Reduced Force Output Research Articles

Are Diminishing Potentiation and Large Extensor Moments the Cause for the Occurrence of the Sticking Region in Maximum Free-Weight Barbell Back Squats among Resistance-Trained Males?

This study compared the kinematics, surface electromyography (sEMG) and kinetics among isometric squats performed at 10 different heights of the upward part and a one-repetition maximum (1-RM) squat. Eleven males (age: 27.5 ± 3.4 years, body mass: 84.9 ± 8.1 kg, body height: 1.79 ± 0.06 m, 1-RM squat: 152.2 ± 20.55 kg) took part in this study. It was found that force output was lowest in the sticking region at around the event of peak deceleration for the 1-RM trial with force output at 2179 ± 212 N. For the isometric trial, the lowest force output occurred at the lowest barbell height (1735 ± 299 N). In addition, for the 1-RM condition hip extension moments peaked at the first four barbell heights (6.5-6.2 Nm/kg) representing the pre-sticking and the sticking region before significantly decreasing during the events representing the post-sticking region. Additionally, the sEMG amplitude peaked for the hip extensors at the barbell heights corresponding to the post-sticking region. Moreover, the sEMG amplitude was significantly higher for the 1-RM condition for all hip extensors, vastus lateralis, and calf muscles (F ≥ 2.7, p ≤ 0.01, ηp2 ≥ 0.25). Therefore, we suggest that the sticking region occurs because of reduced force output in the pre-sticking and the sticking region in back squats among resistance-trained males. The reduced force output is probably a combination of suboptimal internal moment arms, length-tension relationships of the gluteus maximus, hamstring and vastii muscles in the pre-sticking and sticking regions to overcome the large extensor moments together with diminishing potentiation from the pre-sticking to the sticking region.

Volume loss during muscle reinnervation surgery is correlated with reduced CMAP amplitude but not reduced force output in a rat hindlimb model.

Introduction: Muscle reinnervation (MR) surgery offers rehabilitative benefits to amputees by taking severely damaged nerves and providing them with new denervated muscle targets (DMTs). However, the influence of physical changes to muscle tissue during MR surgery on long-term functional outcomes remains understudied. Methods: Our rat hindlimb model of MR surgery utilizes vascularized, directly neurotized DMTs made from the lateral gastrocnemius (LG), which we employed to assess the impact of muscle tissue size on reinnervation outcomes, specifically pairing the DMT with the transected peroneal nerve. We conducted MR surgery with both DMTs at full volume and DMTs with partial volume loss of 500mg at the time of surgery (n = 6 per group) and measured functional outcomes after 100days of reinnervation. Compound motor action potentials (CMAPs) and isometric tetanic force production was recorded from reinnervated DMTs and compared to contralateral naïve LG muscles as positive controls. Results: Reinnervated DMTs consistently exhibited lower mass than positive controls, while DMTs with partial volume loss showed no significant mass reduction compared to full volume DMTs (p = 0.872). CMAP amplitudes were lower on average in reinnervated DMTs, but a broad linear correlation also exists between muscle mass and maximum CMAP amplitude irrespective of surgical group (R2 = 0.495). Surprisingly, neither MR group, with or without volume loss, demonstrated decreased force compared to positive controls. The average force output of reinnervated DMTs, as a fraction of the contralateral LG's force output, approached 100% for both MR groups, a notable deviation from the 9.6% (±6.3%) force output observed in our negative control group at 7days post-surgery. Tissue histology analysis revealed few significant differences except for a marked decrease in average muscle fiber area of reinnervated DMTs with volume loss compared to positive controls (p = 0.001). Discussion: The results from our rat model of MR suggests that tissue electrophysiology (CMAPs) and kinesiology (force production) may recover on different time scales, with volumetric muscle loss at the time of MR surgery not significantly reducing functional outcome measurements for the DMTs after 100days of reinnervation.

Can a chloride channel blocker mitigate muscle fatigue?

Can a chloride channel blocker mitigate muscle fatigue?

Neck linker docking is critical for Kinesin-1 force generation in cells but at a cost to motor speed and processivity.

Kinesin force generation involves ATP-induced docking of the neck linker (NL) along the motor core. However, the roles of the proposed steps of NL docking, cover-neck bundle (CNB) and asparagine latch (N-latch) formation, during force generation are unclear. Furthermore, the necessity of NL docking for transport of membrane-bound cargo in cells has not been tested. We generated kinesin-1 motors impaired in CNB and/or N-latch formation based on molecular dynamics simulations. The mutant motors displayed reduced force output and inability to stall in optical trap assays but exhibited increased speeds, run lengths, and landing rates under unloaded conditions. NL docking thus enhances force production but at a cost to speed and processivity. In cells, teams of mutant motors were hindered in their ability to drive transport of Golgi elements (high-load cargo) but not peroxisomes (low-load cargo). These results demonstrate that the NL serves as a mechanical element for kinesin-1 transport under physiological conditions.

Release of fascial compartment boundaries reduces muscle force output.

Most limb muscles operate within a compartment defined by fascial layers that enclose a muscle or groups of muscles within a defined space. These compartments are important clinically, because fluid accumulation can cause ischemia and tissue necrosis if untreated. Little is known, however, about how fascial enclosures influence healthy muscle function. One previous study showed that removing a fascial covering reduced the force output of a muscle under maximal stimulation. We hypothesized that such reduction in force output was due to a change in the muscle length following fasciotomy and that a reduced force output could be explained by the length-tension relationship of muscle. Thus we predicted that the maximum force across a range of lengths would be unchanged following fasciotomy. We measured maximal tetanic force output in a wing muscle in wild turkeys both before and after removal of fascia that enclosed the muscle in a compartment. Our hypothesis was not supported. The length-tension curve of this muscle showed that removal of fascia reduced maximum force output to 72 ± 10% of the prefascial release condition. Thus a reduction in muscle force following fasciotomy was not explained by a change in muscle length. The mechanism underlying reduction in force is unclear, but it suggests that the assumption underlying most isolated muscle experiments, i.e., removal of a muscle from its situation in vivo does not influence its maximal mechanical output, may need reexamining. NEW & NOTEWORTHY Most limb muscles are enclosed within compartments bound by robust fascial sheets. The mechanical significance of the close packing of muscle and fascia is largely unexplored. We used an animal model to show that removal of a fascial covering reduces the maximal force developed during contraction. These results raise questions about the use of isolated muscles to estimate muscle performance and suggest that a muscle's mechanical surrounding influences performance by mechanisms that are not understood.

Visually-guided saccades attenuate postural sway under non-fatigued, fatigued, and stretched states.

Muscular fatigue, which reduces force output and position sense, often leads to increased sway and potential balance impairments. In contrast, visually-guided saccadic eye movements (saccades) can attenuate sway more than fixating gaze on an external target. The goals of this study were to determine whether the use of saccades could reduce the increased postural sway in a fatigued state and to better understand the contributions to fatigue-induced increased sway. We compared the effects of gazing at a fixation point (FP) and performing saccades (SAC) on various spatial and temporal measures of the center of pressure (CoP) while participants stood as still as possible on a force plate. Participants used either a narrow or wide base of support and performed three trials for each eye movement condition (SAC, FP) in three states (non-fatigued-NF, stretched-S, and fatigued-F). Calf raises to exhaustion induced ankle fatigue. Extreme plantar- and dorsi-flexion induced stretch. SAC significantly decreased sway and increased time-series complexity (sample entropy) compared to FP. F increased sway and decreased time-series complexity compared to NF and S states, which were similar. Reduced force production, which accompanies muscle fatigue and stretching, did not account for increased sway associated with acute bouts of ankle muscle fatigue. Increased position sense often associated with muscle stretching likely compensated for any reduced force output for S, while the decreased position sense associated with F probably explained the increased sway in this state. Performing saccadic eye movements during quiet stance can help reduce sway under various states.

Relative contribution of different altered motor unit control to muscle weakness in stroke: a simulation study

Objective. Chronic muscle weakness impacts the majority of individuals after a stroke. The origins of this hemiparesis is multifaceted, and an altered spinal control of the motor unit (MU) pool can lead to muscle weakness. However, the relative contribution of different MU recruitment and discharge organization is not well understood. In this study, we sought to examine these different effects by utilizing a MU simulation with variations set to mimic the changes of MU control in stroke. Approach. Using a well-established model of the MU pool, this study quantified the changes in force output caused by changes in MU recruitment range and recruitment order, as well as MU firing rate organization at the population level. We additionally expanded the original model to include a fatigue component, which variably decreased the output force with increasing length of contraction. Differences in the force output at both the peak and fatigued time points across different excitation levels were quantified and compared across different sets of MU parameters. Main results. Across the different simulation parameters, we found that the main driving factor of the reduced force output was due to the compressed range of MU recruitment. Recruitment compression caused a decrease in total force across all excitation levels. Additionally, a compression of the range of MU firing rates also demonstrated a decrease in the force output mainly at the higher excitation levels. Lastly, changes to the recruitment order of MUs appeared to minimally impact the force output. Significance. We found that altered control of MUs alone, as simulated in this study, can lead to a substantial reduction in muscle force generation in stroke survivors. These findings may provide valuable insight for both clinicians and researchers in prescribing and developing different types of therapies for the rehabilitation and restoration of lost strength after stroke.

Muscle size explains low passive skeletal muscle force in heart failure patients.

BackgroundAlterations in skeletal muscle function and architecture have been linked to the compromised exercise capacity characterizing chronic heart failure (CHF). However, how passive skeletal muscle force is affected in CHF is not clear. Understanding passive force characteristics in CHF can help further elucidate the extent to which altered contractile properties and/or architecture might affect muscle and locomotor function. Therefore, the aim of this study was to investigate passive force in a single muscle for which non-invasive measures of muscle size and estimates of fiber force are possible, the soleus (SOL), both in CHF patients and age- and physical activity-matched control participants.MethodsPassive SOL muscle force and size were obtained by means of a novel approach combining experimental data (dynamometry, electromyography, ultrasound imaging) with a musculoskeletal model.ResultsWe found reduced passive SOL forces (∼30%) (at the same relative levels of muscle stretch) in CHF vs. healthy individuals. This difference was eliminated when force was normalized by physiological cross sectional area, indicating that reduced force output may be most strongly associated with muscle size. Nevertheless, passive force was significantly higher in CHF at a given absolute muscle length (non length-normalized) and likely explained by the shorter muscle slack lengths and optimal muscle lengths measured in CHF compared to the control participants. This later factor may lead to altered performance of the SOL in functional tasks such gait.DiscussionThese findings suggest introducing exercise rehabilitation targeting muscle hypertrophy and, specifically for the calf muscles, exercise that promotes muscle lengthening.

EP 92. Measuring force in fingertapping: A novel tool to detect motor fatigue

Introduction The Fingertapping-task (FT) is a simple tool to evaluate complex aspects of motor function in a clinical as well as research setting. FT provides information about motor function and deterioration (Shimoyama, 1990). FT tapping rate (FT-speed) is recorded as a correlate for performance with reduced rates corresponding to a decline of function. Motor fatigue (MF) is usually studied by employing a maximum voluntary contraction task and decline of function is defined as reduced force output. Yet, there are few studies focusing on motor fatigue in more complex (repetitive) motor tasks like FT and modified FT-force. We hypothesize reduced FT-force in a standard FT-task will reflect task specific motor fatigue providing additional information to FT-speed. Methods We recorded the FT-force in healthy subjects in a standard FT-task (right and left first dorsal interosseous (FDI), 30 s repetitive sessions) via a morse key equipped with a 1 N force sensor. We hypothesized that as a correlate for motor fatigue the FT-speed and FT-force would decline over time. Results From the continuous force signal data FT tap-intervals were derived and compared to FT tap-intervals defined by a closed circuit (morse key). Results show a very high reliability (intraclass correlation coefficient 0.98). A significant decrease of FT-speed over time in the respective session (intrasession) was seen ( p 0.01 ), that was more prominent in the non-dominant hand. A significant enhancement in FT-force over time was seen within and between FT sessions ( p 0.01 ). This effect was more prominent in the non-dominant hand. Surprisingly, the effects of FT-force and FT-speed were not correlated. Discussion We could show that the assessment of tapping force in the standard FT-task provides additional novel information about motor fatigue not only within, but also between FT-task sessions, that is clearly independent of FT-speed. The increase in FT-force seems counterintuitive, yet is in line with previous definitions of motor fatigue (Edwards, 1980) when interpreted as a correlate for increasing effort and central compensatory mechanisms. Measuring the FT-force in a standard FT-task is a novel and easy to use tool, which might reflect central compensatory mechanisms, could be used in a clinical setting to detect early symptoms of fatigue in various disease entities and we believe deserves further investigation and neurophysiological characterization.

C57BL/6 life span study: age-related declines in muscle power production and contractile velocity.

Quantification of key outcome measures in animal models of aging is an important step preceding intervention testing. One such measurement, skeletal muscle power generation (force * velocity), is critical for dynamic movement. Prior research focused on maximum power (Pmax), which occurs around 30–40 % of maximum load. However, movement occurs over the entire load range. Thus, the primary purpose of this study was to determine the effect of age on power generation during concentric contractions in the extensor digitorum longus (EDL) and soleus muscles over the load range from 10 to 90 % of peak isometric tetanic force (P0). Adult, old, and elderly male C57BL/6 mice were examined for contractile function (6–7 months old, 100 % survival; ~24 months, 75 %; and ~28 months, <50 %, respectively). Mice at other ages (5–32 months) were also tested for regression modeling. We hypothesized and found that power decreased with age not only at Pmax but also over the load range. Importantly, we found greater age-associated deficits in both power and velocity when the muscles were contracting concentrically against heavy loads (>50 % P0). The shape of the force-velocity curve also changed with age (a/P0 increased). In addition, there were prolonged contraction times to maximum force and shifts in the distribution of the myosin light and heavy chain isoforms in the EDL. The results demonstrate that age-associated difficulty in movement during challenging tasks is likely due, in addition to overall reduced force output, to an accelerated deterioration of power production and contractile velocity under heavily loaded conditions.Electronic supplementary materialThe online version of this article (doi:10.1007/s11357-015-9773-1) contains supplementary material, which is available to authorized users.

Vibration Therapy in Management of Delayed Onset Muscle Soreness (DOMS).

Both athletic and nonathletic population when subjected to any unaccustomed or unfamiliar exercise will experience pain 24-72 hours postexercise. This exercise especially eccentric in nature caused primarily by muscle damage is known as delayed-onset muscle soreness (DOMS). This damage is characterized by muscular pain, decreased muscle force production, reduce range of motion and discomfort experienced. DOMS is due to microscopic muscle fiber tears. The presence of DOMS increases risk of injury. A reduced range of motion may lead to the incapability to efficiently absorb the shock that affect physical activity. Alterations to mechanical motion may increase strain placed on soft tissue structures. Reduced force output may signal compensatory recruitment of muscles, thus leading to unaccustomed stress on musculature. Differences in strength ratios may also cause excessive strain on unaccustomed musculature. A range of interventions aimed at decreasing symptoms of DOMS have been proposed. Although voluminous research has been done in this regard, there is little consensus among the practitioners regarding the most effective way of treating DOMS. Mechanical oscillatory motion provided by vibration therapy. Vibration could represent an effective exercise intervention for enhancing neuromuscular performance in athletes. Vibration has shown effectiveness in flexibility and explosive power. Vibration can apply either local area or whole body vibration. Vibration therapy improves muscular strength, power development, kinesthetic awareness, decreased muscle sore, increased range of motion, and increased blood flow under the skin. VT was effective for reduction of DOMS and regaining full ROM. Application of whole body vibration therapy in postexercise demonstrates less pressure pain threshold, muscle soreness along with less reduction maximal isometric and isokinetic voluntary strength and lower creatine kinase levels in the blood.

Changes in Electromyographic Activity after Conditioning Contraction

[Purpose] Post-activation potentiation is the increase in muscle twitch force in response to electrical stimulation after a conditioning contraction (CC) of maximum or near-maximum intensity. It is controversial whether muscular force during low-level voluntary contractions increases after maximum contraction. This study investigated the changes in electromyographic activity after CC. [Subjects and Methods] Fifteen healthy adults participated in this study. In the first experiment, a supramaximal electrical pulse was applied to the tibial nerve, and the isometric twitch force of plantar flexion was measured. After maximum voluntary contraction (MVC), i.e., CC, the twitch force elicited by electrical stimulation was monitored. In the second experiment, the surface electromyogram were recorded at an intensity of 20% MVC before and after CC. [Results] Results indicated the twitch force significantly increased by 30% within 30 s after CC. In the second experiment, the electromyographic activity significantly decreased by 16% for the gastrocnemius muscle but did not change for the soleus. Correlation coefficients between changes in the twitch force and electromyographic activity at 20%MVC were -0.54 for the gastrocnemius muscle and 0.04 for the soleus muscle. [Conclusion] Post-activation potentiation might work for daily physical activities compensating for reduced force output of the fatigued muscle.

The fusimotor and reafferent origin of the sense of force and weight

Signals associated with the command the brain sends to muscles are thought to create the sensation of heaviness when we lift an object. Thus, as a muscle is weakened by fatigue or partial paralysis (neuromuscular blockade), the increase in the motor command needed to lift a weight is thought to explain the increasing subjective heaviness of the lifted object.With different fatiguing contractions we approximately halved the force output of the thumb flexor muscles, which were then used to lift an object. For two deafferented subjects the perceived heaviness of the lifted object approximately doubled, in keeping with the central-signal theory. However, for normal subjects this resulted in objects feeling the same or lighter, inconsistent with the central-signal theory but consistent with the expected effects of the conditioning contractions on the sensitivity of peripheral receptors. In separate experiments we subjected the forearm muscles to complete paralysis with a non-depolarising neuromuscular blocking agent and then allowed them to recover to approximately half-force output. This also resulted in objects feeling lighter when lifted by the semi-paralysed thumb, even though the motor command to the motoneurons must have been greater. This is readily explained by reduced lift-related reafference caused by the prolonged paralysis of muscle spindle intrafusal fibres.We conclude that peripheral signals, including a major contribution from muscle spindles, normally give rise to the sense of exerted force. In concept, however, reafference from peripheral receptors may also be considered a centrally generated signal that traverses efferent and then afferent pathways to feed perceptual centres rather than one confined entirely to the central nervous system. These results therefore challenge the distinction between central- and peripheral-based perception, and the concept that muscle spindles provide only information about limb position and movement.

Changes in joint coupling and variability during walking following tibialis posterior muscle fatigue

BackgroundThe tibialis posterior muscle is believed to play a key role in controlling foot mechanics during the stance phase of gait. However, an experiment involving localised tibialis posterior muscle fatigue, and analysis of discrete rearfoot and forefoot kinematic variables, indicated that reduced force output of the tibialis posterior muscle did not alter rearfoot and forefoot motion during gait. Thus, to better understand how muscle fatigue affects foot kinematics and injury potential, the purpose of this study was to reanalyze the data and investigate shank, rearfoot and forefoot joint coupling and coupling variability during walking.MethodsTwenty-nine participants underwent an exercise fatigue protocol aimed at reducing the force output of tibialis posterior. An eight camera motion analysis system was used to evaluate 3 D shank and foot joint coupling and coupling variability during treadmill walking both pre- and post-fatigue.ResultsThe fatigue protocol was successful in reducing the maximal isometric force by over 30% and a concomitant increase in coupling motion of the shank in the transverse plane and forefoot in the sagittal and transverse planes relative to frontal plane motion of the rearfoot. In addition, an increase in joint coupling variability was measured between the shank and rearfoot and between the rearfoot and forefoot during the fatigue condition.ConclusionsThe reduced function of the tibialis posterior muscle following fatigue resulted in a disruption in typical shank and foot joint coupling patterns and an increased variability in joint coupling. These results could help explain tibialis posterior injury aetiology.

The role of tibialis posterior fatigue on foot kinematics during walking

BackgroundThe purpose of this study was to investigate the effect of localised tibialis posterior muscle fatigue on foot kinematics during walking. It was hypothesised that following fatigue, subjects would demonstrate greater forefoot and rearfoot motion during walking. It was also postulated that the magnitude of the change in rearfoot motion would be associated with standing anatomical rearfoot posture.MethodsTwenty-nine subjects underwent an exercise fatigue protocol aimed at reducing the force output of tibialis posterior. An eight camera motion analysis system was used to evaluate 3D foot kinematics during treadmill walking both pre- and post-fatigue. The anatomical rearfoot angle was measured during standing prior to the fatigue protocol using a goniometer.ResultsPeak rearfoot eversion remained unchanged following the fatigue protocol. Although increases in rearfoot eversion excursion were observed following fatigue, these changes were of a magnitude of questionable clinical significance (<1.0°). The magnitude of the change in rearfoot eversion due to fatigue was not associated with the anatomical measurement of standing rearfoot angle. No substantial changes in forefoot kinematics were observed following the fatigue protocol.ConclusionsThese data indicate that reduced force output of the tibialis posterior muscle did not alter rearfoot and forefoot motion during gait. The anatomical structure of the rearfoot was not associated with the dependence of muscular activity that an individual requires to maintain normal rearfoot kinematics during gait.

Force measurement from viscoelastically recovering Nylon 6,6 fibres

The force generated from viscoelastically stored energy in nylon 6,6 fibres was investigated. A 320 MPa, 24 h tensile creep stress was applied to annealed yarn. Following elastic (short-term) recovery, the yarn was allowed to contract from a loose state to a fixed strain, thereby becoming taut. The resulting force grew to 3.4% of applied creep load over a 2700 h measurement period t, and was predicted to approach a maximum of 3.8% as t → ∞. Increasing humidity reduced force output, but the effect was totally reversible. As the force–time curve follows a Weibull or Kohlrausch–Williams–Watts function, the findings are discussed from a molecular jumps perspective.

Tubular system excitability: an essential component of excitation–contraction coupling in fast-twitch fibres of vertebrate skeletal muscle

The tubular (t-) system is the main interface between the myoplasm and the extracellular environment and is responsible for the rapid inward spread of excitation from the sarcolemma to the inner parts of the skeletal muscle fibre as well as for signal transfer to the sarcoplasmic reticulum to release Ca2+ that, in turn, activates the contractile apparatus. In this review, I explore the insights provided by the mechanically skinned muscle fibre preparation to the better understanding of the importance of the t-system excitability in determining the force response under physiologically relevant conditions. In the mechanically skinned muscle fibre, the t-system seals off after is physically separated from the sarcolemma and its excitability can be investigated by electrical stimulation under controlled conditions. Parameters that can be assessed include the threshold for action potential generation, specific electrical resistance and time constant of the tubular wall, quantity of charge transferred during an action potential, refractory period, length constant and velocity of excitation propagation. Results obtained with mechanically skinned fibres from fast-twitch muscles show that decreased t-system excitability does not necessarily translate into reduced force output, but for any particular set of physiologically relevant conditions there is a level below which a further decrease in t-system excitability markedly decreases the force output. There are several built-in mechanisms linked to the metabolic/energetic state of the muscle fibre which prevent complete action potential failure in the t-system, thus allowing the muscle to respond to nerve stimulation, even if the response becomes markedly attenuated.

Normalized Torque of the Quadriceps Femoris is Similar Between Active Old and Young Adults

2435 Age related losses in maximum force production are well documented. The loss in strength could be attributed to a loss in skeletal muscle mass or a reduced ability to centrally activate available muscle. Previous research has reported mixed results as to whether the force per unit area of skeletal muscle is different between old and young subjects. PURPOSE: To determine if normalized torque (torque per unit area of skeletal muscle) is different between active older and younger adults. METHODS: Skeletal muscle cross sectional area (CSA) and torque from maximum voluntary contraction (MVC) of the m. quadriceps femoris (QF) was determined in 18 individuals. Nine active older subjects (67 ± 7 yrs, 175 ± 10 cm, 81 ± 9 kg, 2 females) were matched with nine young subjects (27 ± 3 yrs, 175 ± 7 cm, 75 ± 15 kg, 2 females) for height, weight, and gender. QF skeletal muscle CSA was measured with magnetic resonance imaging (MRI) and MVC of the knee extensors was measured using the burst superimposition technique. RESULTS: Subjects' height and weight were not different (p > 0.05). Young subjects had higher MVC (248 ± 52 vs. 195 ± 52 Nm, p < 0.05) and larger QF CSA (78 ± 14 vs. 63 ± 12 cm2, p < 0.05). Central activation (%) as determined by the burst superimposition technique was not different in young vs. old subjects (96 ± 2 vs. 97 ± 3 %, respectively, p > 0.05). Normalized torque was not different between groups (3.2 ± 0.4 vs. 3.0 ± 0.6 Nm/cm2, p > 0.05). CONCLUSIONS: These results suggest that the primary reason for reduced force output with aging is skeletal muscle atrophy. Available muscle that can be voluntarily activated by this sample of older subjects has the same force generating capacity as young muscle. Interventions to increase strength in older adults should focus on increasing muscle size. Supported, in part, by NIH grants HD 39676 and 39479 (GAD)

Diaphragm Structure and Function in Emphysematous Hamsters

OBJECTIVE: To determine if muscle fibre injury, reduced force output and fatigue of the diaphragm accompanied hyperinflation induced by experimental emphysema.DESIGN: Controlled and randomized.ANIMALS: Adult male golden Syrian hamsters: seven control and seven experimental emphysema hamsters were studied.INTERVENTIONS: Following anesthesia, experimental emphysema hamsters received a transtracheal injection of elastase. They also received injections of β‐aminopropionitrile every other day for five weeks. After five weeks, all hamsters we re anesthetized , and their diaphragm and lungs were excised.MAIN RESULTS: Measurements of the excised lungs showed an increased residual volume. functional residual capacity and total lung capacity in experimental emphysema hamsters. In vitro physiological study of diaphragm strips showed no difference in force output but a greater fatiguability during repetitive stimulation for 5 mins in experimental emphysema hamsters than in control hamsters. Histological examination did not show significant muscle injury in the diaphragm of experimental emphysema hamsters.CONCLUSION: The greater fatiguability and absence of muscle injury in the diaphragm of emphysematous hamsters may result from hyperinflation decreasing diaphragmatic load. Alternatively, muscle damage not detectable at the light microscopic level may have contributed to increased fatiguability of the diaphragm of experimental emphysematous hamsters.

Changes in force and stiffness induced by fatigue and intracellular acidification in frog muscle fibres.

1. Changes in force and stiffness were recorded simultaneously during 1 s isometric (fixed ends) tetani of single fibres isolated from the anterior tibialis muscle of Rana temporaria (temperature 1-3 degrees C; sarcomere length, 2.10 micron). Stiffness was measured as the change in force that occurred in response to a 4 kHz sinusoidal length oscillation of the fibre. Some experiments were performed in which stiffness was determined from a fast (0.2 ms) length step that was applied to a 'tendon-free' segment of the muscle fibre during the tetanus plateau. 2. A moderate degree of fatigue was produced by decreasing the time between tetani from 300 s (control) to 15 s. By this treatment the maximum tetanic force (Ftet) was reversibly reduced to 70-75% of the control value. Maximum tetanic stiffness (Stet) was related to Ftet according to the following regression (both variables expressed as percentage of their control values): Stet = 0.369 Ftet + 62.91 (correlation coefficient, 0.95; P less than 0.001). A 25% decrease in isometric force during fatigue was thus associated with merely 9% reduction of fibre stiffness. 3. Whereas the rate of rise of force during tetanus was markedly reduced by fatiguing stimulation, the rate of rise of stiffness was only slightly affected. 4. Intracellular acidification (produced by raised extracellular CO2 concentration) largely reproduced the contractile changes observed during fatigue. However, for a given decrease in tetanic force there was a smaller reduction in fibre stiffness during acidosis than during fatigue. 5. Caffeine (0.5 mM) added to the fibre after development of fatigue and intracellular acidosis greatly potentiated the isometric twitch but did not affect maximum tetanic force. This finding provides evidence that the contractile system was fully activated during the tetanus plateau both in the fatigued state and during acidosis. 6. The results suggest that the decrease in contractile strength after frequent tetanization (intervals between tetani, 15 s) is attributable to altered kinetics of cross-bridge function leading to reduced number of active cross-bridges and, most significantly, to reduced force output of the individual bridge. The possible role of increased intracellular H+ concentration in the development of muscle fatigue is discussed.