Longer Muscle Lengths Research Articles

The effects of fatigue on the resultant joint moment, agonist and antagonist electromyographic activity at different angles during dynamic knee extension efforts

Examination of the effects of fatigue on antagonist function can provide information on the role of antagonists in limiting the resultant joint moment and stabilizing the knee. Therefore, the purpose of this study was to examine the moment, agonist and antagonist electromyographic (EMG) activity levels at different angular positions during an isokinetic muscular endurance knee extension test. Fifteen healthy males (age 22.6±1.9 yr) performed 34 maximal isokinetic concentric efforts of the knee extensors at 120° s −1. The EMG activity of vastus medialis and biceps femoris was recorded using surface electrodes. The motion ranged from 90° to 0° of knee flexion. The average moment and average EMG (AEMG) at 10–35°, 36–55° and 56–80° angular position intervals were calculated for each repetition. Twenty eight efforts were further analysed. The moment of force demonstrated a decline of 70% at the end of the test. Two-way repeated measures analysis of variance tests indicated that this decline was significant ( p<0.05). No significant effects of angular position on fatigue moment characteristics were found. The agonist (vastus medialis) AEMG during the first repetition demonstrated a significant increase of 40–60% towards the middle part of the test ( p<0.05). In the second part of the test, the VM AEMG at longer muscle lengths was significantly higher compared to the initial efforts whereas the AEMG at short muscle lengths returned to initial values. The antagonist AEMG at all angular positions did not change significantly during the test. The decline in the resultant joint moment could be attributed to the effects of fatigue on the agonist muscle function. The agonist AEMG fatigue-patterns are dependent on the length of the muscle and may be due to alterations in the motor unit recruitment and/or activation failure in the quadriceps muscle. The biceps femoris maintains constant submaximal (21–33% of the maximum) AEMG activity which may play an important role in the stability of the knee joint. The contribution of antagonist activity to the resultant joint moment increases during the last part of an isokinetic concentric muscle endurance test.

Reduced muscle function in patients with osteoarthritis

The purpose of this study was to determine whether subjects with knee osteoarthritis (OA) had reduced muscle strength at various muscle lengths, endurance, contraction velocity and functional capacity, compared with control subjects and whether the decrease was related to functional capacity. Forty-five men and 45 women with knee OA were compared with a control group (41 males, 63 females) of similar age for functional capacity, maximal isometric strength (in vivo length-tension relationship) and endurance (in vivo force-time relationship) of knee flexion and extension and maximal angular velocity (in vivo force-velocity relationship) of knee extension. The OA subjects had increased difficulty (2.03 +/- 0.53) and pain (1.65 +/- 0.29) for activities of daily living (ADLs) and significantly lower strength for extension (72%) and flexion (56%), endurance for the quadriceps (203%) and hamstrings (214%) and velocity (128%). The reductions were greater at longer muscle lengths. These data demonstrate that patients with knee OA have reduced muscle function and functional capacity compared to controls.

Changes in the mechanical properties of human and amphibian muscle after eccentric exercise

Following a series of eccentric contractions, that is stretching of the muscle while generating active tension, the length-tension relationship of isolated amphibian muscle has been shown to shift towards longer muscle length (Katz 1939; Wood et al. 1993). Here we report observations of electrically stimulated ankle extensor muscles of nine human subjects, demonstrating a similar shift in optimum angle for torque generation [3.9 (1.5) degrees] following exercise on an inclined treadmill that involved eccentric contractions in one leg. (All values are means with the SEMs in parentheses). The shift in the unexercised, control leg was significantly less [mean 0.4 (0.7) degree P < 0.05]. Correlated with this shift was a drop in torque [25.1 (5.6)% for the experimental leg; 1.6 (0.7)% for the control leg, P < 0.002]. Optimum angles returned to pre-exercise values by 2 days post-exercise, while torque took a week to recover. A similar shift in optimum length [12 (1.3)% of rest length] was obtained for five toad (Bufo marinus) sartorius muscles subjected to 25 eccentric contractions. Isometrically contracted control muscles showed a smaller shift [3.5 (1.6)%, n = 5]. Accompanying the shift was a drop in tension of 46 (3)% after the eccentric contractions [control isometric, 23 (6)%, P < 0.0001]. By 5 h after the eccentric contractions the shift had returned to control values, while tension had not recovered. When viewed with an electron microscope, sartorius muscles fixed immediately after the eccentric contractions exhibited many small, and a few larger, regions of myofilament disruption. In muscles fixed 5 h after the contractions, no small regions of disruption were visible, and the number of large regions was no greater than in those muscles fixed immediately after the eccentric contractions. These disruptions are interpreted as the cause of the shift in length-tension relationship.

Quantitative evaluation of a home exercise program on muscle and functional capacity of patients with osteoarthritis.

Rehabilitation of patients with osteoarthritis of the knees is typically based on home exercise. These programs are believed to benefit patients and have been shown to qualitatively improve strength. The purpose of the present study was to quantify the effects of a 3-mo home exercise program on muscle function and functional capacity. The progressive program included flexibility, strength, endurance, active range of motion and functional activities. Nineteen subjects (67.4 +/- 7.5 yr) with osteoarthritis of the knees began the program, with only nine completing it. The subjects initially had significantly reduced muscle function and functional capacity. Maximal isometric strength of knee extension increased significantly at a knee flexion position of 45 degrees for hip flexion positions of 120 degrees and 60 degrees (35%); however, it failed to increase at longer muscle lengths. There were no significant improvements in hamstring strength. Maximal angular velocity improved after 3 mo of exercise (40%). Muscle endurance did not improve significantly. Although there was a slight increase in functional capacity, these data failed to demonstrate significant clinical or statistical improvement in overall function in patients after home exercise.

Factors limiting force during slow, shortening actions of the quadriceps femoris muscle group in vivo

Speed-torque relations of the quadriceps femors muscle group were determined using eight healthy subjects. Isometric or isovelocity, shortening muscle actions were performed at 15 speeds (0-6.28 rad s-1) and torque was measured 0.78 and 0.52 rad below horizontal. Unilateral actions were evoked by surface electrical stimulation or by maximal voluntary effort. Stimulation current evoked a torque equal to approx. 70% of maximal voluntary isometric and was held constant across speeds. For the voluntary or stimulation tests, torque decreased (P < 0.05) with increasing speed. This response in relative terms was greater (P < 0.05) for the stimulation than for the voluntary tests. The difference in the decline in relative torque for the simulation vs. voluntary tests was not influenced by the angle at which torque was measured, and thereby muscle length. Speed-torque data for the two stimulation tests fit a linearized plot of a hyperbolic relation for the higher tests speeds. When torque was measured at the greater joint angle (0.78 rad), and thereby longer muscle length, the equation was Po-P/V = Po 0.724-1.078 (P < 0.05, r2 = 0.95) for speeds greater than 0.70 rad s-1. For the shorter muscle length, the equation for data collected at speeds greater than 1.13 rad s-1 was Po-P/V = Po 0.467 + 4.61 (P < 0.05, r2 = 0.91). Inclusion of data for the next slower speed markedly compromised the 'fit' for either linear relation.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of muscle length on the EMG-force relationship of the cat soleus muscle studied using non-periodic stimulation of ventral root filaments.

The effects of changing the length of the cat soleus muscle on electromyographic (EMG) signals, muscle force and the corresponding EMG-force relationship were assessed using distributed stimulation of ten ventral root filaments and irregular interpulse intervals. EMG-force relationships were first determined for four muscle lengths using a protocol of simultaneous addition and rate modulation of ventral root filaments. In the second test, three submaximal levels of stimulation were applied at eight muscle lengths. EMG signals were obtained using surface and wire electrodes, and force was measured using a strain transducer. For most muscle lengths, the relationships between integrated EMG and mean force obtained using wire and surface electrodes were sigmoid with a linear intermediate region. The effects of muscle length on EMG signals were likely to be associated with movement of the recording electrodes relative to each other and to the active motor units. Mean forces increased with increasing muscle length and with increasing levels of stimulation. Mean force-length relationships obtained using submaximal stimulation were not simply scaled down versions of the force-length relationship obtained using supramaximal stimulation of the soleus nerve, but appeared to be shifted towards longer muscle lengths.

Effects of gadolinium on length-dependent force in guinea-pig papillary muscle.

The degree to which stretch-activated channels operate during physiological length changes in multicellular heart preparations, or how much the channels could contribute to length-dependent activation, is not known. We studied the relationship between muscle length and contractile force in guinea-pig papillary muscles superfused with gadolinium chloride (10 microM), a stretch-activated channel blocker, and compared the effects to those with nifedipine (0.25 microM), a calcium channel blocker. Gadolinium reduced contractile force statistically significantly more at the longer muscle lengths than at the short muscle lengths. This did not apply with nifedipine, although a marginally greater effect at longer lengths was perceptible. The results can only partly be explained by gadolinium having a non-specific action via the calcium channel, or Na(+)-Ca2+ exchange, and are consistent with the possibility that stretch-activated channels contribute to length-dependent activation in cardiac muscle, and thus to 'Starling's Law of the Heart'.

Contraction history modulates isotonic shortening velocity in smooth muscle

The effect of contraction history on the isotonic shortening velocity of canine tracheal smooth muscle was investigated. Muscles were contracted isometrically for 20 s at initial lengths of L(o) (length of maximal active force), 85% L(o), or 70% L(o) using electrical field stimulation. Muscles were then allowed to shorten isotonically under different afterloads either with or without first being subjected to a step decrease in length to 70% L(o). Instantaneous velocities were plotted against instantaneous muscle length during isotonic shortening. Regardless of protocol, the velocity at any muscle length during shortening was lower when the muscle was initially activated at a longer length. The isotonic shortening velocity decreased progressively during shortening at a nearly linear rate with respect to instantaneous muscle length under all conditions. Results suggest that a longer muscle length at the time of activation leads to the development of higher loads on the contractile element during subsequent shortening, resulting in a slower shortening velocity. This plasticity of the force-velocity relationship may result from cytostructural reorganization of the smooth muscle cells in response to contractile activation at different muscle lengths.

Muscle rehabilitation

Abstract Muscle function and functional performance are limited in patients with osteoarthritis (OA). Although aerobic exercise can increase aerobic power and reduce fatigue, it does not appear to improve muscle function. The purpose of this study was to demonstrate the effect of a muscle rehabilitation program on muscle strength, endurance, speed, and function for patients with OA of the knees. Fifteen men (67.6 ± 6.1 years) with OA of the knees underwent a four-month exercise program, three times per week. Muscle strength, endurance, and speed were 50% less in OA patients than in controls. After rehabilitation, there was a significant increase in strength (35%), endurance (35%), and speed (50%). Deficiencies and improvements in the muscles were greater at longer muscle lengths. Increases in muscle function were associated with decreased dependency (10%), difficulty (30%), and pain (40%). The average increase in all measured parameters was 10% and 25% after two and four months of rehabilitation, respectively. Improvements were sustained for eight months after rehabilitation. The muscle rehabilitation program was designed specifically to improve function; the improved muscle function was translated into improved functional performance.

Muscle rehabilitation: its effect on muscular and functional performance of patients with knee osteoarthritis.

Muscle function and functional performance are limited in patients with osteoarthritis (OA). Although aerobic exercise can increase aerobic power and reduce fatigue, it does not appear to improve muscle function. The purpose of this study was to demonstrate the effect of a muscle rehabilitation program on muscle strength, endurance, speed, and function for patients with OA of the knees. Fifteen men (67.6 +/- 6.1 years) with OA of the knees underwent a four-month exercise program, three times per week. Muscle strength, endurance, and speed were 50% less in OA patients than in controls. After rehabilitation, there was a significant increase in strength (35%), endurance (35%), and speed (50%). Deficiencies and improvements in the muscles were greater at longer muscle lengths. Increases in muscle function were associated with decreased dependency (10%), difficulty (30%), and pain (40%). The average increase in all measured parameters was 10% and 25% after two and four months of rehabilitation, respectively. Improvements were sustained for eight months after rehabilitation. The muscle rehabilitation program was designed specifically to improve function; the improved muscle function was translated into improved functional performance.

Slowing of the discharge of secondary endings of cat muscle spindles during fusimotor stimulation

Responses of secondary endings of muscle spindles of the peroneus tertius muscle of the anaesthetized cat have been recorded during repetitive stimulation of functionally single fusimotor fibres that produced slowing of the discharge. In a sample of 125 pairs of single fusimotor fibres and secondary spindle afferents 5 examples of slowing were seen. The amount of slowing became less at longer muscle lengths. Conditioning the spindle by stimulating the muscle nerve at fusimotor strength, at a length 2.5 mm longer than the test length, and then returning to the test length 3 seconds later led to a greater degree of slowing of the discharge than after conditioning stimulation at the test length. With one exception, responses to muscle stretch were reduced during stimulation of a fusimotor fibre that produced slowing. On two occasions stimulating a fusimotor fibre that produced slowing of the response of one secondary ending, led to excitation of two other endings. Two possible explanations for the generation of slowing responses have been considered. The first is that the slowing is the result of contraction of the region of intrafusal fibre directly underlying the secondary sensory ending. The second, which we favour since it accounts for the facts more adequately, is that slowing is the result of shortening of the region of nuclear chain fibres on which the sensory ending lies, produced by movement in an adjacent nuclear bag fibre.

The responses of muscle spindles in the kitten to stretch and vibration

Discharges of muscle spindle afferents from the soleus muscle were studied in kittens aged 1-21 days and in adult cats. Vibration applied longitudinally to the tendon elicited one impulse for each cycle of vibration over the range 1-200 Hz for the kittens and up to 450 Hz for the adult. Threshold amplitudes were generally higher in the kitten than in the adult. In response to large ramp and hold stretches applied at long muscle lengths kitten spindles showed rate saturation during the length change. Dynamic index, that is the peak rate during the length change minus the rate at the final length became progressively smaller at longer muscle lengths. No sign of saturation was seen at comparable muscle lengths in the adult. It is suggested that in the newborn the bag1 intrafusal fibre is not functional and that the dynamic response is produced only by the afferent terminals on the bag2 fibre. Another difference between kitten and adult was the length sensitivity measured under dynamic conditions. This increased much more steeply with stretch rate in the kitten. One possible explanation for the higher dynamic length sensitivity is a lack of elastic fibres surrounding intrafusal fibres of immature spindles.

Chapter 11 Responses of muscle spindles depend on their history of activation and movement

Publisher Summary This chapter discusses the responses of muscle spindles depend on their history of activation and movement. The chapter mentions two experiments designed to test the hypothesis for development of slack in intrafusal fibres. The first considers the effect of altering the amount by which conditioning and test lengths differ. The second experiment described in the chapter considers the effect of varying the time for which the muscle is held at the conditioning length before being returned to the test length. To conclude, the experiments described in the chapter support the explanation of after-effects and are difficult to explain in any other way. The general proposition is that a spindle can develop depression of its responsiveness if it is held passive for a sufficient period and is then shortened. The time necessary for development of this depression is greater at longer muscle lengths, ranging from seconds to tens or even hundreds of seconds.

An explanation for the folding fibers in proboscis retractor muscles of Phascolopsis ( = Golfingia) Gouldi—I. correlation between length-tension data and structure in living muscle

1. 1. Fiber diameters and fold angles were measured at various muscle lengths at rest and during contraction. 2. 2. The length-tension relationship was bimodal, with tension peaks at 1.0 and 1.67 L/ L 0. 3. 3. Fiber folding occurred during contraction at lengths < 1.3 L 0. 4. 4. Diameter increased in folding cells during contraction only at lengths >1.15 L/ L 0. 5. 5. We suggest that two cell types produce the bimodal length-tension curve. 6. 6. We also suggest that folding cells produce force at longer muscle lengths where they are unfolded, but they are passively folded by a second cell type which produces force at shorter muscle lengths.

Effects of deuterium oxide on mechanics and energetics of skeletal muscle contraction.

Mechanics and energetics of contraction were studied in isolated frog semitendinosus muscles (n = 15) in deuterium oxide (D2O) at 5 degrees C. Twitch force and energy liberation (heat + work) in 99.8% D2O-Ringer at lo were 4 +/- 0.8 and 22 +/- 1%, respectively, of control values. Tetanus force was slowed in development, and developed tetanus force was reduced to 89 +/- 7% of control values. With repeated tetanic contractions, the rate of force development increased, and peak force increased above control values in some muscles. The twitch was potentiated 10-fold (10 +/- 2) after repeated tetanic contractions. Energy liberation increased but not in proportion to force. Ca2+ cycling energy liberation or activation heat, estimated from stretched muscles, was 60 +/- 3% of control values. With maximum posttetanic twitch potentiation, the activation heat increased by 25 +/- 3%. Peak twitch force and energy liberation occurred at longer muscle lengths than in normal Ringer. After correcting for small differences in force development at lo and 1.38 +/- 0.01 lo, it was possible to estimate the effect of length per se on energy liberation. Under these conditions, there was an 8 +/- 2% decrease in energy liberation at 1.38 lo.

Changes in the pause in muscle spindle discharge during a sequence of twitches

A study was made of the change in duration of the pauses in discharge from spindle afferents occurring during a series of isometric twitches of the cat's triceps surae muscle. These were induced by stimulation of the muscle nerve at frequencies of 0.1 to 1.0 Hz. At frequencies of 0.2 Hz and higher, the duration of the pause exhibited by group II and tonic and phasic group Ia afferents decreased progressively until stabilizing within 5 to 15 s. The time course of this change was largely independent of the frequency of stimulation, but the eventual duration in the steady state was strongly dependent on the frequency, being shorter at higher rates. At a given frequency, the pause was shorter at longer muscle lengths. The decrease in pause duration was not accompanied by a parallel change in duration or amplitude of the muscle tension responses, indicating that it arose from some alteration intrinsic to the spindle organ. We suggest that the progressive change in sensory discharge could serve the initial reinforcement of repetitive movements.

Length-dependent electromechanical coupling in single muscle fibers.

In single muscle fibers from the giant barnacle, a small decrease in muscle length decreases both the calcium activation and the peak isometric tension produced by a constant current stimulus. The effect is most pronounced if the length change immediately precedes the stimulation. In some cases, the decrease in tension with shortening can be accounted for almost entirely by a decrease in calcium release rather than changes in mechanical factors such as filament geometry. During the constant current stimulation the muscle membrane becomes more depolarized at longer muscle lengths than at the shorter muscle lengths. Under voltage clamp conditions, when the membrane potential is kept constant during stimulation, there is little length dependence of calcium release. Thus, the effect of length on calcium release is mediated through a change in membrane properties, rather than an effect on a subsequent step in excitation-contraction coupling. Stretch causes the unstimulated fiber membrane to depolarize by about l mV while release causes the fiber membrane to hyperpolarize by about the same amount. The process causing this change in potential has an equilibrium potential nearly 10 mV hyperpolarized from the resting level. This change in resting membrane potential with length may account for the length dependence of calcium release.