Lilian Lacourpaille

Supersonic shear imaging provides a reliable measurement of resting muscle shear elastic modulus.

The aim of the present study was to assess the reliability of shear elastic modulus measurements performed using supersonic shear imaging (SSI) in nine resting muscles (i.e. gastrocnemius medialis, tibialis anterior, vastus lateralis, rectus femoris, triceps brachii, biceps brachii, brachioradialis, adductor pollicis obliquus and abductor digiti minimi) of different architectures and typologies. Thirty healthy subjects were randomly assigned to the intra-session reliability (n = 20), inter-day reliability (n = 21) and the inter-observer reliability (n = 16) experiments. Muscle shear elastic modulus ranged from 2.99 (gastrocnemius medialis) to 4.50 kPa (adductor digiti minimi and tibialis anterior). On the whole, very good reliability was observed, with a coefficient of variation (CV) ranging from 4.6% to 8%, except for the inter-operator reliability of adductor pollicis obliquus (CV = 11.5%). The intraclass correlation coefficients were good (0.871 ± 0.045 for the intra-session reliability, 0.815 ± 0.065 for the inter-day reliability and 0.709 ± 0.141 for the inter-observer reliability). Both the reliability and the ease of use of SSI make it a potentially interesting technique that would be of benefit to fundamental, applied and clinical research projects that need an accurate assessment of muscle mechanical properties.

Slack length of gastrocnemius medialis and Achilles tendon occurs at different ankle angles

Although muscle-tendon slack length is a crucial parameter used in muscle models, this is one of the most difficult measures to estimate in vivo. The aim of this study was to determine the onset of the rise in tension (i.e., slack length) during passive stretching in both Achilles tendon and gastrocnemius medialis. Muscle and tendon shear elastic modulus was measured by elastography (supersonic shear imaging) during passive plantarflexion (0° and 90° of knee angle, 0° representing knee fully extended, in a random order) in 9 participants. The within-session repeatability of the determined slack length was good at 90° of knee flexion (SEM=3.3° and 2.2° for Achilles tendon and gastrocnemius medialis, respectively) and very good at 0° of knee flexion (SEM=1.9° and 1.9° for Achilles tendon and gastrocnemius medialis, respectively). The slack length of gastrocnemius medialis was obtained at a significantly lower plantarflexed angle than for Achilles tendon at both 0° (P<0.0001; mean difference=19.4±3.8°) and 90° of knee flexion (P<0.0001; mean difference=25.5±7.6°). In conclusion, this study showed that the joint angle at which the tendon falls slack can be experimentally determined using supersonic shear imaging. The slack length of gastrocnemius medialis and Achilles tendon occurred at different joint angles. Although reporting this result is crucial to a better understanding of muscle-tendon interactions, further experimental investigations are required to explain this result.

Time-course effect of exercise-induced muscle damage on localized muscle mechanical properties assessed using elastography

Changes in muscle stiffness after exercise‐induced muscle damage have been classically inferred from passive torque–angle curves. Elastographic techniques can be used to estimate the shear modulus of a localized muscular area. This study aimed to quantify the changes in shear elastic modulus in different regions of the elbow flexors after eccentric exercise and their relation to muscle length.

Influence of passive muscle tension on electromechanical delay in humans.

Background Electromechanical delay is the time lag between onsets of muscle activation and muscle force production and reflects both electro-chemical processes and mechanical processes. The aims of the present study were two-fold: to experimentally determine the slack length of each head of the biceps brachii using elastography and to determine the influence of the length of biceps brachii on electromechanical delay and its electro-chemical/mechanical processes using very high frame rate ultrasound. Methods/Results First, 12 participants performed two passive stretches to evaluate the change in passive tension for each head of the biceps brachii. Then, they underwent two electrically evoked contractions from 120 to 20° of elbow flexion (0°: full extension), with the echographic probe maintained over the muscle belly and the myotendinous junction of biceps brachii. The slack length was found to occur at 95.5 ± 6.3° and 95.3 ± 8.2° of the elbow joint angle for the long and short heads of the biceps brachii, respectively. The electromechanical delay was significantly longer at 120° (16.9 ± 3.1 ms; p<0.001), 110° (15.0 ± 3.1 ms; p<0.001) and 100° (12.7 ± 2.5 ms; p = 0.01) of elbow joint angle compared to 90° (11.1 ± 1.7 ms). However, the delay between the onset of electrical stimulation and the onset of both muscle fascicles (3.9 ± 0.2 ms) and myotendinous junction (3.7 ± 0.3 ms) motion was not significantly affected by the joint angle (p>0.95). Conclusion In contrast to previous observations on gastrocnemius medialis, the onset of muscle motion and the onset of myotendinous junction motion occurred simultaneously regardless of the length of the biceps brachii. That suggests that the between-muscles differences reported in the literature cannot be explained by different muscle passive tension but instead may be attributable to muscle architectural differences.

Muscle force loss and soreness subsequent to maximal eccentric contractions depend on the amount of fascicle strain in vivo

Defining the origins of muscle injury has important rehabilitation and exercise applications. However, current knowledge of muscle damage mechanics in human remains unclear in vivo. This study aimed to determine the relationships between muscle–tendon unit mechanics during maximal eccentric contractions and the extent of subsequent functional impairments induced by muscle damage.

NON-INVASIVE ASSESSMENT OF MUSCLE STIFFNESS IN PATIENTS WITH DUCHENNE MUSCULAR DYSTROPHY

ABSTRACT: Introduction: Assessment of muscle mechanical properties may provide clinically valuable information for follow‐up of patients with Duchenne muscular dystrophy (DMD) through the course of their disease. In this study we aimed to assess the effect of DMD on stiffness of relaxed muscles using elastography (supersonic shear imaging). Methods: Fourteen DMD patients and 13 control subjects were studied. Six muscles were measured at 2 muscle lengths (shortened and stretched): gastrocnemius medialis (GM); tibialis anterior (TA); vastus lateralis (VL); biceps brachii (BB); triceps brachii (TB); and abductor digiti minimi (ADM). Results: Stiffness was significantly higher in DMD patients compared with controls for all the muscles (main effect for population, P < 0.033 in all cases), except for ADM. The effect size was small (d = 0.33 for ADM at both muscle lengths) to large (d = 0.86 for BB/stretched). Conclusions: Supersonic shear imaging is a sensitive non‐invasive technique to assess the increase in muscle stiffness associated with DMD. Muscle Nerve 51: 284–286, 2015

Massage induces an immediate, albeit short-term, reduction in muscle stiffness

Using ultrasound shear wave elastography, the aims of this study were: (a) to evaluate the effect of massage on stiffness of the medial gastrocnemius (MG) muscle and (b) to determine whether this effect (if any) persists over a short period of rest. A 7‐min massage protocol was performed unilaterally on MG in 18 healthy volunteers. Measurements of muscle shear elastic modulus (stiffness) were performed bilaterally (control and massaged leg) in a moderately stretched position at three time points: before massage (baseline), directly after massage (follow‐up 1), and following 3 min of rest (follow‐up 2). Directly after massage, participants rated pain experienced during the massage. MG shear elastic modulus of the massaged leg decreased significantly at follow‐up 1 (−5.2 ± 8.8%, P = 0.019, d = −0.66). There was no difference between follow‐up 2 and baseline for the massaged leg (P = 0.83) indicating that muscle stiffness returned to baseline values. Shear elastic modulus was not different between time points in the control leg. There was no association between perceived pain during the massage and stiffness reduction (r = 0.035; P = 0.89). This is the first study to provide evidence that massage reduces muscle stiffness. However, this effect is short lived and returns to baseline values quickly after cessation of the massage.

Influence of stimulus intensity on electromechanical delay and its mechanisms

Electromechanical delay (EMD) is the time lag between muscle activation and force development. Using very high frame rate ultrasound, both electrochemical and mechanical processes involved in EMD can be assessed. Percutaneous electrical stimulations at submaximal intensity are often used to stimulate a specific target muscle. The aim of this study was to determine whether stimulus intensity alters the delay between stimulation and the onset of muscle fascicules motion (Dm), the onset of myotendinous junction motion (Dt), and force production (EMD). Ten participants underwent two electrically evoked contractions, with the probe maintained either the biceps brachii muscle belly or the distal myotendinous junction of the biceps brachii, for six stimulus intensities (30%, 50%, 70%, 90%, 110% and 130% of the lowest intensity inducing the maximal involuntary force production, Imax). In addition, inter-day reliability was tested in nine participants at both 70% and 90% of Imax. Dm, Dt and EMD were significantly longer (p < 0.001) at very low (30% and 50% of Imax) compared to higher intensities (70%, 90%, 110% and 130% of Imax). Inter-day reliability of EMD, Dm, and Dt was good (coefficient of variation ranged from 6.8% to 12.5%, i.e. SEM lower than 0.79 ms). These results indicate that the stimulus intensity needs to be standardized to perform longitudinal evaluation and/or to make between-subject comparisons.

Electromechanical delay measured during a voluntary contraction should be interpreted with caution

Electromechanical delay (EMD) is the time lag between muscle activation and muscle force production. It reflects both electrochemical processes (i.e., synaptic transmission, propagation of the action potential, excitation–contraction coupling) and mechanical processes (i.e., force transmission along the active and passive parts of the series elastic component). Consequently, it has been often used (more than 100 articles in PubMed) to evaluate muscle function, such as during fatigue tasks, in response to training programs, after anterior cruciate ligament reconstruction, and in patients with neuropathy or myopathy. EMD can be measured during voluntary, reflex, or electrically induced contraction. It should be considered a valid tool for evaluation of muscle function only if it not influenced by methodological artifacts. In our opinion, EMD measurements performed during voluntary contraction fail to meet this criterion, mainly because the electromyographic (EMG) electrode location may greatly influence detection of onset of muscle activation. In fact, because the onset of force production is due to the first recruited muscle fibers, it is crucial to detect precisely the onset of activation from these fibers. However, the use of classical EMG recordings, in which one pair of electrodes is placed somewhere between the motor point and the tendon, does not allow this to occur. Assuming a muscle fiber conduction velocity of 4 m/s, the propagation of the action potential from the motor point to a pair of electrodes placed 2 cm distal to the motor point would be 5 ms (in a fusiform muscle), which constitutes a relatively high error for EMD values between 40 and 60 ms. Also, due to the spatial heterogeneity in muscle recruitment, one would expect a nonuniform onset of activation within the muscle. In other words, intraand intersubject variations in EMD could be due in part (or totally) to differences in electrode location. To illustrate this drawback, we detected the onset of muscle activation from surface EMG recorded with an electrode array (64 channels; EMG-USB; LISIN-Ottino Biolettronica, Italy) during an isometric elbow flexion performed as quickly as possible. The results showed high variability in the onset times depending on the electrode location (up to 20 ms; Fig. 1). Because the use of electrode arrays would seem appealing, it does not totally solve the drawback just highlighted. The reason for this is that it mainly provides information from motor unit potentials (MUPs) recorded from the surface, where high-threshold motor units are predominant, and it cannot be used in all the synergist muscles involved in the task (e.g., deep muscles). In conclusion, for a proper interpretation of changes in EMD (across subjects and across time), EMD should be measured during electrically induced contraction by taking into account the difference between the stimulation artifact (very easy to detect) and the onset of force FIGURE 1. Experimental setup (A) and results (B). (A) The subject was seated upright. The angle between the upper arm and the forearm was fixed at 90 . He performed isometric elbow flexions as quickly as possible. (B) Electromyographic signals were amplified ( 500), bandpass filtered (10–500 HZ), and digitized at 4096 HZ. The onset of EMG activity was automatically detected (threshold 1⁄4 2 standard deviations beyond mean of baseline activity) and visually checked. The differences from the mean value calculated over 50 EMG channels are depicted. Note that the most proximal and distal lines of channels are not depicted. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

Effects of hip and head position on ankle range of motion, ankle passive torque, and passive gastrocnemius tension

Ankle joint range of motion (ROM) is notably influenced by the position of the hip joint. However, this result remains unexplained. Thus, the aim of this study was to test if the ankle passive torque and gastrocnemius muscle tension are affected by the hip and the head positions. The torque and the muscle shear elastic modulus (measured by elastography to estimate muscle tension) were collected in nine participants during passive ankle dorsiflexions performed in four conditions (by combining hip flexion at 90 or 150°, and head flexed or neutral). Ankle maximum dorsiflexion angle significantly decreased by flexing the hip from 150 to 90° (P < 0.001; mean difference 17.7 ± 2.5°), but no effect of the head position was observed (P > 0.05). Maximal passive torque and shear elastic modulus were higher with the hip flexed at 90° (P < 0.001). During submaximal ROM, no effects of the head and hip positioning (P > 0.05) were found for both torque and shear elastic modulus at a given common ankle angle among conditions. Shifts in maximal ankle angle due to hip angle manipulation are not related neither to changes in passive torque nor tension of the gastrocnemius. Further studies should be addressed to better understand the functional role of peripheral nerves and fasciae in the ankle ROM limits.