Hz Force Research Articles

A Biomechanical Analysis of the Squat Between Competitive Collegiate and High School Powerlifting Teams

PURPOSE: To determine the effect of skill level on performance in powerlifting, a biomechanical analysis of squatting techniques between college (Co) and high school (Hs) powerlifting teams was conducted. METHODS: Powerlifters (N = 12) volunteered for the study and were required to have ranked in the top 5 in powerlifting meets in 2005. Kinematic 2D data of trials (3) at 80% of their 1-RM were recorded using a 60 Hz digital camera and a 2000 Hz force platform was used to sample vertical ground reaction forces (VGRF). RESULTS: Variables analyzed included VGRFs, knee angular displacement, velocity, and accelerations. A statistical analysis using Paired Samples t-test (p<0.05) was performed. Differences were found among Hs and Co groups for the mean peak knee angle (92.16 ± 7.79 and 78.77 ± 4.29(°)), angular acceleration (−932.6 ± 108.4 and −564.3 ± 148.1 (°/s2)), angular velocity (164.3 ± 10.47 and 105.9 ± 21.72 (°/s)) and time-to-peak angular velocity (2.47 ± .31 and 2.14 ± .37 (s)) (p<0.05). CONCLUSIONS: Co and Hs powerlifting teams varied on pattern of performance during the ascent phase with Hs having less efficient patterns of performance. A lower skill level may result in less power and lower overall weight lifted during competition.

Effects of lengthening contraction on calcium kinetics and skeletal muscle contractility in humans

We have tested the hypothesis that the altered muscle contractility after lengthening contractions (LC) is caused by altered calcium (Ca2+) kinetics. Subjects (n = 8) performed 100 drop jumps and muscle contractility was measured pre- and post-exercise by maximal voluntary contraction (MVC) and transcutaneous electrical stimulation (1, 20 and 50 Hz). Muscle biopsies were analysed for muscle metabolites, rates of SR Ca(2+) uptake (CaU) and release (CaR) and myosin heavy chain (MHC) composition. The rates of torque relaxation and CaU were positively related to muscle fibre type composition (% MHC II). Muscle creatine (Cr) decreased and the ratio between phosphocreatine (PCr) and Cr increased 3 and 24 h post-exercise (P < 0.05 vs. pre-exercise). LC resulted in reduced MVC (-19%), twitch torque (-41%) and 20/50 Hz torque ratio (-30%) and a faster relaxation rate (P < 0.05). The contractile parameters recovered partially but remained altered 24 h post-exercise (P < 0.05). The average CaR was unchanged after LC (P > 0.05). However, the response varied between subjects and the relative post-exercise CaR was significantly related to the degree of LFF (post/pre 20/50 Hz force ratio) and to the decline in twitch force (post/pre twitch ratio). CaU was lower in seven of eight subjects after LC (P > 0.05). The decline in torque after LC could not be explained by metabolic factors since PCr/Cr ratio increased. The relation between CaR and fatigue suggests that the mechanism of fatigue in part may be attributed to intrinsic changes in the SR Ca2+ release channel. The faster torque relaxation after LC could not be explained by an increased rate of CaU.

Changes in force, surface and motor unit EMG during post-exercise development of low frequency fatigue in vastus lateralis muscle

We investigated the effects of low frequency fatigue (LFF) on post-exercise changes in rectified surface EMG (rsEMG) and single motor unit EMG (smuEMG) in vastus lateralis muscle (n = 9). On two experimental days the knee extensors were fatigued with a 60-s-isometric contraction (exercise) at 50% maximal force capacity (MFC). On the first day post-exercise (15 s, 3, 9, 15, 21 and 27 min) rsEMG and electrically-induced (surface stimulation) forces were investigated. SmuEMG was obtained on day two. During short ramp and hold (5 s) contractions at 50% MFC, motor unit discharges of the same units were followed over time. Post-exercise MFC and tetanic force (100 Hz stimulation) recovered to about 90% of the pre-exercise values, but recovery with 20 Hz stimulation was less complete: the 20-100 Hz force ratio (mean +/- SD) decreased from 0.65+/-0.06 (pre-exercise) to 0.56+/-0.04 at 27 min post-exercise (P<0.05), indicative of LFF. At 50% MFC, pre-exercise rsEMG (% pre-exercise maximum) and motor unit discharge rate were 51.1 +/- 12.7% and 14.1 +/- 3.7 (pulses per second; pps) respectively, 15 s post-exercise the respective values were 61.4 +/- 15.4% (P<0.05) and 13.2 +/- 5.6 pps (P>0.05). Thereafter, rsEMG (at 50% MFC) remained stable but motor unit discharge rate significantly increased to 17.7 +/- 3.9 pps 27 min post-exercise. The recruitment threshold decreased (P<0.05) from 27.7 +/- 6.6% MFC before exercise to 25.2 +/- 6.7% 27 min post-exercise. The increase in discharge rate was significantly greater than could be expected from the decrease in recruitment threshold. Thus, post-exercise LFF was compensated by increased motor unit discharge rates which could only partly be accounted for by the small decrease in motor unit recruitment threshold.

Athletics

The aim of this study was to establish the functions of the support leg in the long jump take‐off with a three‐element mechanical model spring, damper, and actuator. The take‐off motions of eleven male long jumpers, with personal bests from 6.45 to 7.99 m, were videotaped at 250 Hz and ground reaction forces were simultaneously recorded at 1 kHz. A two‐dimensional 14‐segment linked model was used to collect basic kinematic parameters. The spring, damper and actuator forces were determined from the displacement and velocity of the centre of mass and from ground reaction forces. Large spring and damper forces were exerted, and absorbed the impact force immediately after the touch‐down. The spring force was also exerted from 25 to 75% of the take‐off phase. The actuator force was dominant in the latter two‐thirds of the take‐off phase. Statistically significant correlations were found between the spring force impulse and the knee flexion during the take‐off phase (r = 0.699, p < 0.05), and between the knee flexion and the angular velocity of the thigh at the touch‐down (r = 0.726, p < 0.05). These results indicated that the jumper should retain less flexion of the take‐off leg knee to increase the spring force, after a fast extension of the hip, and use a more extended knee at the touchdown to prevent excessive knee flexion.

Low-frequency fatigue, post-tetanic potentiation and their interaction at different muscle lengths following eccentric exercise

Low-frequency fatigue (LFF) and post-tetanic potentiation (PTP) were quantified at different muscle lengths in rat medial gastrocnemius (GM) muscle. In situ experiments were performed on GM muscle-tendon complexes of anaesthetised (urethane, 1.5 g kg(-1) i.p.) Wistar rats (N=8). Force-length characteristics were determined at maximal (200 Hz) and submaximal (60 Hz) stimulation. Data for submaximally stimulated muscle were obtained in a non-potentiated and in a potentiated condition. LFF was induced by a series of 40 eccentric contractions. Post-exercise (40-80 min), data for the force-length relationships were obtained once more. Whereas force loss at 200 Hz-stimulation was least at optimum muscle length, L(0,200 Hz), (17.0+/-1.4%, mean +/-S.E.M.), force loss at 60 Hz-stimulation was maximal near L(0,200 Hz) (55.1+/-4.3% at L(0,200 Hz)-1 mm). When the muscle was potentiated, force loss at 60 Hz-stimulation was maximal at short muscle length: L(0,200 Hz)-4 mm (53.5+/-3.8%). The extent of LFF, quantified by a decrease in the 60:200 Hz force ratio, varied with muscle length: LFF increased with decreasing muscle lengths when muscles were potentiated. However, in the non-potentiated condition, LFF was maximal at a length just below L(0,200 Hz); the 60:200 Hz force ratio had decreased to 54.6+/-5.9% of the pre-exercise ratio at L(0,200 Hz)-1 mm. Compared with the non-potentiated condition, LFF was less pronounced in the potentiated condition. PTP counteracted LFF particularly at long muscle lengths. However, at short muscle lengths, LFF was still observed in potentiated muscles.

Effect of low cytoplasmic [ATP] on excitation-contraction coupling in fast-twitch muscle fibres of the rat.

In this study we investigated the roles of cytoplasmic ATP as both an energy source and a regulatory molecule in various steps of the excitation-contraction (E-C) coupling process in fast-twitch skeletal muscle fibres of the rat. Using mechanically skinned fibres with functional E-C coupling, it was possible to independently alter cytoplasmic [ATP] and free [Mg2+]. Electrical field stimulation was used to elicit action potentials (APs) within the sealed transverse tubular (T-) system, producing either twitch or tetanic (50 Hz) force responses. Measurements were also made of the amount of Ca2+ released by an AP in different cytoplasmic conditions. The rate of force development and relaxation of the contractile apparatus was measured using rapid step changes in [Ca2+]. Twitch force decreased substantially (approximately 30%) at 2 mm ATP compared to the level at 8 mm ATP, whereas peak tetanic force only declined by approximately 10% at 0.5 mm ATP. The rate of force development of the twitch and tetanus was slowed only slightly at [ATP] > or = 0.5 mm, but was slowed greatly (> 6-fold) at 0.1 mm ATP, the latter being due primarily to slowing of force development by the contractile apparatus. AP-induced Ca2+ release was decreased by approximately 10 and 20% at 1 and 0.5 mm ATP, respectively, and by approximately 40% by raising the [Mg2+] to 3 mm. Adenosine inhibited Ca2+ release and twitch responses in a manner consistent with its action as a competitive weak agonist for the ATP regulatory site on the ryanodine receptor (RyR). These findings show that (a) ATP is a limiting factor for normal voltage-sensor activation of the RyRs, and (b) large reductions in cytoplasmic [ATP], and concomitant elevation of [Mg2+], substantially inhibit E-C coupling and possibly contribute to muscle fatigue in fast-twitch fibres in some circumstances.

Changes of the force–frequency relationship in human tibialis anterior at fatigue

This work estimates the influence of the single twitch (ST) parameters changes on specific regions of the force–frequency relationship (FFR) in fatigued human tibialis anterior (TA). In 20 subjects (age 20–40) the TA underwent three stimulation phases: (a) five STs at 1 Hz followed by 5 s stimulation with increasing rate (1–50 Hz, to obtain FFR); (b) fatiguing stimulation (35 Hz for 40 s); (c) same as in “a”. By the average STs (mean of the five responses) of a and c phases, the peak twitch (Pt) was calculated. Moreover, after ST normalization to Pt, the maximum contraction rate (MCR) and the maximum relaxation rate (MRR) were computed. By the FFR, normalized to the 50 Hz force, we first defined the threshold frequency (TF) when the force oscillation presented the same value in (a) and (c), and then the areas below the FFR in the 1 Hz–TF and in the TF–50 Hz ranges. Results: In unfatigued and fatigued muscle Pt, and MRR changed from 6.12±3.08 to 3.27±1.16 N and from 0.87±0.13 to 0.65±0.09% Pt/ms, respectively. MCR did not change significantly. The 1 Hz-TF area ratio (c/a) was >1 for muscles having fatigued Pt>60% of its basal value. The TF–50 Hz area ratio (c/a) was mostly below 1. Conclusions: At fatigue, MRR reduction, leading to a better fusion of muscle mechanical output, is able to compensate, in the 1 Hz–TF frequency range, up to 40% Pt loss; beyond TF, the changes of FFR are related to the degree of force loss indicated by the fatigued Pt.

Low-frequency fatigue is fibre type related and most pronounced after eccentric activity in rat medial gastrocnemius muscle.

Effects of fibre type composition and type of contraction on low-frequency fatigue (LFF) were investigated in isolated rat medial gastrocnemius (GM) muscle. Fast oxidative or fast glycolytic GM muscle parts of anaesthetised male Wistar rats (n=18) were activated selectively by maximal electrical stimulation of the nerve after selective cutting of sub-branches. LFF was induced by a series of 40 isometric, concentric or eccentric contractions. Post exercise (55 min), the force-frequency curves differed significantly from the pre-exercise curves. Decreased forces were exerted mainly at the lower frequencies. This effect was significantly greater for glycolytic than oxidative muscle parts and following eccentric compared to isometric and concentric exercise. Seventy minutes following eccentric exercise, the relative values of the 60:200 Hz force ratios for the oxidative compared to the glycolytic parts were 65.6+/-2.2% and 43.6+/-4.6% (mean+/-SE) of the pre-fatigue values (=100%), respectively. In conclusion, for conditions of identical activation, eccentric exercise led to significantly more LFF than isometric and concentric exercise. In addition, and independent of the exercise type, fast glycolytic muscle parts were more susceptible to LFF than fast oxidative muscle parts.

Fatigue of Paralyzed and Control Thenar Muscles Induced by Variable or Constant Frequency Stimulation

Muscles paralyzed by chronic (>1 yr) spinal cord injury fatigue readily. Our aim was to evaluate whether the fatigability of paralyzed thenar muscles (n = 10) could be reduced by the repeated delivery of variable versus constant frequency pulse trains. Fatigue was induced in four ways. Intermittent supramaximal median nerve stimulation (300-ms-duration trains) was delivered at 1) constant high frequency (13 pulses at 40 Hz each second for 2 min); 2) variable high frequency (each second for 2 min). The first two intervals of each variable frequency train were 5 and 20 ms. The remaining pulses were evenly distributed in time across 275 ms. The number of pulses varied for each subject such that the force time integral in the unfatigued state matched that evoked by a constant 40-Hz train; 3) constant low frequency (7 pulses at 20 Hz each second for 4 min); and 4) variable low frequency (each second for 4 min). The pulse pattern was the same as that for variable high frequency except that the force-time integral was matched to that produced by the constant low-frequency stimulation. These same experiments were performed on the thenar muscles of five able-bodied control subjects. The variable high-frequency trains used to fatigue paralyzed and control muscles had an average (+/- SE) of 12 +/- 2 and 10 +/- 1 pulses, respectively. Variable low-frequency trains had 7 +/- 1 and 6 +/- 1 pulses, respectively. Significant mean force declines of comparable magnitude (to 20-25% initial fatigue force or to 13-21% initial 50 Hz force) were seen in paralyzed muscles with all four stimulation protocols. The force reductions in paralyzed muscles were always accompanied by significant increases in half-relaxation time and decreases in force-time integral, irrespective of the stimulation protocol. Significant force decreases also occurred in control muscles during each fatigue test. Again, these force declines were similar whether constant or variable pulse patterns were used at high or low frequencies (to 40-60% initial fatigue force or to 29-36% initial 50 Hz force). The force reductions in control muscles were significantly less than those seen in paralyzed muscles, except when constant high-frequency stimulation was used. The variations in stimulation frequency, pulse pattern, and pulse number used in this study therefore had little influence on thenar muscle fatigue in control subjects or in spinal cord-injured subjects with chronic paralysis.

A rigid body model of the dynamic posteroanterior motion response of the human lumbar spine

Background: Clinicians apply posteroanterior (PA) forces to the spine for both mobility assessment and certain spinal mobilization and manipulation treatments. Commonly applied forces include low-frequency sinusoidal oscillations (<2 Hz) as used in mobilization, single haversine thrusts (<0.5 seconds) as imparted in high-velocity, low-amplitude (HVLA) manipulation, or very rapid impulsive thrusts (<5 ms) such as those delivered in mechanical-force, manually-assisted (MFMA) manipulation. Little is known about the mechanics of these procedures. Reliable methods are sought to obtain an adequate understanding of the force-induced displacement response of the lumbar spine to PA forces. Objective: The objective of this study was to investigate the kinematic response of the lumbar spine to static and dynamic PA forces. Design: A 2-dimensional modal analysis was performed to predict the dynamic motion response of the lumbar spine. Methods: A 5-degree-of-freedom, lumped equivalent model was developed to predict the PA motion of the lumbar spine. Lumbar vertebrae were modeled as masses, massless-spring, and dampers, and the resulting equations of motion were solved by using a modal analysis approach. The sensitivity of the model to variations in the spring stiffness and damping coefficients was examined, and the model validity was determined by comparing the results to oscillatory and impulsive force measurements of vertebral motion associated with spine mobilization and 2 forms of spinal manipulation. Results: Model predictions, based on a damping ratio of 0.15 (moderate damping) and PA spring stiffness coefficient ranging from 25 to 60 kN/m, showed good agreement with in vivo human studies. Quasi-static and low-frequency (<2.0 Hz) forces at L3 produced L3 segmental and L3-L4 intersegmental displacements up to 8.1 mm and 3.0 mm, respectively. PA oscillatory motions were over 2.5-fold greater for oscillatory forces applied at the natural frequency. Impulsive forces produced much lower segmental displacements in comparison to static and oscillatory forces. Differences in intersegmental displacements resulting from impulsive, static, and oscillatory forces were much less remarkable. The latter suggests that intersegmental motions produced by spinal manipulation may play a prominent role in eliciting therapeutic responses. Conclusions: The simple analytical model presented in this study can be used to predict the static, cyclic, and impulsive force PA displacement response of the lumbar spine. The model provides data on lumbar segmental and intersegmental motion patterns that are otherwise difficult to obtain experimentally. Modeling of the PA motion response of the lumbar spine to PA forces assists in the understanding the biomechanics of therapeutic PA forces applied to the lumbar spine and may ultimately be used to validate chiropractic technique procedures and minimize risk to patients receiving spinal manipulative therapy. (J Manipulative Physiol Ther 2002;25:485-96)

Human neuromuscular fatigue is associated with altered Na+-K+-ATPase activity following isometric exercise.

The purpose of this study was to investigate the hypothesis that reductions in Na+-K+- ATPase activity are associated with neuromuscular fatigue following isometric exercise. In control (Con) and exercised (Ex) legs, force and electromyogram were measured in 14 volunteers [age, 23.4 +/- 0.7 (SE) yr] before and immediately after (PST0), 1 h after (PST1), and 4 h after (PST4) isometric, single-leg extension exercise at ~60% of maximal voluntary contraction for 30 min using a 0.5 duty cycle (5-s contraction, 5-s rest). Tissue was obtained from vastus lateralis muscle before exercise in Con and after exercise in both the Con (PST0) and Ex legs (PST0, PST1, PST4), for the measurements of Na+-K+-ATPase activity, as determined by the 3-O-methylfluorescein phosphatase (3-O-MFPase) assay. Voluntary (maximal voluntary contraction) and elicited (10, 20, 50, 100 Hz) force was reduced 30-55% (P < 0.05) at PST0 and did not recover by PST4. Muscle action potential (M-wave) amplitude and area (measured in the vastus medialis) and 3-O-MFPase activity at PST0-Ex were less than that at PST0-Con (P < 0.05) by 37, 25, and 38%, respectively. M-wave area at PST1-Ex was also less than that at PST1-Con (P < 0.05). Changes in 3-O-MFPase activity correlated to changes in M-wave area across all time points (r = 0.38, P < 0.05, n = 45). These results demonstrate that Na+-K+- ATPase activity is reduced by sustained isometric exercise in humans from that in a matched Con leg and that this reduction in Na+-K+-ATPase activity is associated with loss of excitability as indicated by M-wave alterations.

Role of intracellular calcium in fatigue in single skeletal muscle fibers isolated from the rat

The aim of the present study was to demonstrate the role of intracellular calcium ([Ca 2+] i ) in the performance of fatigued muscle fibers isolated from the skeletal muscle of the rat. We measured developed tension of a single myocyte during short tetanic electrical stimulation of various intensities along with [Ca 2+] i dynamics by fura-2. The performance of individual muscle fiber was assessed by developed tension during 100 Hz tetanic stimulation (‘100 Hz force’). We regarded the muscle fiber fatigued when, after repeated tetanic stimulations, the developed tension declined to 50% of the initial level. When fatigue was induced by maximal stimulation (100 Hz tetani), the 100 Hz force measured immediately following completion of fatigue was considerably decreased (48% of control). This change in the muscle performance was associated with significant increase in the resting [Ca 2+] i (280% of control) and decrease in Ca 2+ transient (54% of control). The 50% relaxation time after cessation of tetanic stimulation (RT 50) was also prolonged. In contrast, when fatigue was induced by low frequency electrical stimulation (30 Hz tetani), neither the 100 Hz force, RT 50, nor Ca 2+ transient in fatigue were significantly different from the controls, while the resting [Ca 2+] i increased only slightly. These findings suggest a tight relationship between [Ca 2+] i and the performance of fatigued single isolated skeletal muscles. Also, the results show that performance of the fatigued muscle fiber may in part depend on the protocol used to produce muscle fatigue.

Electrical stimulation of human tibialis anterior: (A) contractile properties are stable over a range of submaximal voltages; (B) high- and low-frequency fatigue are inducible and reliably assessable at submaximal voltages.

To investigate the validity and reliability of submaximal voltage stimulation for assessing the 'fresh' contractile properties of human tibialis anterior muscle (TA) and the efficacy of such stimulation in inducing and assessing high- and low-frequency fatigue. (A) Contractile properties of fresh TA were assessed in six normal volunteers using multifrequency stimulation trains (comprising 2 seconds at each of 10, 20 and 50 Hz, arranged contiguously) over a range of submaximal voltages. (B) On three separate occasions, fatigue was induced in the TA of 10 normal volunteers by means of a 3-minute unbroken sequence of the described multifrequency stimulation trains, delivered at a 'standardized' submaximal voltage. This fatiguing protocol was preceded by discrete multifrequency stimulation trains, at the same standardized voltage, but followed by discrete multifrequency trains delivered over a range of submaximal voltages (which included the standardized voltage). In experiment A the 10:50 Hz and 20:50 Hz force ratios were analysed for between-voltages variability using coefficients of variation (CVs), and for trends using Friedman tests and post-hoc Wilcoxon tests. In experiment B low-frequency fatigue was detected using 10:50 Hz and 20:50 Hz force ratios derived from the discrete multifrequency trains. High-frequency fatigue was calculated from the decline in high-frequency force which occurred during the fatiguing protocol itself. Each parameter was assessed for between-days repeatability using CVs. In experiment A the 'fresh' 10:50 Hz force ratio was clearly unreliable at voltages which generated <10% of maximal voluntary contractile force (MVC) (CV< or =29.7%), but was reasonably reliable at voltages which generated 20-30% of MVC (CV < or = 11.5%; p = 0.847). The 'fresh' 20:50 Hz force ratio was,in contrast, extremely reliable throughout the tested voltage range (CV< or =5.8%; p = 0.636) in fresh muscle. In experiment B paired t-tests indicated that the fatiguing protocol induced significant high-frequency fatigue (p <0.0037) and low-frequency fatigue (p <0.0008 for 'fresh' versus 'fatigued' 10:50 Hz force ratio; p <0.0001 for 'fresh' versus 'fatigued' 20:50 Hz force ratio). In muscle thus fatigued, the 20:50 Hz force ratio was extremely reliable in the 20-33% of MVC range (CV < or =7.3%; p = 0.847). Between-days repeatability was poor for the 10:50 Hz force ratio in both fresh and fatigued muscle (CV < or =23.8 and 44.4% respectively), but was highly acceptable for both voluntary and stimulated fatigue indices and for the 20:50 Hz force ratio, the latter in both fresh and fatigued muscle. These results confirm the validity and reliability of submaximal voltages in assessing contractile properties (including low-frequency fatiguability) and inducing fatigue of human TA.

Myocardial length-force relationship in end stage dilated cardiomyopathy and normal human myocardium: analysis of intact and skinned left ventricular trabeculae obtained during 11 heart transplantations.

The Frank-Starling-mechanism (FSM) was analyzed in isolated intact and skinned human left ventricular myocardium obtained from 11 heart transplantations (normal donor hearts (NDH), n = 8; dilated cardiomyopathy (DCM), n = 11). The new technique to utilize muscle strips from normal donor hearts which were actually implanted is described in detail. I) In electrically stimulated left ventricular trabeculae (37 degrees C, oxygenated Krebs-Henseleit solution, supramaximal electrical stimulation, frequency 1 Hz) force development was analyzed as a function of muscle length (NDH = 8; DCM = 11). II) In an additional series left ventricular myocardium was demembranized ("skinned") by Triton-X-100. At different sarcomere lengths and calcium concentrations corresponding to pCa values of 4.3, 5.5, and 8.0 force development was measured (DCM = 11; NDH = 9). I) Developed force increased up to an optimum as a function of muscle length in intact NDH- and DCM-myocardium. However, the relative increment of developed force after any length step was smaller in DCM than in NDH. Near "Lmax" (muscle length associated with maximum developed force) passive resting tension was considerably elevated in DCM, indicating significantly increased diastolic stiffness. II) In skinned left ventricular DCM- and NDH-myocardium developed force depended on sarcomere length with an optimum near 2.2 microns. However, a reduction of activator calcium concentration from pCa 4.3 to pCa 5.5 produces a smaller percent decline in force at short sarcomere lengths in DCM than it does in NDH. The present study shows that except for diastolic stiffness and a smaller relative force increment after any length step in DCM the Frank Starling mechanism is still present in isolated human left ventricular DCM- as in NDH-myocardium. The current study does not allow to decide whether in skinned myocardium the smaller percent decline in force after reduction of activator calcium concentrations in DCM is caused by an increased calcium sensitivity at short sarcomere lengths or decreased sensitivity at long sarcomere lengths.

Effects of reduced muscle glycogen concentration on force, Ca2+ release and contractile protein function in intact mouse skeletal muscle.

1. The purpose of this study was to examine the effects of reduced glycogen concentration on force, Ca2+ release and myofibrillar protein function during fatigue in skeletal muscle. Force and intracellular free Ca2+ concentration ([Ca2+]i) were measured in single mammalian skeletal muscle fibres during fatigue and recovery. Glycogen was measured in bundles of 20-40 fibres from the same muscle under the same conditions. 2. Fatigue was induced by repeated maximum tetani until force was reduced to 30% of initial. This was associated with a reduction in muscle glycogen to 27 +/- 6% of control values. In fibres allowed to recover for 60 min in the presence of 5.5 mM glucose (n = 6), tetanic (100 Hz) force recovered fully but tetanic [Ca2+]i remained at 82 +/- 8% of initial values. This prolonged depression in Ca2+ release was not associated with decreased muscle glycogen since glycogen had recovered to pre-fatigue levels (157 +/- 42%). 3. To examine the responses under conditions of reduced muscle glycogen concentration, fibres recovered from fatigue for 60 min in the absence of glucose (n = 6). After glucose-free recovery, the decreases in tetanic force and [Ca2+]i were only partially reversed (to 64 +/- 8% and 57 +/- 7% of initial values, respectively). These alterations were associated with a sustained reduction in muscle glycogen concentration (27 +/- 4% of initial values). 4. In another set of fibres, fatigue was followed by 50 Hz intermittent stimulation for 22.6 +/- 4 min. With this protocol, tetanic force and [Ca2+]i partially recovered to 76 +/- 9% and 55 +/- 6% of initial levels, respectively. These changes were associated with a recovery of muscle glycogen (to 85 +/- 10%). 5. During fatigue, Ca2+ sensitivity and maximum Ca(2+)-activated force (Fmax) were depressed but these alterations were fully reversed when muscle glycogen recovered. When glycogen did not recover, Ca2+ sensitivity remained depressed but Fmax partially recovered. The altered myofibrillar protein function is probably due to alterations in inorganic phosphate levels or other metabolites associated with reduced levels of muscle glycogen. 6. These data indicate that the reductions in force, Ca2+ release and contractile protein inhibition observed during fatigue are closely associated with reduced muscle glycogen concentration. These findings also suggest that the changes in Ca2+ release associated with fatigue and recovery have two components-one which is glycogen dependent and another which is independent of glycogen but depends on previous activity.

Changes in human skeletal muscle contractile function following stimulated eccentric exercise.

Indices of human skeletal muscle contractile function were examined in nine subjects for up to 9 days following a single bout of stimulated eccentric exercise. Eccentric muscle actions of the knee extensor muscles were evoked by percutaneous electrical myostimulation (PES). Delayed onset muscle soreness (DOMS), elevated serum creatine kinase activity, chronic force loss, and a decline in the 20:100 Hz force ratio were observed in the days postexercise. The exercised knee extensor muscles demonstrated an impaired ability to respond to PES. This was evident by an increased time delay between the start of 100 Hz PES and the onset of contraction immediately postexercise [22.3 (SD 15.9)%, P < 0.01] and 3 days postexercise [14.9 (SD 18.1)%, P < 0.05]. Muscle relaxation rates appeared unaffected by the eccentric exercise protocol, where the muscles showed no differences in the time between the end of PES and the onset of relaxation (P > 0.05). During the days following the exercise, no significant differences were observed in the time between the start of contraction and attainment of 70% of the mean tetanic force following a single 1-s pulse of PES. Similarly, no significant differences were observed in the time between the start of relaxation and attainment of 70% of the total relaxation during the same time. The increased delay in excitation-contraction coupling observed immediately postexercise and 3 days after the exercise, may reflect a damage-induced delay in action potential propagation. Muscle relaxation rates postexercise remained unchanged, which would seem to indicate normal functioning of the sarcoplasmic reticulum, suggesting this was not the site of failure in excitation-contraction coupling.

Contractile activation and measurements of intracellular Ca2+ concentration in cane toad twitch fibres in the presence of 2,3-butanedione monoxime.

The effects of 2,3-butanedione monoxime (BDM, 0.5-20 mM) on Ca(2+)-activated force in skinned muscle fibres, and force and Ca2+ responses in aequorin-injected intact fibres from the iliofibularis muscle of the cane toad Bufo marinus were investigated. Peak twitch force responses progressively decreased to 3% of the control response with increasing [BDM] up to 20 mM. Peak twitch aequorin light responses decreased to 65% of the control response in 10 mM BDM, but increased again to control values in 20 mM BDM. The duration of the twitch aequorin light response increased by up to 60% above 5 mM BDM. Tetanic (170 Hz) force and aequorin light responses reversibly decreased in a dose-dependent fashion to about 50% of the control response in 10 mM BDM. Failure of tetanic (170 Hz) stimulation was observed in the presence of 20 mM BDM. Intracellular [Ca2+] could be modified by changing the frequency of tetanic stimulation in the presence of BDM, permitting a study of the dependence of isometric force on intracellular [Ca2+] at different concentrations of BDM. In 10 mM BDM, the rate of force development in intact fibres was slower by a factor of two at saturating [Ca2+], and was up to one order of magnitude slower at non-saturating [Ca2+], when compared with control responses. At a similar intracellular [Ca2+] steady-state isometric force was reduced to about 85 and 50% of the control responses in 2 and 10 mM BDM, respectively. The effect of BDM on maximum Ca2+-activated force in skinned fibres paralleled the decrease in tetanic (170 Hz) force observed in intact fibres. The rate of force development in skinned fibres decreased with an increase in [BDM] at constant [Ca2+], and the sensitivity of the contractile apparatus to Ca2+ was shifted to a higher [Ca2+] by BDM. The results suggest that BDM reduces contractility in cane toad iliofibularis muscle by direct inhibition of the contractile apparatus, and reduction of the release of activator Ca2+ from the sarcoplasmic reticulum. Furthermore, BDM may be a useful tool to help study the relationship between force and [Ca2+] in intact muscle fibres.

Detection and severity of low frequency fatigue in the human adductor pollicis muscle

The sensitivity of electrical stimulation tests for detecting low frequency fatigue (LFF) and its duration were examined in the adductor pollicis muscle of ten normal subjects. Supramaximal stimulation was applied to the ulnar nerve at the wrist and values for test stimulation force ratios of 10:100 Hz, 15:100 Hz and 20:50 Hz (2 sec at each frequency with 5 min rest between each ratio) were obtained for fresh muscle. Fatigue was then induced by voluntary isometric contractions at 50% maximal voluntary force (MVC) repeated until only 30% MVC could be achieved. Contractions lasted 10 sec with 5 sec rest between each. The three test ratios were then repeated to monitor recovery at intervals up to 72 h after activity. High frequency forces returned to fresh values by 24 h but low frequency forces were all still significantly reduced. Forces at 10 and 15 Hz were still significantly reduced at 48 and 72 h (10 Hz > fatigue than 15 Hz). The low/high frequency ratios, calculated once 50 and 100 Hz forces had recovered, also demonstrated differences in revoery rates. Repeatability tests indicated that 10 Hz force was more variable than other frequencies and forces at all stimulation frequencies were repeatable on different days with a coefficient of variation of < 15%. Values for the 15:100 Hz ratio from fresh muscle in 22 normal subjects were 0.48±8. The 15:100 Hz ratio is suggested as the most appropriate test ratio for detecting LFF since 15 Hz force is more sensitive than 20 Hz and more stable than 10 Hz.

Force-frequency relationships of human thenar motor units

1. Force-frequency relationships were examined in 30 human thenar single motor units. The technique of intraneural stimulation was used to stimulate the motor axon in the median nerve proximal to the elbow with a tungsten microelectrode. 2. The stimulation consisted of either single shocks or trains of pulses (1 or 2 s duration) at constant rates varying between 5 and 100 Hz. To control various mechanical artifacts, the stimuli were delivered after electronically resetting the force baseline, and the stimuli were phased to the pulse pressure wave. Thumb flexion and abduction force components were recorded and the magnitude and direction of the resultant force calculated. Electromyographic responses (EMG) were recorded from both the proximal and distal thenar muscle surfaces. 3. For all units, twitch force began to fuse between 5 and 8 Hz and maximum tetanic force was achieved between 30 and 100 Hz. Half-maximum tetanic force was produced at 12 +/- 4 (SD) Hz, as assessed by interpolation using the nearly linear force-frequency relationship between 8 and 30 Hz (on logarithmic frequency coordinates). 4. For the majority of units (n = 19), the strongest force changes in response to variations in stimulation frequency occurred between 5 and 10 Hz (sensitivity 6 +/- 1 mN/Hz). Fewer units showed highest force-frequency sensitivity between 8 and 15 Hz (n = 7; 4 +/- 3 mN/Hz) or 10 and 20 Hz (n = 4; 5 +/- 2 mN/Hz).(ABSTRACT TRUNCATED AT 250 WORDS)

Ca 2+ activation properties of myofibrillar atpase from fatigued rat plantaris

1. I. Muscle fatigue following long-duration rhythmic activity is often characterized by reduced force following a single impulse and at low-frequencies of stimulation. 2. 2. Although this response is generally attributed to an alteration in excitation-contraction coupling, the possibility that the responsiveness of myofibrillar proteins to a given Ca 2+ signal is altered has never been ruled out. 3. 3. In this study, rat plantaris muscles were subjected to an in situ regimen of contractions (100 Hz, lasting 100msec, once every 750msec, for 1 hr), and allowed to recover for 15 min. 4. 4. Twitch, 100 Hz, and 200 Hz forces were reduced by 79%, 49% and 17% respectively, at this time. 5. 5. In myofibrils isolated from these muscles, maximum activity of Ca 2+ activated myofibrillar ATPase, Ca 2+ sensitivity (pCa 50), and co-operatively (Hill n), were not different from non-fatigued muscles. 6. 6. It appears, therefore, that the Ca 2+ activation properties of myofibrillar ATPase do not contribute to this pattern of fatigue.