Estimation of cat medial gastrocnemius fascicle lengths during dynamic contractions

In typical muscle models, it is often assumed that the contractile element (fascicle) length depends exclusively on the instantaneous muscle-tendon length and the instantaneous muscle force. In order to test whether the instantaneous fascicle length during dynamic contractions can be predicted from muscle-tendon length and force, fascicle lengths, muscle-tendon lengths, and muscle forces were directly measured in cat medial gastrocnemii during isometric and dynamic contractions. Two theoretical muscle models were developed: model A was based on force-time data obtained during the activation phase and model D on force-time data obtained during the deactivation phase of isometric contractions. To test the models, instantaneous fascicle lengths were predicted from muscle-tendon lengths and forces during dynamic contractions that simulated cat locomotion for speeds ranging from 0.4 to 1.6m/s. The theoretically predicted fascicle lengths were compared with the experimentally measured fascicle lengths. It was found that fascicle lengths were not uniquely associated with muscle-tendon lengths and forces; that is, for a given muscle-tendon length and force, fascicle lengths varied depending on the contractile history. Consequently, models A and D differed in fascicle length predictions; model D (maximum average error=8.5%) was considerably better than model A (maximum average error=22.3%). We conclude from this study that it is not possible to predict the exact fascicle lengths from muscle-tendon lengths and forces alone, however, adequate predictions seem possible based on such a model. The relationship between fascicle length and muscle force and muscle-tendon length is complex and highly non-linear, thus, it appears unlikely that accurate fascicle length predictions can be made without some reference contractions in which fascicle length, muscle-tendon length, and force are measured simultaneously.     

In vivo determination of fascicle curvature in contracting human skeletal muscles.

Fascicle curvature of human medial gastrocnemius muscle (MG) was determined in vivo by ultrasonography during isometric contractions at three (distal, central, and proximal) locations (n = 7) and at three ankle angles (n = 7). The curvature significantly (P < 0.05) increased from rest to maximum voluntary contraction (MVC) (0.4-5.2 m(-1)). In addition, the curvature at MVC became larger in the order dorsiflexed, neutral, plantar flexed (P < 0.05). Thus both contraction levels and muscle length affected the curvature. Intramuscular differences in neither the curvature nor the fascicle length were found. The direction of curving was consistent along the muscle: fascicles were concave in the proximal side. Fascicle length estimated from the pennation angle and muscle thickness, under the assumption that the fascicle was straight, was underestimated by ~6%. In addition, the curvature was significantly correlated to pennation angle and muscle thickness. These findings are particularly important for understanding the mechanical functions of human skeletal muscle in vivo.     

Automatic tracking of medial gastrocnemius fascicle length during human locomotion

During human locomotion lower extremity muscle-tendon units undergo cyclic length changes that were previously assumed to be representative of muscle fascicle length changes. Measurements in cats and humans have since revealed that muscle fascicle length changes can be uncoupled from those of the muscle-tendon unit. Ultrasonography is frequently used to estimate fascicle length changes during human locomotion. Fascicle length analysis requires time consuming manual methods that are prone to human error and experimenter bias. To bypass these limitations, we have developed an automatic fascicle tracking method based on the Lucas-Kanade optical flow algorithm with an affine optic flow extension. The aims of this study were to compare gastrocnemius fascicle length changes during locomotion using the automated and manual approaches and to determine the repeatability of the automated approach. Ultrasound was used to examine gastrocnemius fascicle lengths in eight participants walking at 4, 5, 6, and 7 km/h and jogging at 7 km/h on a treadmill. Ground reaction forces and three dimensional kinematics were recorded simultaneously. The level of agreement between methods and the repeatability of the automated method were quantified using the coefficient of multiple correlation (CMC). Regardless of speed, the level of agreement between methods was high, with overall CMC values of 0.90 ± 0.09 (95% CI: 0.86-0.95). Repeatability of the algorithm was also high, with an overall CMC of 0.88 ± 0.08 (95% CI: 0.79-0.96). The automated fascicle tracking method presented here is a robust, reliable, and time-efficient alternative to the manual analysis of muscle fascicle length during gait.     

Reliability and accuracy of an automated tracking algorithm to measure controlled passive and active muscle fascicle length changes from ultrasound

Manual tracking of muscle fascicle length changes from ultrasound images is a subjective and time-consuming process. The purpose of this study was to assess the repeatability and accuracy of an automated algorithm for tracking fascicle length changes in the medial gastrocnemius (MG) muscle during passive length changes and active contractions (isometric, concentric and eccentric) performed on a dynamometer. The freely available, automated tracking algorithm was based on the Lucas–Kanade optical flow algorithm with an affine optic flow extension, which accounts for image translation, dilation, rotation and shear between consecutive frames of an image sequence. Automated tracking was performed by three experienced assessors, and within- and between-examiner repeatability was computed using the coefficient of multiple determination (CMD). Fascicle tracking data were also compared with manual digitisation of the same image sequences, and the level of agreement between the two methods was calculated using the coefficient of multiple correlation (CMC). The CMDs across all test conditions ranged from 0.50 to 0.93 and were all above 0.98 when recomputed after the systematic error due to the estimate of the initial fascicle length on the first ultrasound frame was removed from the individual fascicle length waveforms. The automated and manual tracking approaches produced similar fascicle length waveforms, with an overall CMC of 0.88, which improved to 0.94 when the initial length offset was removed. Overall results indicate that the automated fascicle tracking algorithm was a repeatable, accurate and time-efficient method for estimating fascicle length changes of the MG muscle in controlled passive and active conditions.     

In vivo operational fascicle lengths of vastus lateralis during sub-maximal and maximal cycling

Instantaneous contractile characteristics of skeletal muscle, during movement tasks, can be determined and related to steady state mechanical properties such as the force–length relationship with the use of ultrasound imaging. A previous investigation into the contractile characteristics of the vastus lateralis (VL) during cycling has shown that fascicles operate on the “weak” descending limb of the force–length relationship, thus not taking advantage of the “strong” plateau region. The purpose of this study was to investigate if VL fascicle lengths change from sub-maximal to maximal cycling conditions, and if maximal cycling results in VL fascicle lengths which operate across the plateau of the force–length relationship. Fifteen healthy male subjects (age 20.9±1.8 yr, wt. 67.0±6.3 kg, ht. 176.7±7.2 cm) were tested to establish the maximal force–length relationship for the VL through ten maximal isometric contractions at various knee angles. Subjects then cycled on an SRM cycle ergometer at cadences of 50 and 80 revolutions per minute at 100 W, 250 W, and maximal effort. Fascicle lengths were determined at crank angles of 0, 90, and 180°. Fascicles operated at or near the plateau of the maximal force–length relationship for maximal cycling, while operating on the descending limb during sub-maximal conditions for both cadences. However, when comparing the fascicle operating range for the sub-maximal cycling conditions to the corresponding sub-maximal force–length relationships, the VL now also operated across the plateau region. We concluded from these results that regardless of cycling effort, the VL operated through the ideal plateau region of the corresponding force–length relationship, hence always working optimally. We hypothesize that this phenomenon is due to the coupling of series elastic compliance and length dependent calcium sensitivity in the VL.     

Reports of the length dependence of fatigue are greatly exaggerated.

Relative force depression associated with muscle fatigue is reported to be greater when assessed at short vs. long muscle lengths. This appears to be due to a rightward shift in the force-length relationship. This rightward shift may be caused by stretch of in-series structures, making sarcomere lengths shorter at any given muscle length. Submaximal force-length relationships (twitch, double pulse, 50 Hz) were evaluated before and after repetitive contractions (50 Hz, 300 ms, 1/s) in an in situ preparation of the rat medial gastrocnemius muscle. In some experiments, fascicle lengths were measured with sonomicrometry. Before repetitive stimulation, fascicle lengths were 11.3 +/- 0.8, 12.8 +/- 0.9, and 14.4 +/- 1.2 mm at lengths corresponding to -3.6, 0, and 3.6 mm where 0 is a reference length that corresponds with maximal active force for double-pulse stimulation. After repetitive stimulation, there was no change in fascicle lengths; these lengths were 11.4 +/- 0.8, 12.6 +/- 0.9, and 14.2 +/- 1.2 mm. The length dependence of fatigue was, therefore, not due to a stretch of in-series structures. Interestingly, the rightward shift that was evident when active force was calculated in the traditional way (subtraction of the passive force measured before contraction) was not seen when active force was calculated by subtracting the passive force that was associated with the fascicle length reached at the peak of the contraction. This calculation is based on the assumption that passive force decreases as the fascicles shorten during a fixed-end contraction. This alternative calculation revealed similar postfatigue absolute active force depression at all lengths. In relative terms, a length dependence of fatigue was still evident, but this was greatly diminished compared with that observed when active force was calculated with the traditional method.     

Musculotendon parameters and musculoskeletal pathways within the human foot

In order to create a flexible model of the foot for dynamic musculoskeletal models, anthropometric data combined with geometric information describing the intrinsic musculature are needed. In this study, the left feet of two male and two female cadavers were dissected to expose the intrinsic musculotendon pathways. Three-dimensional coordinates of bony landmarks, tendon origins, insertions, and via points were digitized to submillimeter accuracy. Muscle architectural parameters were also measured, including volume, weight, and pennation angle and sarcomere, fascicle, and free tendon lengths. Optimal muscle fascicle lengths, pen-nation angles at optimal length, physiological cross-sectional areas (PCSA), and tendon slack lengths were calculated from the directly measured values. Fascicle length and pennation angle varied greatly within each subject. Average fascicle lengths normalized by optimal fascicle length varied between 0.73 and 1.25, with 75% of the formalin-preserved muscles being found in a shortened state. The muscle volume and PCSA also had a large variability within subjects but less variation between subjects. The ratio of tendon slack length to optimal fascicle length was found to vary between 1.05 and 9.56. Using this data, a deformable model of the foot can now be created. It is envisioned that deformable feet will significantly improve stability and realism in models of gait, posture, and sporting activities.     

Determination of fascicle length and pennation in a contracting human muscle in vivo

Fukunaga, Tetsuo, Yoshiho Ichinose, Masamitsu Ito, YasuoKawakami, and Senshi Fukashiro. Determination of fascicle lengthand pennation in a contracting human muscle in vivo.J. Appl. Physiol. 82(1): 354-358, 1997.We have developed a technique to determine fascicle length inhuman vastus lateralis muscle in vivo by using ultrasonography. Whenthe subjects had the knee fully extended passively from a position of110° flexion (relaxed condition), the fascicle length decreasedfrom 133 to 97 mm on average. During static contractions at 10% ofmaximal voluntary contraction strength (tensed condition), fascicleshortening was more pronounced (from 126 to 67 mm), especially when theknee was closer to full extension. Similarly, as the knee was extended, the angle of pennation (fascicle angle, defined as the angle between fascicles and aponeurosis) increased (relaxed, from 14 to 18°; tensed, from 14 to 21°), and a greater increase in the pennation angle was observed in the tensed than in the relaxed condition when theknee was close to extension (<40°). We conclude that there aredifferences in fascicle lengths and pennation angles when the muscle isin a relaxed and isometrically tensed conditions and that thedifferences are affected by joint angles, at least at thesubmaximal contraction level.

     

Automated tracking of muscle fascicle orientation in B-mode ultrasound images

B-mode ultrasound can be used to non-invasively image muscle fascicles during both static and dynamic contractions. Digitizing these muscle fascicles can be a timely and subjective process, and usually studies have used the images to determine the linear fascicle lengths. However, fascicle orientations can vary along each fascicle (curvature) and between fascicles. The purpose of this study was to develop and test two methods for automatically tracking fascicle orientation. Images were initially filtered using a multiscale vessel enhancement (a technique used to enhance tube-like structures), and then fascicle orientations quantified using either the Radon transform or wavelet analysis. Tests on synthetic images showed that these methods could identify fascicular orientation with errors of less than 0.06°. Manual digitization of muscle fascicles during a dynamic contraction resulted in a standard deviation of angle estimates of 1.41° across ten researchers. The Radon transform predicted fascicle orientations that were not significantly different from the manually digitized values, whilst the wavelet analysis resulted in angles that were 1.35° less, and reasons for these differences are discussed. The Radon transform can be used to identify the dominant fascicular orientation within an image, and thus used to estimate muscle fascicle lengths. The wavelet analysis additionally provides information on the local fascicle orientations and can be used to quantify fascicle curvatures and regional differences with fascicle orientation across an image.     

Architectural and functional features of human triceps surae muscles during contraction

Architecturalproperties of the triceps surae muscles were determined in vivo for sixmen. The ankle was positioned at 15° dorsiflexion (15°)and 0, 15, and 30° plantar flexion, with the knee set at 0, 45, and90°. At each position, longitudinal ultrasonic images of the medial(MG) and lateral (LG) gastrocnemius and soleus (Sol) muscles wereobtained while the subject was relaxed (passive) and performed maximalisometric plantar flexion (active), from which fascicle lengths andangles with respect to the aponeuroses were determined. In the passivecondition, fascicle lengths changed from 59, 65, and 43 mm (knee,0°; ankle, 15°) to 32, 41, and 30 mm (knee, 90°ankle, 30°) for MG, LG, and Sol, respectively. Fascicle shorteningby contraction was more pronounced at longer fascicle lengths. MG hadgreatest fascicle angles, ranging from 22 to 67°, and was in a verydisadvantageous condition when the knee was flexed at 90°,irrespective of ankle positions. Different lengths and angles offascicles, and their changes by contraction, might be related todifferences in force-producing capabilities of the muscles and elasticcharacteristics of tendons and aponeuroses.

     

A novel method of studying fascicle architecture in relaxed and contracted muscles

A muscle’s architecture, described by geometric variables such as fascicle pennation angles or lengths, plays a crucial role in its functionality. Usually, single parameters are used to estimate force vectors or lengthening rates, thereby assuming that they represent the architecture properly and are constant during contraction. To describe muscle architecture in more detail and compare relaxed and contracted states, we developed and validated a new approach. The m. soleus of the laboratory rat was shock-frozen while relaxed and under isometric contraction, reconstructed three-dimensionally from histological sections, and fascicle lengths, curvatures and pennation angles, as well as the shape of the aponeuroses were analysed. Remarkable differences in volume distribution and the shapes of the aponeuroses as well as locally varying changes in the fascicle architecture were observed. While the mean pennation angle increased by only 2° due to contraction, local changes of up to 4° were observed. Fascicle curvature increased in the distal but remained unchanged in the proximal parts. Our approach may help to identify functional subunits within the muscle, i.e., regions with homogeneous architectural properties. Our results are discussed regarding the input parameters essential for realistic muscle modelling and challenge maximum isometric force estimations that are based on the physiological cross-sectional area or the Hill-model.     

The length dependence of muscle active force: considerations for parallel elastic properties.

The purpose of this study was to choose between two popular models of skeletal muscle: one with the parallel elastic component in parallel with both the contractile element and the series elastic component (model A), and the other in which it is in parallel with only the contractile element (model B). Passive and total forces were obtained at a variety of muscle lengths for the medial gastrocnemius muscle in anesthetized rats. Passive force was measured before the contraction (passive A) or was estimated for the fascicle length at which peak total force occurred (passive B). Fascicle length was measured with sonomicrometry. Active force was calculated by subtracting passive (A or B) force from peak total force at each fascicle or muscle length. Optimal length, that fascicle length at which active force is maximized, was 13.1 +/- 1.2 mm when passive A was subtracted and 14.0 +/- 1.1 mm with passive B (P < 0.01). Furthermore, the relationship between double-pulse contraction force and length was broader when calculated with passive B than with passive A. When the muscle was held at a long length, passive force decreased due to stress relaxation. This was accompanied by no change in fascicle length at the peak of the contraction and only a small corresponding decrease in peak total force. There is no explanation for the apparent increase in active force that would be obtained when subtracting passive A from the peak total force. Therefore, to calculate active force, it is appropriate to subtract passive force measured at the fascicle length corresponding to the length at which peak total force occurs, rather than passive force measured at the length at which the contraction begins.     

Demonstration of extended field-of-view ultrasound’s potential to increase the pool of muscles for which in vivo fascicle length is measurable

Static, B-mode ultrasound is the most common method of measuring fascicle length in vivo. However, most forearm muscles have fascicles that are longer than the field-of-view of traditional ultrasound (T-US). As such, little work has been done to quantify in vivo forearm muscle architecture. The extended field-of-view ultrasound (EFOV-US) method, which fits together a sequence of B-mode images taken from a continuous ultrasound scan, facilitates direct measurements of longer, curved fascicles. Here, we test the validity and reliability of the EFOV-US method for obtaining fascicle lengths in the extensor carpi ulnaris (ECU). Fascicle lengths from images of the ECU captured in vivo with EFOV-US were compared to lengths from a well-established method, T-US. Images were collected in a joint posture that shortens the ECU such that entire fascicle lengths were captured within a single T-US image. Resulting measurements were not significantly different (p = 0.18); a Bland-Altman test demonstrated their agreement. A novice sonographer implemented EFOV-US in a phantom and in vivo on the ECU. The novice sonographer’s measurements from the ultrasound phantom indicate that the combined imaging and analysis method is valid (average error = 2.2 ± 1.3 mm) and the in vivo fascicle length measurements demonstrate excellent reliability (ICC = 0.97). To our knowledge, this is the first study to quantify in vivo fascicle lengths of the ECU using any method. The ability to define a muscle’s architecture in vivo using EFOV-US could lead to improvements in diagnosis, model development, surgery guidance, and rehabilitation techniques.     

Passive knee movement-induced modulation of the soleus H-reflex and alteration in the fascicle length of the medial gastrocnemius muscle in humans

In humans, an inhibitory via Ia afferent pathway from the medial gastrocnemius (MG) to the soleus (SOL) motoneuron pool has been suggested. Herein, we examined the relation between MG fascicle length changes and the SOL H-reflex modulation during passive knee movement. Twelve subjects performed static and passive (5° s^?1) knee movement tasks with the ankle immobilized using an isokinetic dynamometer in sitting posture. The maximal H- and M-waves were measured at four target angles (20°, 40°, 60°, and 80° flexion from full knee extension). The MG fascicles length and velocity were measured using a B-mode ultrasonic apparatus. Results demonstrated that the SOL Hmax/Mmax; i.e., ratio of the maximal H- to M-waves, was attenuated with increasing MG fascicle length in static tasks. The SOL Hmax/Mmax at 20° was significantly attenuated compared with 60° and 80° with increasing MG fascicle length and lengthening velocity in passive knee extension. However, no significant differences in the SOL Hmax/Mmax were found across the target angles in the passive knee flexion task. In conclusion, as muscle spindles increase their discharge with lengthening fascicle velocity, but keep silent when fascicles shorten, our data suggest that lengthening the MG facilitates an inhibitory Ia pathway from MG to SOL, and modulates SOL motoneuron activity during movements.     

Effect of different ankle- and knee-joint positions on gastrocnemius medialis fascicle length and EMG activity during isometric plantar flexion

The purpose of this study was to provide evidence on the fact that the observed decrease in EMG activity of the gastrocnemius medialis (GM) at pronounced knee flexed positions is not only due to GM insufficiency, by examining muscle fascicle lengths during maximal voluntary contractions at different positions. Twenty-two male long distance runners (body mass: 78.5+/-6.7 kg, height: 183+/-6 cm) participated in the study. The subjects performed isometric maximal voluntary plantar flexion contractions (MVC) of their left leg at six ankle-knee angle combinations. To examine the resultant ankle joint moments the kinematics of the left leg were recorded using a Vicon 624 system with 8 cameras operating at 120 Hz. The EMG activity of GM, gastrocnemius lateralis (GL), soleus (SOL) and tibialis anterior (TA) were measured using surface electromyography. Synchronously, fascicle length and pennation angle values of the GM were obtained at rest and at the plateau of the maximal plantar flexion using ultrasonography. The main findings were: (a) identifiable differences in fascicle length of the GM at rest do not necessarily imply that these differences would also exist during a maximal isometric plantar flexion contraction and (b) the EMG activity of the biarticular GM during the MVC decreased at a pronounced flexed knee-joint position (up to 110 degrees ) despite of no differences in GM fascicle length. It is suggested that the decrease in EMG activity of the GM at pronounced knee flexed positions is due to a critical force-length potential of all three muscles of the triceps surae.     

Behavior of human muscle fascicles during shortening and lengthening contractions in vivo.

The aim of the present study was to investigate the behavior of human muscle fascicles during dynamic contractions. Eight subjects performed maximal isometric dorsiflexion contractions at six ankle joint angles and maximal isokinetic concentric and eccentric contractions at five angular velocities. Tibialis anterior muscle architecture was measured in vivo by use of B-mode ultrasonography. During maximal isometric contraction, fascicle length was shorter and pennation angle larger compared with values at rest (P < 0.01). During isokinetic concentric contractions from 0 to 4.36 rad/s, fascicle length measured at a constant ankle joint angle increased curvilinearly from 49.5 to 69.7 mm (41%; P < 0.01), whereas pennation angle decreased curvilinearly from 14.8 to 9.8 degrees (34%; P < 0.01). During eccentric muscle actions, fascicles contracted quasi-isometrically, independent of angular velocity. The behavior of muscle fascicles during shortening contractions was believed to reflect the degree of stretch applied to the series elastic component, which decreases with increasing contraction velocity. The quasi-isometric behavior of fascicles during eccentric muscle actions suggests that the series elastic component acts as a mechanical buffer during active lengthening.     

Resonance in the human medial gastrocnemius muscle during cyclic ankle bending exercise.

We investigated the behavior of the muscle tendon unit (MTU) of the medial gastrocnemius muscle during cyclic ankle bending exercise at eight different frequencies (ranging from 1.33 to 3.67 Hz). The changes in the length of fascicle in the muscle during the exercises were determined by real-time ultrasound imaging. The coordinates of anatomical references and the ground reaction force were determined from video recording and a force plate, respectively. The length change of the MTU (the distance from the origin to insertion of the muscle) was calculated from changes in the knee and ankle joint angles. It was found that the amplitude ratio and phase difference between the fascicle and MTU lengths were both dependent on the movement frequency. At lower frequencies, the fascicle lengths varied almost in phase with the MTU length, whereas they varied out of phase at the higher frequencies. At intermediate frequency, the amplitude of the fascicle became very small compared with that of the MTU, which is considered resonance. We constructed a mechanical model of the MTU based on a notion of forced oscillation in a mass-spring system. The obtained data were well explained by the model. It was concluded that the behavior of the MTU highly depends on the movement frequency due to the viscoelasticity of the MTU.     

Muscle fiber and tendon length changes in the human vastus lateralis during slow pedaling.

Muscle fascicle lengths of vastus lateralis (VL) muscle were measured in five healthy men during slow pedaling to investigate the interaction between muscle fibers and tendon. Subjects cycled at a pedaling rate of 40 rpm (98 W). During exercise, fascicle lengths changed from 91 +/- 7 (SE) to 127 +/- 5 mm. It was suggested that fascicles were on the descending limb of their force-length relationship. The average shortening velocity of fascicle was greater than that of muscle-tendon complex in the first half of the knee extension phase and was less in the second half. The maximum shortening velocity of fascicle in the knee extension phase was less than that of muscle-tendon complex by 22 +/- 9%. These discrepancies in velocities were mainly caused by the elongation of the tendinous tissue. It was suggested that the elasticity of VL tendinous tissue enabled VL fascicles to develop force at closer length to their optimal length and kept the maximum shortening velocity of VL fascicles low during slow pedaling.     

Automated regional analysis of B-mode ultrasound images of skeletal muscle movement

To understand the functional significance of skeletal muscle anatomy, a method of quantifying local shape changes in different tissue structures during dynamic tasks is required. Taking advantage of the good spatial and temporal resolution of B-mode ultrasound imaging, we describe a method of automatically segmenting images into fascicle and aponeurosis regions and tracking movement of features, independently, in localized portions of each tissue. Ultrasound images (25 Hz) of the medial gastrocnemius muscle were collected from eight participants during ankle joint rotation (2° and 20°), isometric contractions (1, 5, and 50 Nm), and deep knee bends. A Kanade-Lucas-Tomasi feature tracker was used to identify and track any distinctive and persistent features within the image sequences. A velocity field representation of local movement was then found and subdivided between fascicle and aponeurosis regions using segmentations from a multiresolution active shape model (ASM). Movement in each region was quantified by interpolating the effect of the fields on a set of probes. ASM segmentation results were compared with hand-labeled data, while aponeurosis and fascicle movement were compared with results from a previously documented cross-correlation approach. ASM provided good image segmentations (<1 mm average error), with fully automatic initialization possible in sequences from seven participants. Feature tracking provided similar length change results to the cross-correlation approach for small movements, while outperforming it in larger movements. The proposed method provides the potential to distinguish between active and passive changes in muscle shape and model strain distributions during different movements/conditions and quantify nonhomogeneous strain along aponeuroses.     

Heterogeneous fascicle behavior within the biceps femoris long head at different muscle activation levels

Magnetic resonance and ultrasound imaging have shown hamstring strain injuries occur most often in the biceps femoris long head (BFLH), and particularly in the proximal vs. distal region of this muscle. Animal research and musculoskeletal modeling (MSK) have detected heterogeneous fascicle behavior within muscle regions, and within fascicles. Understanding architectural behavior differences during muscle contractions may help to discern possible mechanisms behind proximal BFLH injuries. The purpose of our study was to assess the magnitude of shortening of the proximal and distal fascicles of the BFLH under a range of muscle activation levels under isometric conditions using ultrasound imaging (US). Thirteen healthy adults performed targeted sustained isometric contractions while US were taken of the entire BFLH. Measurements of fascicle lengths in both muscle regions were compared at 20%, 30%, 50%, and 67% MVIC. The results showed that while both regions shortened significantly with activation, the proximal fascicles were significantly longer, regardless of activation level (~38%), and shortened significantly more than the distal fascicles overall (~40%), and cumulatively at higher activation levels (30% and above). No significant strain differences were found between the two regions. These data suggest heterogeneous fascicle behavior exists in an absolute sense; however, differences in behavior are eliminated when normalized (strain). Coupled with MSK literature, the absence of regional fascicle strain differences in this study may indicate strain heterogeneity is not detectable at the whole fascicle level. Further knowledge of this commonly strained muscle?s regional behavior during dynamic movements could provide evidence of proximal hamstring strain predisposition.