Inversion–eversion moment arms of gastrocnemius and tibialis anterior measured in vivo

In this study, the frontal plane moment arms of tibialis anterior (TA) and the lateral and medial heads of gastrocnemius (LG and MG) were determined using ultrasonography of ten healthy subjects. Analysis of variance was performed to investigate the effects of frontal plane angle, muscle activity, and plantarflexion angle on inversion–eversion moment arm for each muscle. The moment arms of each muscle were found to vary with frontal plane angle (all p<0.001). TA and LG exhibited eversion moment arms when the foot was everted, but MG was found to have a slight inversion moment arm in this position. As the ankle rotated from 0° to 20° inversion, the inversion moment arm of each increased, indicating that the three muscles became increasingly effective inverters. In neutral position, the inverter moment arm of MG was greater than that of LG (p=0.001). Muscle activity had a significant effect on both LG and MG moment arm at all frontal plane positions (all p0.005). These results demonstrate the manner in which frontal plane moment arms of gastrocnemius and TA differ across the frontal plane range of motion in healthy subjects. This method for assessing muscle action in vivo used in this study may prove useful for subject-specific planning of surgical treatments for frontal plane foot and ankle deformities.     

Optimal pennation angle of the primary ankle plantar and dorsiflexors: variations with sex, contraction intensity, and limb

Ultrasonography was used to measure the pennation angle of the human tibialis anterior (TA), lateral gastrocnemius (LG), medial gastrocnemius (MG), and soleus (Sol). The right and left legs of 8 male and 8 female subjects were tested at rest and during maximum voluntary contraction (MVC). Joint angles were chosen to control muscle tendon lengths so that the muscles were near their optimal length within the length-tension relationship. No differences in pennation angle were detected between the right and left legs. Another consistent finding was that the pennation angle at MVC was significantly greater than at rest for all muscles tested. Optimal pennation angles for the TA, MG, and Sol were significantly greater for the men than for the women. Optimal pennation angles for the TA, LG, MG, and Sol for the male subjects were 14.3 degrees, 23.7 degrees, 34.6 degrees, and 40.1 degrees respectively, whereas values of 12.1 degrees, 16.3 degrees, 27.3 degrees, and 26.3 degrees were recorded for the female subjects. The results of this study suggest the following: (1) similar values for pennation angle can be used for the right and left TA, LG, MG, and Sol; (2) pennation angle is significantly greater at MVC than at rest for all muscles tested; and (3) sex-specific values for optimal pennation angle should be used when modeling the force-generating potential of the primary muscles responsible for ankle plantar and dorsiflexion.     

Electromechanical delay estimated by using electromyography during cycling at different pedaling frequencies

The purpose of this study was to propose a new method that can be used to calculate electromechanical delay (EMD) without the measurement of forces. A secondary purpose, as an example of the importance of measuring EMD, was to predict muscle force development events based on the EMG activity of selected muscles during cycling at different pedaling frequencies. EMD was estimated using newly derived equations based on activation dynamics hypothesis. Tibialis anterior (TA) and soleus (SL) muscles of 16 male participants were studied while subjects pedaled at targeted cadences of 60, 80, and 100 revolutions per minute. The estimated EMDs of TA and SL were significantly different from each other with means of 68.1 and 88.7 ms, respectively. The average crank angle for the initiation and time to peak TA contraction was estimated at 75±35° and 26±15° before the crank reached top-dead-center (TDC), while the contraction ended at 31±19° after the TDC on average. The projected starting, peak and end angles of SL contraction activity were 45±18°, 123±13°, and 218±35° after the TDC, respectively. There was no difference among different pedaling cadences observed for these mechanical events. The proposed method was proven to be effective in studying EMD and estimate muscle contraction patterns during cycling.     

The shear modulus of lower-leg muscles correlates to intramuscular pressure

Shear wave elastography (SWE) is emerging as an innovative tool to evaluate muscle properties and function. It has been shown to correlate with both passive and active muscle forces, and is sensitive to physiological processes and pathological conditions. Similarly, intramuscular pressure (IMP) is an important parameter that changes with passive and active muscle contraction, body position, exercise, blood pressure, and several pathologies. Therefore, the objective of this study was to quantify the dependency of shear modulus within the lower-leg muscles on IMP in healthy individuals. Nineteen healthy individuals (age: Mean age ± SD, 23.84 ± 6.64 years) were recruited. Shear modulus was measured using ultrasound SWE on the tibialis anterior (TA) and peroneus longus (PL) muscles using pressure cuff inflation around the thigh at 40 mmHg, 80 mmHg, and 120 mmHg. Changes in IMP were verified using a catheter connected to a blood pressure monitor. It was found that IMP was correlated to TA and PL shear modulus (spearman's rank correlation = 0.99 and 0.99, respectively). Applying a gradual increase of cuff pressure from 0 to 120 mmHg increased the shear modulus of the TA and PL muscles from 15.83 (2.46) kPa to 21.88 (4.33) kPa and from 9.64 (1.97) kPa to 12.88 (5.99) kPa, respectively. These results demonstrate that changes of muscle mechanical properties are dependent on IMP. This observation is important to improve interpretation of ultrasound elastograms and to potentially use it as a biomarker for more accurate diagnosis of pathologies related to increased IMP.     

Can pennation angles be predicted from EMGs for the primary ankle plantar and dorsiflexors during isometric contractions?

Ultrasonography was used to measure pennation angle and electromyography (EMG) to record muscle activity of the human tibialis anterior (TA), lateral gastrocnemius (LG), medial gastrocnemius (MG), and soleus (SOL) muscles during graded isometric ankle plantar and dorsiflexion contractions done on a Biodex dynamometer. Data from 8 male and 8 female subjects were collected in increments of approximately 25% of maximum voluntary contraction (MVC) ranging from rest to MVC. A significant positive linear relationship (p<0.05) between normalized EMG and pennation angle for all muscles was observed when subject specific pennation angles at rest and MVC were included in the analysis. These were included to account for gender differences and inter-subject variability in pennation angle. The coefficient of determination, R(2), ranged between 0.76 for the TA and 0.87 for the SOL. The EMG-pennation angle relationships have ramifications for use in EMG-driven models of muscle force. The regression equations can be used to characterize fiber pennation angle more accurately and to determine how it changes with contraction intensity, thus providing improved estimates of muscle force when using musculoskeletal models.     

Changes in human skeletal muscle architecture and function induced by extended spaceflight

The aim of this study was to quantitatively describe the relationships between joint angles and muscle architecture (lengths (L_f) and angles (Θ_f) of fascicles) of human triceps surae [medial (MG) and lateral (LG) gastrocnemius and soleus (SOL) muscles] in vivo for three men-cosmonaut after long-duration spaceflight. Sagittal sonographs of MG, LG, SOL were taken at ankle was positioned at 15° (dorsiflexion), 0° (neutral position), +15°, and +30° (plantarflexion), with the knee at 90° at rest and after a long-duration spaceflight. At each position, longitudinal ultrasonic images of the MG and LG and SOL were obtained while the cosmonauts was relaxed from which fascicle lengths and angles with respect to the aponeuroses were determined. After space flight plantarflexor force declined significantly (26%; p < 0.001). The internal architecture of the GM, and LG, and SOL muscle was significantly altered. In the passive condition, L_f changed from 45, 53, and 39 mm (knee, 0°, ankle, −15°) to 26, 33, and 28 mm (knee, 90° ankle, 30°) for MG, LG, and SOL, respectively. Different lengths and angles of fascicles, and their changes by contraction, might be related to differences in force-producing capabilities of the muscles and elastic characteristics of tendons and aponeuroses. The three heads of the triceps surae muscle substantially differ in architecture, which probably reflects their functional roles. Differences in fiber length and pennation angle that were observed among the muscles and could be associated with differences in force production and in elastic properties of musculo-tendinous complex and aponeuroses.     

Shear modulus of multifidus and longissimus muscles measured using shear wave elastography correlates with muscle activity,but depends on image quality

Shear wave elastography (SWE) estimates shear modulus in muscle. This is interpreted as an index of muscle stiffness, but depends on muscle characteristics. This study evaluated relationship between shear modulus and myoelectric activity of lumbar multifidus and longissimus muscles to assess its validity. Intramuscular electromyography (EMG) of multifidus (deep [DM], superficial [SM] fibres) at L4/5, longissimus [LG] at L2, were recorded in nine healthy participants. Participants performed isometric trunk extension in side-lying from 0 to 30% maximal voluntary contraction (MVC) with EMG amplitude feedback. Using SWE, two regions of interest (ROI) were investigated in each muscle. Generally, shear modulus was moderately correlated with root mean squared (RMS) EMG (r = 0.50–0.78). Univariate and multiple regression analyses showed ultrasound/SWE features of ‘B-mode quality’ (24.5%), ‘%Void pixels’ (17.9%) and ‘Connective tissue’ (16.2%) explained most variation in the shear modulus/EMG relationship. Regression prediction scores generated using imaging features were correlated with r-coefficients of shear modulus/EMG relationship. When analysis was restricted to high quality data (i.e., regression prediction score above an a priori defined threshold), the shear modulus/EMG relationship increased to r = 0.70–0.96. Although a linear relationship between shear modulus/EMG was confirmed, supporting validity of SWE measures in anatomically distinct back muscles, this depends on image quality.     

Muscle-driven finite element simulation of human foot movements

This paper describes a finite element scheme for realistic muscle-driven simulation of human foot movements. The scheme is used to simulate human ankle plantar flexion. A three-dimensional anatomically detailed finite element model of human foot and lower leg is developed and the idea of generating natural foot movement based entirely on the contraction of the plantar flexor muscles is used. The bones, ligaments, articular cartilage, muscles, tendons, as well as the rest soft tissues of human foot and lower leg are included in the model. A realistic three-dimensional continuum constitutive model that describes the biomechanical behaviour of muscles and tendons is used. Both the active and passive properties of muscle tissue are accounted for. The materials for bones and ligaments are considered as homogeneous, isotropic and linearly elastic, whereas the articular cartilage and the rest soft tissues (mainly fat) are defined as hyperelastic materials. The model is used to estimate muscle tissue deformations as well as stresses and strains that develop in the lower leg muscles during plantar flexion of the ankle. Stresses and strains that develop in Achilles tendon during such a movement are also investigated.     

Muscle-driven finite element simulation of human foot movements

This paper describes a finite element scheme for realistic muscle-driven simulation of human foot movements. The scheme is used to simulate human ankle plantar flexion. A three-dimensional anatomically detailed finite element model of human foot and lower leg is developed and the idea of generating natural foot movement based entirely on the contraction of the plantar flexor muscles is used. The bones, ligaments, articular cartilage, muscles, tendons, as well as the rest soft tissues of human foot and lower leg are included in the model. A realistic three-dimensional continuum constitutive model that describes the biomechanical behaviour of muscles and tendons is used. Both the active and passive properties of muscle tissue are accounted for. The materials for bones and ligaments are considered as homogeneous, isotropic and linearly elastic, whereas the articular cartilage and the rest soft tissues (mainly fat) are defined as hyperelastic materials. The model is used to estimate muscle tissue deformations as well as stresses and strains that develop in the lower leg muscles during plantar flexion of the ankle. Stresses and strains that develop in Achilles tendon during such a movement are also investigated.     

Different effects of ATP on the contractile activity of mice diaphragmatic and skeletal muscles

Apart from acetyl-choline (Ach), adenosine-5′-trisphosphate (ATP) is thought to play a role in neuromuscular function, however little information is available on its cellular physiology. As such, effects of ATP and adenosine on contractility of mice diaphragmatic and skeletal muscles (m. extensor digitorum longa—MEDL) have been investigated in in vitro experiments. Application of carbacholine (CCh) in vitro in different concentrations led to pronounced muscle contractions, varying from 9.15 ± 4.76 to 513.13 ± 15.4 mg and from 44.65 ± 5.01 to 101.46 ± 9.11 mg for diaphragm and MEDL, respectively. Two hundred micromolars of CCh in both muscles caused the contraction with the 65% (diaphragm) to 75% (MEDL) of maximal contraction force—this concentration was thus used in further experiments. It was found that application of ATP (100 μM) increased the force of diaphragmatic contraction caused by CCh (200 μM) from 335.2 ± 51.4 mg (n = 21) in controls to 426.5 ± 47.8 mg (n = 10; P < 0.05), but decreased the contractions of MEDL of CCh from 76.6 ± 6.5 mg (n = 26) in control to 40.2 ± 9.0 mg (n = 8; P < 0.05). Application of adenosine (100 μM) had no effect on CCh-induced contractions of these muscles.

Resting membrane potential (MP) measurements using sharp electrodes were done at 10, 20 and 30 min after the application of ATP and adenosine. Diaphragm showed depolarization from 75 ± 0.6 down to 63.2 ± 1.05, 57.2 ± 0.96 and 53.6 ± 1.1 mV after 10, 20 and 30 min of exposition, respectively (20 fibers from 4 muscles each, P < 0.05 in all three cases). Adenosine showed no effect on diaphragmatic MP. Both agents were ineffective in case of MEDL.

The effects of ATP in both tissues were abolished by suramin (100 μM), a P2-receptor antagonist, and chelerythrin (50 μM), a specific protein-kinase C (PKC) inhibitor, but were not affected by 1H-[1,2,4]-oxadiazolo-[4,3-]-quinoxalin-1-one (ODQ, 1 μM), a guanylyl-cyclase inhibitor, or by adenosine-3,5-monophosphothioate (Rp-cAMP, 1 μM), a protein-kinase A (PKA) inhibitor.

Besides the action on contractile activity, ATP (100 μM) led to a significant (P < 0.001) depolarization of diaphragm muscle fibers from 74.5 ± 2.3 down to 64 ± 2.1, 58.2 ± 2.2 and 54.3 ± 2.4 mV after 10, 20 and 30 min of incubation, respectively. Incubation of MEDL with the same ATP concentration showed no significant change of MP.

Denervation of the muscles for 28 days led to a decrease of CCh-induced contractions of diaphragm down to 171.1 ± 34.5 mg (n = 11, P < 0.05), but increased the contractile force of MEDL up to 723.9 ± 82.3 mg (n = 9, P < 0.01). Application of ATP elevated the contractility of denervated diaphragm caused by CCh up to normal values (311.1 ± 79.7 mg, n = 6, P > 0.05 versus control), but did not significantly affect of contractility of MEDL, which became 848.1 ± 62.7 mg (n = 6).

These results show that the effects of ATP on both diaphragmatic and skeletal muscles are mediated through P2Y receptors coupled to chelerytrin-sensitive protein-kinase C.     

Fatigue of the plantar intrinsic foot muscles increases navicular drop

Our purpose was to assess the effect of foot intrinsic muscle fatigue on pronation, as assessed with navicular drop, during static stance. Twenty-one healthy young adults participated. Navicular drop was measured before and after fatiguing exercise of the plantar foot intrinsic muscles. Surface electromyography of the abductor hallucis muscle was recorded during maximum voluntary isometric contractions (MVIC) in order to find the baseline median frequency (MedF). Subjects then performed sets of 75 repetitions of isotonic flexion contractions of the intrinsic foot muscles against a 4.55 kg weight on a custom pulley system. After each set an MVIC was performed to track shifts in MedF. After a MedF shift of at least 10%, navicular drop measurements were repeated. Subjects exhibited 10.0 ± 3.8 mm of navicular drop at baseline and 11.8 ± 3.8 mm after fatigue (p < 0.0005). The change in navicular drop was significantly correlated with change in MedF (r = .47, p = .03). The intrinsic foot muscles play a role in support of the medial longitudinal arch in static stance. Disrupting the function of these muscles through fatigue resulted in an increase in pronation as assessed by navicular drop.     

Ankle flexor muscles in the cat: Length-active tension and muscle unit properties as related to locomotion

The mechanical properties of the whole muscle and fast-twitch muscle units of the cat hindlimb pretibial flexors have been explored and related to normal locomotion. Tibialis anterior (TA) is parallel-fibered and functionally crosses a single joint, the ankle, whereas extensor digitorum longus (EDL) is pinnate and spans the ankle, knee, metatarsophalangeal and interphalangeal joints. The active tetanic tension of TA remains near its peak value over a range of muscle lengths associated with normal ankle movement. In contrast, the length-tension curve of EDL is sharply peaked. However, normal corollary action of the knee, ankle and metatarsophalangeal joints during stepping minimizes EDL's excursion and maintains it at or near a length optimal for peak tension development. EDL is capable of producing synchronous but sterotyped digit and ankle movements while TA provides for independent ankle flexion at all relevant joint angles. The mechanical properties of 84 TA and 98 EDL fast-twitch muscle units were studied by measuring twitch contraction time (≤45 msec), peak tetanic tension, response to repetitive stimulation, and contractile fatigue resistance during electrical stimulation of single alpha axons, functionally isolated from ventral root filaments. These mechanical properties were essentially similar for both muscles with the exception of mean peak tetanic tension which was 30% lower for TA units (14 gm-wt) than for EDL units (20 gm-wt). A high proportion of units in both muscles demonstrated fatigue resistance which is reflective of the repetitive, phasic demand upon these muscles during locomotion.     

Ageing and dexamethasone associated sarcopenia: peculiarities of regeneration

The purpose of this study was to assess the development of ageing- and glucocorticoid-related sarcopenia on the level of myofibrillar apparatus, paying attention to the synthesis (SR) and degradation rate (DR) of contractile proteins, muscle strength, and daily motor activity. We also wanted to test the effect of ageing and dexamethasone (Dex) excess on the regeneration peculiarities of skeletal muscle autografts. Four and 30-month-old male rats of the Wistar strain were used. Ageing associated sarcopenia was calculated from gastrocnemius muscle relative mass decrease (from 5.6 ± 0.08 to 3.35 ± 0.04; p < 0.001). The SR of MyHC in old rats was 30% and actin 23% lower than in young rats. Dex treatment decreased SR of two main contractile proteins significantly in both age groups (p < 0.001) and increased DR during ageing from 2.11 ± 0.15 to 4.09 ± 0.29%/day (p < 0.001). Hindlimb grip strength in young rats was 5.90 ± 0.35 N/100 g bw and 2.64 ± 0.2 N/100 g bw (p < 0.001) in old rats.

Autografts of old rats have a higher content of adipose tissue 14.9 ± 1.1% in comparison with young rats 6.8 ± 0.51% (p < 0.001) and less muscle tissue 39.8 ± 2.6% and 48.3 ± 2.8%, respectively (p < 0.05).

Both, ageing and dex-caused sarcopenic muscles have diminished capacity for regeneration.     

Aging effects on posture-related modulation of stretch reflex excitability in the ankle muscles in humans

The purpose of this study was to examine the effects of aging on posture-related changes of the stretch reflex excitability in the ankle extensor, soleus (SOL), and flexor, tibialis anterior (TA) muscles. Fourteen neurologically normal elderly (mean 68 ± 6 years) and 12 young (mean 27 ± 3 years) subjects participated. Under two postural conditions, upright standing (STD) and sitting (SIT), stretch reflex electromyographic (EMG) responses in the SOL/TA muscle were elicited by imposing rapid ankle dorsi-/plantar-flexion. Under the SIT condition, subjects were asked to keep the SOL background EMG level, which is identical to that under the STD condition. In the SOL muscle, both groups showed significant enhancement of the short-latency stretch reflex (SLR) response when the posture changed from SIT to STD. In the TA muscle, the young group showed significant enhancement of the middle- (MLR) and long-latency stretch reflex (LLR) when the posture changed from SIT to STD; no such modulation was observed in the elderly group. Since the TA stretch reflex responses under the STD condition were comparable in the young and elderly groups, the lack of posture-related modulation of the TA muscle in the elderly group might be explained by augmented stretch reflex excitability under the SIT condition. The present results suggest that the (1) SOL SLR responses are modulated both in the young and elderly subjects when the posture is changed from SIT to STD, (2) TA MLR and LLR responses are not modulated in the elderly subjects when the posture is changed from SIT to STD, while each response is same between the young and elderly in STD, and (3) the effect of aging on the posture-related stretch reflex differs in the SOL and TA muscles.     

Functional electrical stimulation using microstimulators to correct foot drop: a case study

This paper presents a case study that tested the feasibility and efficacy of using injectable microstimulators (BIONs) in a functional electrical stimulation (FES) device to correct foot drop. Compared with surface stimulation of the common peroneal nerve, stimulation with BIONs provides more selective activation of specific muscles. For example, stimulation of the tibialis anterior (TA) and extensor digitorum longus (EDL) muscles with BIONs produces ankle flexion without excessive inversion or eversion of the foot (i.e., balanced flexion). Efficacy was assessed using a 3-dimensional motion analysis of the ankle and foot trajectories during walking with and without stimulation. Without stimulation, the toe on the affected leg drags across the ground. BION stimulation of the TA muscle and deep peroneal nerve (which innervates TA and EDL) elevates the foot such that the toe clears the ground by 3 cm, which is equivalent to the toe clearance in the less affected leg. The physiological cost index (PCI) measured effort during walking. The PCI equals the change in heart rate (from rest to activity) divided by the walking speed; units are beats per metre. The PCI is high without stimulation (2.29 +/- 0.37, mean +/- SD) and greatly reduced with surface (1.29 +/- 0.10) and BIONic stimulation (1.46 +/- 0.24). Also, walking speed increased from 9.4 +/- 0.4 m/min without stimulation to 19.6 +/- 2.0 m/min with surface and 17.8 +/- 0.7 m/min with BIONic stimulation. These results suggest that FES delivered by a BION is an alternative to surface stimulation and provides selective control of muscle activation.     

Structural role of tropomyosin in muscle regulation: analysis of the x-ray diffraction patterns from relaxed and contracting muscles

Previous work has shown that there are significant differences in the X-ray diffraction patterns obtained from relaxed and contracting muscles. We show that some of these changes can be explained in terms of a small movement (~ 5 to 15 Å) of the tropomyosin molecules in the groove of the actin helix. The position of the tropomyosin in relaxed skeletal muscle is such that it might physically block or at least structurally alter the cross-bridge attachment site on actin, whereas in contracting skeletal muscle the tropomyosin moves to a position well clear of the attachment site. The movement of the tropomyosin molecules is apparently smaller in molluscan muscles during tonic contraction than in vertebrate skeletal muscle. We suggest a possible relationship between the smaller movement of the tropomyosin and the “catch” response of molluscan muscles.We also show that any increase of intensity on the 59 Å and 51 Å layer-lines is most likely to be associated with some extra mass (HMM S-1) attaching to the actin molecules. Such a change cannot be explained in terms of a change in tropomyosin structure or in the order within the thin filaments. Since changes on these two layer-lines have been observed during contraction, this provides good evidence for cross-bridge attachment to actin in contracting muscles.     

The isometric length-force models of nine different skeletal muscles.

The length-force relations of nine different skeletal muscles in the hindlimb of the cat were determined experimentally, with electrical stimulation of the sciatic nerve as the activation mode. It was shown that the active-, passive-, and total-force patterns varied widely among the muscles. The tibialis posterior (TP), medial and lateral gastrocnemius (MG, LG) and flexor digitorum longus (FDL) had a symmetric active-force curve, whereas the tibialis anterior (TA), peroneus brevis (PB), peroneus longus (PL), extensor digitorum longus (EDL), and soleus (SOL) had an asymmetric curve which exhibits about 25% of the maximal isometric force at extreme lengths. The SOL, EDL, and LG had a low-level passive force which appeared at short muscle length, whereas all other muscles exhibited initial passive force just before the optimal length. The total force was rising quasi-linearly for the SOL, whereas the other muscles exhibited an intermediate plateau about the optimal length. The LG and FDL had a substantial but temporary intermediate dip in the total force as the muscle was elongated past the optimal length. The elongation range of the various muscles also varied, ranging from +/- 15 to +/- 30% of the optimal length. The elongation range was symmetric for the FDL, LG, MG, TP, SOL, and EDL, and asymmetric for the PL, PB, and TA, being -12 to + 17%, -12 to + 17%, and -35 to + 12%, respectively. Two different models which incorporate muscle architecture were successfully fitted to the experimental data of the muscles except for the MG and TA. The architecture of these two muscles is highly nonhomogeneous and contains compartments with two pennation patterns or two different optimal lengths. New models, which add spatially and temporally the individual characteristics of each compartment of the muscles, were constructed for these two muscles. The new models demonstrated high correlation to the experimental data obtained from the MG and TA. It was concluded that the length-force relation varies widely among various skeletal muscles and is probably dependent on the primary function of the muscle in the context of integrated movement; this is a manifestation of architectural factors such as fiber pennation pattern and angle, cross-sectional area, ratio of muscle to tendon length, distribution of the fiber length within the muscle and compartmental pennation.     

Muscle activation during cycling at different cadences: Effect of maximal strength capacity

The purpose of this study was to examine the influence of maximal strength capacity on muscle activation, during cycling, at three selected cadences: a low cadence (50 rpm), a high cadence (110 rpm) and the freely chosen cadence (FCC). Two groups of trained cyclists were selected on the basis of the different maximal isokinetic voluntary contraction values (MVCi) of their lower extremity muscles as follow: F_min (lower MVCi group) and F_max (higher MVCi group). All subjects performed three 4-min cycling exercises at a power output corresponding to 80% of the ventilatory threshold under the three cadences. Neuromuscular activity of vastus lateralis (VL), rectus femoris (RF) and biceps femoris (BF) was studied quantitatively (integrated electromyography, IEMG) and qualitatively (timing of muscle bursts during crank cycle). Cadence effects were observed on the EMG activity of VL muscle and on the burst onset of the BF, VL and RF muscles. A greater normalized EMG activity of VL muscle was observed for the F_min group than the F_max group at all cadences (respectively F_min vs. F_max at 50 rpm: 17 ± 5% vs. 38 ± 6%, FCC: 22 ± 7% vs. 44 ± 5% and 110 rpm: 21 ± 6% vs. 45 ± 6%). At FCC and 110 rpm, the burst onset of BF and RF muscles of the F_max group started earlier in the crank cycle than the F_min group These results indicate that in addition to the cadence, the maximal strength capacity influences the lower extremity muscular activity during cycling.     

Fiber architecture of canine abdominal muscles.

During respiration, abdominal muscles experience loads, not only in the muscle-fiber direction but also transverse to the fibers. We wondered whether the abdominal muscles exhibit a fiber architecture that is similar to the diaphragm muscle, and, therefore, we chose two adjacent muscles: the internal oblique (IO), with about the same muscle length as the diaphragm, and the transverse abdominis (TA), which is twice as long as the diaphragm. First, we used acetylcholinesterase staining to examine the distribution of neuromuscular junctions on both surfaces of the TA and IO muscles in six dogs. A maximum of four irregular bands of neuromuscular junctions crossed the IO, and as many as six bands crossed the TA, which is consistent with a discontinuous fiber architecture. In six additional dogs, we examined fiber architecture of these muscles by microdissecting 103 fascicles from the IO and 139 from the TA. Each fascicle contained between 20 and 30 muscle fibers. The mean length of nonspanning fibers (NSF) ranged from 2.8 +/- 0.3 cm in the IO to 4.3 +/- 0.5 cm in the TA, and the mean length of spanning fibers ranged from 4.3 +/- 0.5 cm in the IO to 7.6 +/- 1.4 cm in the TA. NSF accounted for 89.6 +/- 1.5% of all fibers dissected from the IO and 99.1 +/- 0.2% of all fibers dissected from the TA. The percentage of NSF with both ends tapered was 6.2 +/- 1.0 and 41.0 +/- 2.3% for IO and TA, respectively. These data show that fiber architecture in either IO or TA is discontinuous, with much more short-tapered fibers in the TA than in the IO. When abdominal muscles are submaximally activated, as during both normal expiration and maximal expiratory efforts, muscle force could be transmitted to the cell membrane and to the extracellular intramuscular connective tissue by shear linkage, presumably via structural transmembrane proteins.     

Non-uniform electromyographic activity during fatigue and recovery of the vastus medialis and lateralis muscles

The aim of the study was to investigate EMG signal features during fatigue and recovery at three locations of the vastus medialis and lateralis muscles. Surface EMG signals were detected from 10 healthy male subjects with six 8-electrode arrays located at 10%, 20%, and 30% of the distance from the medial (for vastus medialis) and lateral (vastus lateralis) border of the patella to the anterior superior spine of the pelvic. Subjects performed contractions at 40% and 80% of the maximal force (MVC) until failure to maintain the target force, followed by 20 2-s contractions at the same force levels every minute for 20 min (recovery). Average rectified value, mean power spectral frequency, and muscle fiber conduction velocity were estimated from the EMG signals in 10 epochs from the beginning of the contraction to task failure (time to task failure, mean ± SD, 70.7 ± 25.8 s for 40% MVC; 27.4 ± 16.8 s for 80% MVC) and from the 20 2 s time intervals during recovery. During the fatiguing contraction, the trend over time of EMG average rectified value depended on location for both muscles (P < 0.05). After 20-min recovery, mean frequency and conduction velocity of both muscles were larger than in the beginning of the fatigue task (P < 0.05) (supernormal values). Moreover, the trend over time of mean frequency during recovery was affected by location and conduction velocity values depended on location for both muscles (P < 0.05). The results indicate spatial dependency of EMG variables during fatigue and recovery and thus the necessity of EMG spatial sampling for global muscle assessment.