Injury and repair of the soleus muscle after electrical stimulation of the sciatic nerve in the rat.

To study injury and subsequent changes in skeletal muscles, the rat sciatic nerve was electrically stimulated at 50 Hz and muscle contraction was induced for 30 min. Muscle damage was classified into five types (hypercontraction, hyperstretching, Z band disorders, misalignment of myofilament and regions of scarce myofilaments) by electron microscopy and quantified by ultrastructural assessment. After electrical nerve stimulation, the percentages of the injured areas of the soleus muscle were 18.8 +/- 15.8% (mean +/- SD) at 0 h, 9.7 +/- 1.0% at 6 h, 22.0 +/- 23.6% at 12 h, 13.1 +/- 3.2% at 24 h, 4.9 +/- 6.0% at 3 days and 0.5 +/- 0.4% at 7 days. At 0 h, the vast majority of ultrastructural alterations were sarcomere hypercontraction. At 6 h, hypercontraction was not recognizable and sarcomere hyperstretching and Z band disarrangement constituted the major findings. At 12 h, when the injury reached its maximum, myofilament disorganization and hyperstretching were predominant. At 24 h or afterwards, the injury began to decrease and recovered to almost normal conditions by 7 days. There were very few necrotic muscle fibers in all specimens. It is considered that the muscle lesions in the present study were reversible, and recovered through changes in various types of sarcomere alterations. Z band streaming and free ribosomes were frequently found at 12 and 24 h, which may indicate repair processes rather than newly formed lesions.     

Comparison of sarcomere alterations after muscle contraction and tension loading in the rat soleus muscle

Muscle contraction induced by 30 min of continuous nerve stimulation at 50 Hz resulted in sarcomere changes of the soleus muscle in the rat in our previous study. To further investigate the cause of sarcomere alterations, the sciatic nerve was electrically stimulated intermittently for 30 min. Nerve stimulation was also conducted after cutting the tendons of the soleus, gastrocnemius and plantaris muscles in order to prevent imposing tension on these muscles as a result to their own contractions. In addition, the muscles were pulled by weights via their tendons to load high tension for 30 min without nerve stimulation. Sarcomere alterations immediately after treatments were quantified by electron microscopy. The percentages of aberrant sarcomere areas of the soleus muscle were 25.7 ± 16.4% (mean ± SD) in the group of intermittent nerve stimulation with intact tendons and 21.1 ± 35.4% in the group of tenotomy and continuous nerve stimulation, which were roughly equal to or more severe than the group of continuous nerve stimulation with intact tendons (18.8 ± 15.8%) in our previous study. Sarcomere alterations consisted mainly of hypercontraction in these groups. Almost all sarcomere changes in the tension-loaded (pulled) soleus muscles were scarce myofilaments (1.7 ± 1.0% by 600 g; 4.5 ± 2.9% by 1200 g), and hypercontraction was not observed. These findings indicate that neither high tension nor a decrease of muscle blood flow during continuous contraction seems to be the primary cause of sarcomere alterations in the present study. There are probably other causes that produce aberrant sarcomeres.     

Functional maximal strength training induces neural transfer to single-joint tasks

The purpose of this study was to investigate whether neural adaptations following functional multiple-joint leg press training can induce neural adaptations to the plantar flexor muscles in a single-joint contraction task. Subjects were randomised to a maximal strength training (MST) (n = 10) or a control group (n = 9). MST consisted of 24 sessions (8 weeks) of 4 × 4 repetitions of horizontal leg press using maximal intended velocity in the concentric phase with the movement ending in a plantar flexion. Neural adaptations in the soleus and gastrocnemius medialis (GM) were assessed by surface electromyographic activity and V-waves during maximum voluntary isometric contraction (MVIC), and also by H-reflexes in the soleus during rest and 20% MVIC. One repetition maximum leg press increased by 44 ± 14% (mean ± SD; P < 0.01). Plantar flexion MVIC increased by 20 ± 14% (P < 0.01), accompanied by 13 ± 19% (P < 0.05) increase in soleus, but not GM surface electromyography. Soleus V/M_SUP increased by 53 ± 66% and in GM by 59 ± 64% (P < 0.05). Normalised soleus H-reflexes remained unchanged by training. No changes occurred in the control group. These results suggest that leg press MST can induce neural adaptations in a single-joint plantar flexion MVIC task.     

Transition in the thin-filament arrangement in rat skeletal muscle

The transition in thin-filament arrangement from tetragonal near the Z-band to trigonal in the A-band was investigated by computer-assisted analysis of thin-filament arrangement in serially cross-sectioned rat muscle. Extensor digitorum longus (EDL; fast) muscle from adult rats and adult, 9-day and 3-day neonatal soleus (slow) muscle were serially cross-sectioned from the H-zone of one sarcomere to the H-zone of an adjacent sarcomere. Thin-filament arrangement was analysed throughout the I-band, and particularly at three levels of the sarcomere: in the I-band, one section (0.06 microns) from the Z-band; six sections (0.36 microns) from the Z-band; in the A-band, two sections from the A-I junction (0.72 microns from the Z-band). Data for radical distributions and annular distributions were obtained by computer. In all muscles studies, thin-filament arrangement exhibited four-neighbour ordering throughout the I-band. Thin-filament arrangement exhibited three-neighbour ordering only in the A-band. The transition in thin-filament arrangement from four-neighbour to three-neighbour ordering occurred within 0.12 microns of the A-I junction in muscles fixed at rest length. In adult soleus that had been stretched 20% so that the A-I junction moved away from the N2-line, the transition in thin-filament arrangement occurred in the I-band within a region 0.4-0.5 microns from the Z-band. This region corresponds to the N2-line region of the I-band. Thus, the transition from four-neighbour to three-neighbour ordering occurs in the I-band independent of the thin filament-thick filament interaction. We conclude that some inherent feature of the I-band or thin filament-thin filament interaction imposes a four-neighbour ordering on thin filaments from the Z-band to the N2-line.     

Architectural analysis and predicted functional capability of the human latissimus dorsi muscle

The latissimus dorsi is primarily considered a muscle with actions at the shoulder, despite its widespread attachments at the spine. There is some dispute regarding the potential contribution of this muscle to lumbar spine function. The architectural design of a muscle is one of the most accurate predictors of muscle function; however, detailed architectural data on the latissimus dorsi muscle are limited. Therefore, the aim of this study was to quantify the architectural properties of the latissimus dorsi muscle and model mechanical function in light of these new data. One latissimus dorsi muscle was removed from each of 12 human cadavers, separated into regions, and micro‐dissected for quantification of fascicle length, sarcomere length, and physiological cross‐sectional area. From these data, sarcomere length operating ranges were modelled to determine the force–length characteristics of latissimus dorsi across the spine and shoulder ranges of motion. The physiological cross‐sectional area of latissimus dorsi was 5.6 ± 0.5 cm² and normalized fascicle length was 26.4 ± 1.0 cm, indicating that this muscle is designed to produce a moderate amount of force over a large range of lengths. Measured sarcomere length in the post‐mortem neutral spine posture was nearly optimal at 2.69 ± 0.06 μm. Across spine range of motion, biomechanical modelling predicted latissimus dorsi acts across both the ascending and descending limbs of the force–length curve during lateral bend, and primarily at or near the plateau region (where maximum force generation is possible) during flexion/extension and axial twist. Across shoulder range of motion, latissimus dorsi acts primarily on the plateau region and descending limbs of the force length curve during both flexion/extension and abduction/adduction. These data provide novel insights into the ability of the latissimus dorsi muscle to generate force and change length throughout the spine and shoulder ranges of motion. In addition, these findings provide an improved understanding of the spine and shoulder positions at which the force‐generating capacity of this muscle can become jeopardized, and consequently how this may affect its spine‐stabilizing ability.     

Energy metabolism in human calf muscle performing isometric plantar flexion superimposed by 20-Hz vibration

Vibration training is commonly expected to induce an active muscle contraction via a complex reflex mechanism. In calf muscles of 20 untrained subjects, the additional energy consumption in response to vibration superimposed on an isometric contraction was examined by ³¹P magnetic resonance spectroscopy and by near infrared spectroscopy. Subjects performed 3 min of isometric plantar flexion exercise at 40% MVC under four conditions: with (VIB) and without (CON) superimposed 20 Hz vibration at ±2 mm amplitude, both combined with or without arterial occlusion (AO). After contraction under all conditions, the decreases in oxygenated haemoglobin were not significantly different. After VIB + AO consumption of ATP was increased by 60% over CON + AO, visible by significant decreases in [PCr] and intracellular pH (P < 0.05). The additional energy consumption by vibration was not detectable under natural perfusion. Probably without AO the additional energy consumption by vibration was compensated by oxidative phosphorylation enabled by additional perfusion.     

Mechanomyographic responses in human biceps brachii and soleus during sustained isometric contraction

The purpose of this study was to elucidate the responses of the mechanomyogram (MMG) from two apparently different muscles (biceps brachii and soleus) during a sustained voluntary contraction at 50% maximum voluntary contraction. The MMG and surface electromyogram (EMG) were recorded from human biceps brachii and soleus during sustained elbow flexion and plantar flexion, respectively. Results indicated that the slope coefficient of rise in EMG amplitude as a function of time for the biceps was significantly greater than that for the soleus (P<0.001). On the contrary, the MMG amplitude of the biceps showed a significant increase during the initial phase of sustained contraction (P<0.05); however, when exhaustion was approached the amplitude declined significantly (P<0.05). In the soleus muscle the decrease in MMG amplitude toward exhaustion occurred to a much lesser extent than that observed in the biceps. This difference could be attributed to the nature of the fusion state of the underlying muscle fibers. That is, the great extent of fusion observed in the biceps may be as a result of a greater quantity of fatigable motor units. In addition, the absence of MMG reduction in the soleus would indicate the absence of fatigue-induced slowing of contractile machinery and/or the lack of full activation (tetanus) of muscle fibers even at the exhaustion phase of plantar flexion.     

The influence of muscle length on the fatigue-related reduction in joint range of motion of the human dorsiflexors

The fatigue-related reduction in joint range of motion (ROM) during dynamic contraction tasks may be related to muscle length-dependent alterations in torque and contractile kinetics, but this has not been systematically explored previously. Twelve young men performed a repetitive voluntary muscle shortening contraction task of the dorsiflexors at a contraction load of 30% of maximum voluntary isometric contraction (MVC) torque, until total 40° ROM had decreased by 50% at task failure (POST) to 20° ROM. At both a short (5° dorsiflexion) and long muscle length (35° plantar flexion joint angle relative to a 0° neutral ankle joint position), voluntary activation, MVC torque, and evoked tibialis anterior contractile properties of a 52.8 Hz high-frequency isometric tetanus [peak evoked torque, maximum rate of torque development (MRTD), maximum rate of relaxation (MRR)] were evaluated at baseline (PRE), at POST, and up to 10 min of recovery. At POST, we measured similar fatigue-related reductions in torque (voluntary and evoked) and slowing of contractile kinetics (MRTD and MRR) at both the short and long muscle lengths. Thus, the fatigue-related reduction in ROM could not be explained by length-dependent fatigue. Although torque (voluntary and evoked) at both muscle lengths was depressed and remained blunted throughout the recovery period, this was not related to the rapid recovery of ROM at 0.5 min after task failure. The reduction in ROM, however, was strongly related to the reduction in joint angular velocity (R ²  = 0.80) during the fatiguing task, although additional factors cannot yet be overlooked.     

Calf and shin muscle oxygenation patterns and femoral artery blood flow during dynamic plantar flexion exercise in humans

The effects of dynamic plantar flexion exercise [40, 60, and 80 contractions·min^–1 (cpm)] on calf and shin muscle oxygenation patterns and common femoral artery blood flow ( ) were examined in six female subjects [mean age 21 (SD 1) years] who exercised for 1 min at 33% of their maximal voluntary contraction at ankle angles between 90° and 100°. Spatially resolved near-infrared spectroscopy was used to measure medial gastrocnemius, lateral soleus (synergist) and anterior tibialis (antagonist) muscle oxygen saturation (SO₂, %). was measured by ultrasound Doppler. The SO₂ changed significantly only in the medial gastrocnemius and its decrease (up to about 30%) was independent of the contraction frequencies examined. The increase in , at the end of exercise, was highest at 80 cpm. When the exercise at 60 cpm was prolonged until exhaustion [mean 2.7 (SD 1.1) min], medial gastrocnemius SO₂ decreased, reaching its minimal value [mean 30 (SD 10)%] within the 1st min, and had partially recovered before the end of the exercise with concomitant increases in total haemoglobin content and . These results suggest that the medial gastrocnemius is the muscle mostly involved in dynamic plantar flexion exercise and its oxygen demand with increases in contraction frequency and duration is associated with an up-stream increase in . Electronic Publication     

Hypertrophy of mature Xenopus muscle fibres in culture induced by synergy of albumin and insulin

The aim of this study was to investigate effects of albumin and insulin separately as well as in combination on mature muscle fibres during long-term culture. Single muscle fibres were dissected from m. iliofibularis of Xenopus laevis and attached to a force transducer in a culture chamber. Fibres were cultured in a serum-free medium at slack length (mean sarcomere length 2.3 μm) for 8 to 22 days. The medium was supplemented with (final concentrations): (1) bovine insulin (6 nmol/L or 200–600 nmol/L), (2) 0.2% bovine albumin or (3) 0.2% bovine albumin in combination with insulin (120 nmol/L). In culture medium with insulin, 50% of the muscle fibres became in-excitable within 7–12 days, whereas the other 50% were stable. Caffeine contractures of in-excitable muscle fibres produced 80.4 ± 2.4% of initial peak tetanic force, indicating impaired excitation–contraction (E–C) coupling in in-excitable fibres. In the presence of albumin, all cultured muscle fibres were stable for at least 10 days. Muscle fibres cultured in medium with insulin or albumin exclusively did not hypertrophy or change the number of sarcomeres in series. In contrast, muscle fibres cultured with both albumin and insulin showed an increase in tetanic force and fibre cross-sectional area of 19.6 ± 2.8% and 32.5 ± 4.9%, respectively, (means ± SEM.; P = 0.007) after 16.3 ± 1.7 days, whereas the number of sarcomeres in series remained unchanged. We conclude that albumin prevents muscle fibre damage and preserves E–C coupling in culture. Furthermore, albumin is important in regulating muscle fibre adaptation by a synergistic action with growth factors like insulin.     

Evidence for reduced efficacy of the Ia-pathway during shortening plantar flexions with increasing effort

To determine whether the soleus (SOL) H-reflex is modulated during shortening contractions in a manner that has been observed for isometric contractions, SOL H-reflexes and M-waves were elicited via percutaneous electrical stimulation to the tibial nerve at an intensity that evoked an H-reflex at 50% of its maximum in 11 healthy subjects. Paired electrical stimuli were delivered as the ankle angle passed through 90° at an interval of 400 ms while the subject performed shortening contractions at levels of plantar flexion torque ranging between 2 and 30% of that during a maximal voluntary contraction (MVC). H-reflexes were also recorded during the performance of isomeric contractions of plantar flexors at similar levels of plantar flexion torque and at the same joint angle (muscle length) in an additional five healthy subjects. Correlations were examined between the peak-to-peak amplitude of the first H-reflexes, M-waves and plantar flexion torques in both protocols. It was revealed that no significant correlation was found between the SOL H-reflex and increasing plantar flexion torque during shortening contractions (ρ = −0.07, P = 0.15), while a strong positive correlation was observed for the isometric conditions (ρ = 0.99, P < 0.01). No significant change was observed in the SOL M-wave for either contraction type. Furthermore, the H-reflexes elicited via paired stimuli with the same background activity in voluntary shortening contractions showed almost identical amplitudes, suggesting that the level of homosynaptic post-activation depression did not change in response to the varying levels of activation in voluntary shortening contractions. Therefore, the lack of increase in the H-reflex during shortening contractions at increasing intensities is possibly due to a centrally regulated increase in presynaptic inhibition. Such a downward modulation of the reflex suggests that Ia-excitatory input onto the SOL motoneurone pool needs to be reduced during the performance of shortening contractions.     

The Z-band lattice in skeletal muscle before,during and after tetanic contraction

Summary Electron micrographs and optical diffraction patterns of the Z-band were studied in rat soleus muscle fixed before, during, and after tetanic contraction. We compared the morphology (small square or basketweave pattern) and dimensions of the Z-lattice of control and tetanized muscles near rest length. Z-bands of muscle fixed at rest and of muscle allowed to rest after a tetanic contraction exhibited the small square pattern. Z-bands from muscle fixed during tetanic contraction exhibited the basketweave pattern. Concomitant with the transition to basketweave, we observed an average increase of 20% in spacing between the axial filaments of the Z-lattice. Optical diffraction measurements of the A-bandd ₁₀ spacing revealed that the Z/A ratio remained constant during the transition. We have modelled the small square to basketweave transformation as resulting from a change of curvature of constant length cross-connecting Z-filaments when the axial filaments increase their separation.     

Changes in human skeletal muscle length during stimulated eccentric muscle actions

Following eccentric exercise, increases in muscle length alter the length-tension relation of skeletal muscle. However, its unclear if this change occurs during eccentric exercise. Therefore, 70 eccentric actions of the knee extensors of one leg (with superimposed electrical stimulation) were performed at 100°/s, from full extension to full flexion. Angle-specific eccentric force was recorded throughout. Force decreased at all angles although this was non-uniform. At 70°, force decreased by 25%, whereas at 130°, force decreased by 41%. Initial peak force was recorded at 100° (590 ± 232 N); the exercise bout induced a 21% decrease in peak force and a 10° shift in the position of peak force production to 90°. The rightward shift in the muscle length-tension relation thus occurred during eccentric exercise, where greater force loss at short muscle lengths suggested an eccentric-induced over-stretching of sarcomeres.     

Post-exercise facilitation of compound muscle action potentials evoked by transcranial magnetic stimulation in healthy subjects

Post-exercise facilitation (PEF) of motor evoked potentials (MEPs) was studied by transcranial magnetic stimulation in 15 healthy subjects following standardized and controlled isometric contraction of the biceps brachii muscle. PEF was highly dependent on the time delay (TD) from muscle relaxation to delivery of the magnetic stimulus and only to a minor degree on the duration of the maintained muscular contraction of 2, 4, and 6 s. In addition, PEF was unaffected by the contraction levels of 25%, 50%, and 100% of maximal voluntary contraction (MVC). There was a linear relationship between the log amplitude of the post-exercise MEPs and the TD. The time point at which PEF had vanished was calculated to be 15.2 s. In order to challenge the question whether segmental and/or suprasegmental mechanisms are primarily responsible for PEF, MEPs and H-reflexes were recorded from the soleus muscle following a sustained plantar flexion at the ankle joint in three healthy subjects. PEF of MEPs was present at a TD of 1000 ms following a sustained contraction of 6 s at a level of 50% of MVC. It was accompanied by a pronounced decrease in the soleus H-reflex amplitude at a TD of 1000 ms.     

Erratum to: Estimation of elbow flexion force during isometric muscle contraction from mechanomyography and electromyography

Mechanomyography (MMG) is the muscle surface oscillations that are generated by the dimensional change of the contracting muscle fibers. Because MMG reflects the number of recruited motor units and their firing rates, just as electromyography (EMG) is influenced by these two factors, it can be used to estimate the force exerted by skeletal muscles. The aim of this study was to demonstrate the feasibility of MMG for estimating the elbow flexion force at the wrist under an isometric contraction by using an artificial neural network in comparison with EMG. We performed experiments with five subjects, and the force at the wrist and the MMG from the contributing muscles were recorded. It was found that MMG could be utilized to accurately estimate the isometric elbow flexion force based on the values of the normalized root mean square error (NRMSE = 0.131 ± 0.018) and the cross-correlation coefficient (CORR = 0.892 ± 0.033). Although MMG can be influenced by the physical milieu/morphology of the muscle and EMG performed better than MMG, these experimental results suggest that MMG has the potential to estimate muscle forces. These experimental results also demonstrated that MMG in combination with EMG resulted in better performance estimation in comparison with EMG or MMG alone, indicating that a combination of MMG and EMG signals could be used to provide complimentary information on muscle contraction.     

Non-uniform distribution of strain during stretch of relaxed skeletal muscle fibers from rat soleus muscle

Tension and regional average sarcomere length (L_s) behavior were examined during repeated stretches of single, permeabilized, relaxed muscle fibers isolated from the soleus muscles of rats. We tested the hypothesis that during stretches of single permeabilized fibers, the global fiber strain is distributed non-uniformly along the length of a relaxed fiber in a repeatable pattern. Each fiber was subjected to eight constant-velocity stretch and release cycles with a strain of 32% and strain rate of 54% s⁻¹. Stretch-release cycles were separated by a 4.5 min interval. Throughout each stretch-release cycle, sarcomere lengths were measured using a laser diffraction technique in which 20 contiguous sectors along the entire length of a fiber segment were scanned within 2 ms. The results revealed that: (1) the imposed length change was not distributed uniformly along the fiber, (2) the first stretch-release cycle differed from subsequent cycles in passive tension and in the distribution of global fiber strain, and (3) a characteristic “signature” for the L_s response emerged after cycle 3. The findings support the conclusions that longitudinal heterogeneity exists in the passive stiffness of individual muscle fibers and that preconditioning of fibers with stretch-release cycles produces a stable pattern of sarcomere strains.     

Increased nitric oxide synthase activity and Hsp90 association in skeletal muscle following chronic exercise

Exercise training results in dynamic changes in skeletal muscle blood flow and metabolism. Nitric oxide (NO) influences blood flow, oxidative stress, and glucose metabolism. Hsp90 interacts directly with nitric oxide synthases (NOS), increasing NOS activity and altering the balance of superoxide versus NO production. In addition, Hsp90 expression increases in various tissues following exercise. Therefore, we tested the hypothesis that exercise training increases Hsp90 expression as well as Hsp90/NOS association and NOS activity in skeletal muscle. Male, Sprague–Dawley rats were assigned to either a sedentary or exercise trained group (n = 10/group). Exercise training consisted of running on a motorized treadmill for 10 weeks at 30 m/min, 5% grade for 1 h. Western blotting revealed that exercise training resulted in a 1.9 ± 0.1-fold increase in Hsp90 expression in the soleus muscle but no increase in neuronal nitric oxide synthase (nNOS), inducible nitric oxide synthase, or endothelial nitric oxide synthase (eNOS). Exercise training also resulted in a 3.4 ± 1.0-fold increase in Hsp90 association with nNOS, a 2.3 ± 0.4-fold increase association with eNOS measured by immunoprecipitation as well as a 1.5 ± 0.3-fold increase in eNOS phosphorylation at Ser-1179. Total NOS activity measured by the rate of conversion of L-[¹⁴C]arginine to L-[¹⁴C]citrulline was increased by 1.42 ± 0.9 fold in soleus muscle following exercise training compared to controls. In summary, a 10-week treadmill training program in rats results in a significant increase in total NOS activity in the soleus which may be due, in part, to increased NOS interaction with Hsp90 and phosphorylation. This interaction may play a role in altering muscle blood flow and skeletal muscle redox status.     

Post‐contractile BOLD contrast in skeletal muscle at 7 T reveals inter‐individual heterogeneity in the physiological responses to muscle contraction

Muscle blood oxygenation‐level dependent (BOLD) contrast is greater in magnitude and potentially more influenced by extravascular BOLD mechanisms at 7 T than it is at lower field strengths. Muscle BOLD imaging of muscle contractions at 7 T could, therefore, provide greater or different contrast than at 3 T. The purpose of this study was to evaluate the feasibility of using BOLD imaging at 7 T to assess the physiological responses to in vivo muscle contractions. Thirteen subjects (four females) performed a series of isometric contractions of the calf muscles while being scanned in a Philips Achieva 7 T human imager. Following 2 s maximal isometric plantarflexion contractions, BOLD signal transients ranging from 0.3 to 7.0% of the pre‐contraction signal intensity were observed in the soleus muscle. We observed considerable inter‐subject variability in both the magnitude and time course of the muscle BOLD signal. A subset of subjects (n = 7) repeated the contraction protocol at two different repetition times (T_R: 1000 and 2500 ms) to determine the potential of T₁‐related inflow effects on the magnitude of the post‐contractile BOLD response. Consistent with previous reports, there was no difference in the magnitude of the responses for the two T_R values (3.8 ± 0.9 versus 4.0 ± 0.6% for T_R = 1000 and 2500 ms, respectively; mean ± standard error). These results demonstrate that studies of the muscle BOLD responses to contractions are feasible at 7 T. Compared with studies at lower field strengths, post‐contractile 7 T muscle BOLD contrast may afford greater insight into microvascular function and dysfunction.     

The effect of sarcomere length on triad locationin intact feline caudofemoralis muscle fibres

The location of triads within a mammalian skeletal muscle sarcomere has traditionally been defined as ‘at the A-I junction’. We attempted to verify this statement by examining systematically the location of triads within the sarcomere over the physiological range of sarcomere lengths. This study was conducted using intact feline muscle fibres from caudofemoralis – an exclusively fast-twitch muscle from the hindlimb. Our results in intact fibres indicate that the distance between the Z-band and triad (ZT) is relatively constant over the range of sarcomere lengths (SLs) examined in this study (1.8–3.4μm). The slope between ZT and SL was measured to be 0.06 ± 0.01 (r=0.36, p < 0.001) while the slope between the M-line to triad distance (MT) and SL was measured to be 0.440.01 (r > 0.9, p < 0.001). The mean ZT was 0.52 ± 0.07μm, which corresponds to a triad location approximately halfway along the thin filaments. These results do not support the traditional statement regarding triad location. Nor do these results support a similar recent study conducted using chemically skinned muscle fibres from rat extensor digitorum longus (also a homogeneously fast-twitch muscle of the hindlimb), in which a slope of 0.25 was observed between ZT and SL (r > 0.9, p < 0.01). These results are, however, in qualitative agreement with results using intact fibres from fast-twitch rat semitendinosus. Based upon known morphology, we suggest that the only structure supporting triad position is the SR itself, and that a non-homogeneous distribution of the SR within the sarcomere might be responsible for maintaining triad location near the mid-region of the thin filaments. We also suggest that there might be optimal design reasons for locating the triads at the mid-region of the thin filaments. This revised version was published online in July 2006 with corrections to the Cover Date.