60 ms time phase. For all age groups, sub-maximal hopping was associated with significantly greater background muscle activity and short-latency stretch reflex activity in the soleus and vastus lateralis, when compared to maximal hopping (P < 0.001). Results suggest that as children mature, they become more reliant on supra-spinal feed forward input and short latency stretch reflexes to regulate greater levels of leg stiffness and RSI when hopping.     

Mechanomyography and electromyography force relationships during concentric, isometric and eccentric contractions.

The purpose of this study was to investigate systematically if complementary knowledge could be obtained from the recordings of electromyography (EMG) and mechanomyography (MMG) signals. EMG and MMG activities were recorded from the first dorsal interosseous muscle during slow concentric, isometric, and eccentric contraction at 0, 25, 50, 75 and 100% of the maximal voluntary contraction (MVC). The combination of the EMG and MMG recordings during voluntary concentric-isometric-eccentric contraction showed significant different non-linear EMG/force and MMG/force relationships (P<0.001). The EMG root mean square (rms) values increased significantly from 0 to 50% MVC during concentric and isometric contraction and up to 75% MVC during eccentric contraction (P<0.05). The MMG rms values increased significantly from 0 to 50% MVC during concentric contraction (P<0.05). The non-linear relationships depended mainly on the type and the level of contraction together with the angular velocity. Furthermore, the type of contraction, the contraction level, and the angular velocity influenced the electromechanical efficiency evaluated as the MMG to EMG ratio (P<0.05). These results highlight that EMG and MMG provide complementary information about the electrical and mechanical activity of the muscle. Different activation strategies seem to be used during graded isometric and anisometric contraction.     

Effects of different dropping intensities on fascicle and tendinous tissue behavior during stretch-shortening cycle exercise.

This study examined whether the elasticity of the tendinous tissues plays an important role in human locomotion by improving the power output and efficiency of skeletal muscle. Ten subjects performed one-leg drop jumps (DJ) from different dropping heights with a constant rebound height. The fascicle length of the vastus lateralis muscle was measured by using real-time ultrasonography during DJ. In the braking phase of the DJ, fascicle lengthening decreased and the tendinous tissue lengthening increased with increased dropping intensity. In the subsequent push-off phase, the shortening of tendinous tissues increased with higher dropping intensity. The averaged electromyographic activities of the preactivation and braking phases increased and those of the push-off phase decreased as the drop height was increased. With higher dropping height but constant submaximal rebound jump, the stretched tendinous tissue length increased with less stretched fascicle during the braking phase. In the subsequent push-off phase, the recoil of tendinous tissues became greater. These results suggest that the increased prestretch intensity has considerable influence on the process of storage and subsequent recoil of the elastic energy during the stretch-shortening cycle action.     

Force-velocity relationship and stretch-shortening cycle function in sprint and endurance athletes

This study examined the torque-velocity and power-velocity relationships of quadriceps muscle function, stretch shortening cycle function, and leg-spring stiffness in sprint and endurance athletes. Isokinetic maximal knee extension torque was obtained from 7 sprinters and 7 endurance athletes using a Con-trex isokinetic dynamometer. Torque and power measures were corrected for lean-thigh cross-sectional area and lean-thigh volume, respectively. Stretch-shortening cycle function and muscle stiffness measurements were obtained while subjects performed single-legged squat, countermovement, and drop-rebound jumps on an inclined sledge and force-plate apparatus. The results indicated that sprinters generated, on average, 0.15 +/- 0.05 N.m.cm(-2) more torque across all velocities compared with endurance athletes. Significant differences were also found in the power-velocity relationships between the 2 groups. The sprinters performed significantly better than the endurance athletes on all jumps, but there were no differences in prestretch augmentation between the groups. The average vertical leg stiffness during drop jumps was significantly higher for sprinters (5.86 N.m(-1)) compared with endurance runners (3.38 N.m(-1)). The findings reinforce the need for power training to be carried out at fast contraction speeds but also show that SSC function remains important in endurance running.     

Relationships between muscle power output using the stretch-shortening cycle and eccentric maximum strength

This study aimed to examine the relationships between muscle power output using the stretch-shortening cycle (SSC) and eccentric maximum strength under elbow flexion. Eighteen young adult males pulled up a constant light load (2 kg) by ballistic elbow flexion under the following two preliminary conditions: 1) the static relaxed muscle state (SR condition), and 2) using the SSC with countermovement (SSC condition).Muscle power was determined from the product of the pulling velocity and the load mass by a power measurement instrument that adopted the weight-loading method. We assumed the pulling velocity to be the subject's muscle power parameters as a matter of convenience, because we used a constant load. The following two parameters were selected in reference to a previous study: 1) peak velocity (m x s(-1)) (peak power) and 2) 0.1-second velocity during concentric contraction (m x s(-1)) (initial power). Eccentric maximum strength by elbow flexion was measured by a handheld dynamometer.Initial power produced in the SSC condition was significantly larger than that in the SR condition. Eccentric maximum strength showed a significant and high correlation (r = 0.70) with peak power in the SSC condition but not in the SR condition. Eccentric maximum strength showed insignificant correlations with initial power in both conditions. In conclusion, it was suggested that eccentric maximum strength is associated with peak power in the SSC condition, but the contribution of the eccentric maximum strength to the SSC potentiation (initial power) may be low.     

Processing of vimentin occurs during the early stages of adenovirus infection.

Evidence is presented that a fraction of vimentin, a component of cytoskeleton recently found to be associated with intracytoplasmic, migrating adenovirus type 2 (Ad2), is processed into smaller polypeptides at early times after infection. The extent of vimentin cleavage appears to depend upon both the multiplicity of infection and the adenovirus serotype. Ad2, Ad5, Ad4, and Ad9 induced similar vimentin cleavage in infected cells, whereas Ad3, Ad7, and Ad12, for which most infecting particles are found sequestered within phagosomes, induced very little, if any, vimentin breakdown. This suggests that vimentin processing is in some way related to the number of virus particles migrating through the cytoplasm. Experiments performed in vitro and in vivo with adenovirus temperature-sensitive mutants H2 ts1 and H2 ts112 and UV-inactivated wild-type Ad2 indicated that vimentin processing is due to a nonvirion, cytoskeleton-associated, proteolytic enzyme activated by adenovirus and sharing characteristics with the protease described by Nelson and Traub (W.J. Nelson and P. Traub, J. Cell Sci. 57:25-49, 1982). The activity of this protease appears to be required for productive infection by adenovirus serotypes 2 and 5 (subgroup C), 4 (subgroup E), and 9 (subgroup D) but not by the oncogenic serotypes 3 and 7 (subgroup B) and 12 (subgroup A).     

Effects of muscle damage on stretch-shortening cycle function and muscle stiffness control

This experiment examined the effect of eccentric contraction-induced muscle damage on the stretch-shortening cycle and vertical leg spring stiffness during jumping activities. Ten moderately active male and female adult volunteers participated in this study (aged 23 +/- 2.3 years). Temporary muscle damage to the knee extensors was administered by a bout of eccentric contractions on an isokinetic dynamometer. Measurements were obtained of maximum voluntary force and of take-off velocities for single-leg countermovement jumps (CMJs), squat jumps (SJs), and drop jumps (DJs), performed on a specially constructed sledge and force plate apparatus. These measurements were obtained before and after the damage intervention, and the undamaged leg was used as a control. The results indicated that eccentric muscle damage significantly affected stretch-shortening cycle performance by causing relatively greater reductions in SJ performance than CMJ or DJ. The muscle damage intervention also significantly increased leg-spring stiffness, which indicates that the changes in leg stiffness may be an important adaptation resulting from eccentric exercise.     

Effect of 15 days of betaine ingestion on concentric and eccentric force outputs during isokinetic exercise

The purpose of this study was to examine the efficacy of 15 days of betaine supplementation on peak concentric (CON) and eccentric (ECC) force during isokinetic exercise in active college-aged men. Eleven men volunteered for this study (21.7 ± 5.1 years; height: 178.5 ± 6.4 cm; body mass: 79.8 ± 10.3 kg). Subjects were randomly assigned to either a supplement (BET) or placebo (PL) group. Supplementation occurred for 15 days. Subjects reported to the Human Performance Laboratory on 5 occasions during this period, separated by 72 hours, for a testing and exercise session on an isokinetic chest press device. After each exercise protocol, subjects rated their fatigue and muscle soreness on a 15-cm visual analog scale. Subjects then consumed no daily BET for 4 weeks but maintained familiarity with the exercise device once per week. After the washout period, subjects resumed the BET protocol using the opposite drink and repeated the same 15-day protocol. No differences were noted in maximum CON force output between pre (335.9 ± 78.3 and 321.6 ± 63.6 N) and post (330.3 ± 74.8 and 330.2 ± 71.6 N) workouts in both BET and PL, respectively. In addition, no differences were noted in maximum ECC force output between pre (352.0 ± 90.6 and 324.4 ± 85.2 N) and post (353.2 ± 98.2 and 366.9 ± 128.5 N) workouts in BET and PL, respectively. No differences in subjective measures of soreness and fatigue were seen, but a significant reduction in Δ fatigue was observed in BET compared to PL. In conclusion, 15 days of betaine supplementation did not increase peak CON or ECC force outputs during an isokinetic chest press but did appear to reduce subjective measures of fatigue to the exercise protocol.     

Acoustic myography reflects force changes during dynamic concentric and eccentric contractions of the human biceps brachii muscle.

The relationship between acoustic myography (AMG), electromyography (EMG) and force during submaximal dynamic contractions was examined in the biceps brachii muscles of eight healthy males (aged 17-26 years). Different weights were lifted and lowered at a constant speed, using a wall pulley system, to perform concentric and eccentric contractions, respectively. Integrated AMG (iAMG) and integrated EMG (iEMG) activity both increased linearly with force during concentric (iAMG r = 0.94; iEMG r = 0.99) and eccentric (iAMG r = 0.90; iEMG r = 0.94) contractions. The slopes of the concentric regression lines were significantly different from the eccentric slopes (P less than 0.01) for both iAMG and iEMG with concentric contractions showing greater levels of activity. The results indicated that AMG can be used to detect changes in force during dynamic contractions which has important implications for the use of AMG in rehabilitation. The differences in iAMG activity between concentric and eccentric contractions are discussed in relationship to the origin of the AMG signal.     

Task-irrelevant learning occurs only when the irrelevant feature is weak

The role of attention in perceptual learning has been controversial. Numerous studies have reported that learning does not occur on stimulus features that are irrelevant to a subject's task [1,2] and have concluded that focused attention on a feature is necessary for a feature to be learned. In contrast, another line of studies has shown that perceptual learning occurs even on task-irrelevant features that are subthreshold, and concluded that attention on a feature is not required to learn that feature [3-5]. Here we attempt to reconcile these divergent findings by systematically exploring the relation between signal strength of the motion stimuli used during training and the resultant magnitude of perceptual learning. Our results show that performance improvements only occurred for the motion-stimuli trained at low, parathreshold, coherence levels. The results are in accord with the hypothesis that weak task-irrelevant signals fail to be 'noticed', and consequently to be suppressed, by the attention system and thus are learned, while stronger stimulus signals are detected, and suppressed [6], and are not learned. These results provide a parsimonious explanation of why task-irrelevant learning is found in some studies but not others, and could give an important clue to resolving a long-standing controversy.     

Motor unit activation patterns during isometric, concentric and eccentric actions at different force levels.

Motor unit activation patterns were studied during four different force levels of concentric and eccentric actions. Eight male subjects performed concentric and eccentric forearm flexions with the movement range from 100 degrees to 60 degrees in concentric and from 100 degrees to 140 degrees elbow angle in eccentric actions. The movements were started either from zero preactivation or with isometric preactivation of the force levels of 20, 40, 60 and 80% MVC. The subjects were then instructed to maintain the corresponding relative force levels during the dynamic actions. Intramuscular and surface EMG was recorded from biceps brachii muscle. Altogether 28 motoneuron pools were analyzed using the intramuscular spike-amplitude frequency (ISAF) analysis technique of Moritani et al. The mean spike amplitude was lower and the mean spike frequency higher in the isometric preactivation phase than in the consequent concentric and eccentric actions. When the movements started with isometric preactivation the mean spike amplitude increased significantly (P<0.001) up to 80% in isometric and concentric actions but in eccentric actions the increase continued only up to 60% (P<0.01). The mean spike frequency in isometric preactivation and in concentric action with preactivation was lower only at the 20% force level (P<0.01) as compared to the other force levels while in eccentric action with preactivation the increase between the force levels was significant (P<0.01) up to 60%. When the movement was started without preactivation the mean spike amplitude at 20% and at 40% force level was higher (P<0.01) in eccentric action than in concentric actions. It was concluded that the recruitment threshold may be lower in dynamic as compared to isometric actions. The recruitment of fast motor units may continue to higher force levels in isometric and in concentric as in eccentric actions which, on the other hand, seems to achieve the higher forces by increasing the firing rate of the active units. At the lower force levels mean spike amplitude was higher in eccentric than in concentric actions which might indicate selective activation of fast motor units. This was, however, the case only when the movements were started without isometric preactivation.     

Follicular development during early pregnancy and the estrous cycle of the sow

The objective of this study was to monitor and compare follicle populations and follicular development in pregnant and nonpregnant sows from Day 3 to Day 20 after breeding. Twenty-four sows were paired within parity on the day of artificial insemination and were randomly allocated within pair for insemination with either killed (n=12) or live spermatozoa (n=12). All the sows were artificially inseminated with the pooled ejaculate of the same boar. From Day 3 through Day 20 post estrus, ovarian follicles were scanned daily by ultrasonography. Ultrasound images were recorded on videotape and were retrospectively analyzed. Follicles were mapped to indentify the existence of follicular waves. The follicles were then classified as small (< 3 mm), medium (3-5 mm), or large (>/=5 mm). Pregnancy diagnosis was performed on Day 21 by ultrasonography. Pregnant sows maintained a constant proportion of the follicle population in the small, medium and large follicle categories. However, in the nonpregnant sows, the proportion of follicles in the various size categories remained constant until Day 15. Thereafter, the proportion of small follicles decreased (P < 0.05) from Day 15 to 20, and the proportions of medium and large follicles increased (P < 0.05). The predictability of pregnancy status on Day 20 based on follicle populations in any of the 3 follicle categories was low. Moreover, there was no evidence of follicular waves during the estrous cycle or early pregnancy. In conclusion, the proportion of small follicles decreased while medium and large follicle increased from Day 15 through Day 20 of the estrous cycle, but not during a similar stage of pregnancy. This latter finding concurs with follicle recruitment from the pool of small follicles for ovulation following PGF2alpha secretion to induce luteolysis, which reduces progesterone concentrations and thereby allows for the stimulation of the pool of small follicles by gonadotropins.     

The history dependence of force production in mammalian skeletal muscle following stretch-shortening and shortening-stretch cycles

The purpose of this study was to determine the history dependence of force production during and following stretch-shortening and shortening-stretch cycles in mammalian skeletal muscle. Thirty-three different isometric, stretch, shortening, stretch-shortening and shortening-stretch experiments were preformed in cat soleus (n=8) using previously established methods. Stretch-shortening and shortening-stretch cycles are not commutative with respect to the isometric forces following the length changes. Whereas force depression following shortening is virtually unaffected by previous stretching of the muscle, force enhancement following stretch depends in a dose-dependent manner on the amount of muscle shortening preceding the stretch. The history dependence of isometric force following shortening-stretch cycles can conveniently be modelled using an elastic (compressive and tensile) element that engages at the length of muscle activation. Such an "elastic" mechanism has been proposed by Edman and Tsuchiya (1996) (Edman, K.A. P., Tsuchiya, T., 1996. Strain of passive elements during force enhancement by stretch in frog mucle fibres. Journal of Physiology 490. 1, 191-205) based on experimental observations, and has been implemented theoretically in a rheological model of muscle (Forcinito et al., 1997) (Forcinito, M., Epstein, M., Herzog, W., 1997. Theoretical considerations on myofibril stiffness. Biophysics Journal 72, 1278-1286). The history dependence of isometric force following stretch-shortening cycles appears independent of the stretch preceding the shortening, except perhaps, if stretching occurs at very high speeds (i.e. 6-10 times fibre length per second). The results of this study are hard to reconcile with the two major mechanisms associated with history dependence of force production: sarcomere length non-uniformity (Edman et al., 1993) and stress-induced cross-bridge inhibition (Maréchal and Plaghki, 1979) (Maréchal, G., Plaghki, L., 1979. The deficit of the isometric tetanic tension redeveloped after a relase of frog muscle at a constant velocity. Journal of General Physiology 73, 453-467). It appears that studying the history dependence of force production under more functionally relevant conditions than has been done to date may provide new information that contributes to our understanding of possible mechanisms associated with force depression and force enhancement following muscular length changes.     

Proteolytic degradation of the nuclear isoform of uracil-DNA glycosylase occurs during the S phase of the cell cycle

Uracil-DNA glycosylases are enzymes that remove uracil from DNA and initiate base-excision repair. These enzymes play a key role in maintaining genomic integrity by reducing the mutagenic events caused by G:C to A:T transition mutations. The recent finding that a family of RNA editing enzymes (APOBECs) can deaminate cytosine in DNA has raised the interest in these base-excision repair enzymes. This research focuses on the regulation of the nuclear isoform of uracil-DNA glycosylase, a 36000 Da protein that contains a unique 44 amino acid N-terminus. In synchronized HeLa cells, UDG1A protein levels decrease to barely detectable levels during the S phase of the cell cycle. Immunoblot analysis of immunoprecipitated or affinity-isolated UDG1A reveals ubiquitin-conjugated UDG1A when proteolysis is inhibited using N-acetyl-leu-leu-norleu-al or MG132, inhibitors of proteosomal dependent protein degradation. Transient transfection experiments, with histidine-tagged ubiquitin, were used to confirm that endogenous UDG1A is ubiquitinated in vivo. Addition of the nuclear export inhibitor, leptomycin B, prevents ubiquitination and degradation of UDG1A. This indicates that translocation from the nucleus may be a step in UDG1A turnover. Finally, UDG1A protein degradation is prevented when cells are incubated with the cyclin-dependent kinase inhibitor, roscovitine. These results suggest that protein phosphorylation and/or nuclear export participate in the post-translational regulation of UDG1A protein levels.