Muscle in Variable Gravity: “I Do Not Know Where I Am, But I Know What to Do”

Purpose: Fascicle and sarcomere lengths are important predictors of muscle mechanical performance. However, their regulation during stretch-shortening cycle (SSC) activities in usual and challenging conditions is poorly understood. In this study, we aimed to investigate muscle fascicle and sarcomere behavior during drop jumps (a common SSC activity) in conditions of variable gravity.Methods: Fifteen volunteers performed repeated drop jumps in 1 g, hypo-gravity (0 to 1 g), and hyper-gravity (1 to 2 g) during a parabolic flight. Gastrocnemius medialis (GM) electromyographic activity and fascicle length (Lf) were measured at drop-off, ground contact (GC), minimum ankle joint angle (MAJ), and push-off. GM sarcomere number was estimated by dividing Lf, measured by ultrasound at rest, by published data on GM sarcomere length, and measured in vivo at the same joint angle. Changes in sarcomere length were estimated by dividing GM Lf in each jump phase by sarcomere number calculated individually. The sarcomere force-generating capacity in each jump phase was estimated from the sarcomere length-tension relationship previously reported in the literature.Results: The results showed that, regardless of the gravity level, GM sarcomeres operated in the ascending portion of their length-tension relationship in all the jump phases. Interestingly, although in hypo-gravity and hyper-gravity during the braking phase (GC-MAJ) GM fascicles and sarcomeres experienced a stretch (as opposed to the quasi-isometric behavior in 1 g), at MAJ they reached similar lengths as in 1 g, allowing sarcomeres to develop about the 70% of their maximum force.Conclusion: The observed fascicle behavior during drop jumping seems useful for anchoring the tendon, enabling storage of elastic energy and its release in the subsequent push-off phase for effectively re-bouncing in all gravity levels, suggesting that an innate neuromuscular wisdom enables to perform SSC movements also in challenging conditions.

Serial sarcomere number is substantially decreased within the paretic biceps brachii in individuals with chronic hemiparetic stroke

A muscle’s structure, or architecture, is indicative of its function and is plastic; changes in input to or use of the muscle alter its architecture. Stroke-induced neural deficits substantially alter both input to and usage of individual muscles. We combined in vivo imaging methods (second-harmonic generation microendoscopy, extended field-of-view ultrasound, and fat-suppression MRI) to quantify functionally meaningful architecture parameters in the biceps brachii of both limbs of individuals with chronic hemiparetic stroke and in age-matched, unimpaired controls. Specifically, serial sarcomere number (SSN) and physiological cross-sectional area (PCSA) were calculated from data collected at three anatomical scales: sarcomere length, fascicle length, and muscle volume. The interlimb differences in SSN and PCSA were significantly larger for stroke participants than for participants without stroke (P = 0.0126 and P = 0.0042, respectively), suggesting we observed muscle adaptations associated with stroke rather than natural interlimb variability. The paretic biceps brachii had ∼8,200 fewer serial sarcomeres and ∼2 cm2 smaller PCSA on average than the contralateral limb (both P < 0.0001). This was manifested by substantially smaller muscle volumes (112 versus 163 cm3), significantly shorter fascicles (11.0 versus 14.0 cm; P < 0.0001), and comparable sarcomere lengths (3.55 versus 3.59 μm; P = 0.6151) between limbs. Most notably, this study provides direct evidence of the loss of serial sarcomeres in human muscle observed in a population with neural impairments that lead to disuse and chronically place the affected muscle at a shortened position. This adaptation is consistent with functional consequences (increased passive resistance to elbow extension) that would amplify already problematic, neurally driven motor impairments.

The influence of training-induced sarcomerogenesis on the history dependence of force

The increase or decrease in isometric force following active muscle lengthening or shortening, relative to a reference isometric contraction at the same muscle length and level of activation, are referred to as residual force enhancement (rFE) and residual force depression (rFD), respectively. The purpose of these experiments was to gain further mechanistic insight into the trainability of rFE and rFD, on the basis of serial sarcomere number (SSN) alterations to length-dependent properties. Maximal rFE/rFD measures from the soleus and extensor digitorum longus (EDL) of rats were compared after 4 weeks of uphill/downhill running and a no running control. Serial sarcomere numbers adapted to the training: soleus serial sarcomere number was greater with downhill compared to uphill running, while EDL demonstrated a trend towards more serial sarcomeres for downhill compared to no running. In contrast, absolute and normalized rFE/rFD did not differ across training groups for either muscle. As such, it appears that training-induced SSN adaptations do not modify rFE/rFD at the whole-muscle level.Summary StatementThe addition and subtraction of serial sarcomeres induced by downhill and uphill running, respectively, did not influence the magnitude of stretch-induced force enhancement and shortening-induced force depression.

Serial sarcomere number is substantially decreased within the paretic biceps brachii in individuals with chronic hemiparetic stroke

ABSTRACTA muscle’s structure, or architecture, is indicative of its function and is plastic; changes in input to or use of the muscle alter its architecture. Stroke-induced neural deficits substantially alter both input to and usage of individual muscles. Here, we combined novel in vivo imaging methods (second harmonic generation microendoscopy, extended field-of-view ultrasound, and fat-supression MRI) to quantify functionally meaningful muscle architecture parameters in the biceps brachii of both limbs of individuals with chronic hemiparetic stroke and in age-matched, unimpaired controls. Specifically, serial sarcomere number and physiological cross-sectional area were calculated from data collected at three anatomical scales: sarcomere length, fascicle length, and muscle volume. Our data indicate that the paretic biceps brachii had ~8,500 fewer serial sarcomeres compared to the contralateral limb (p=0.0044). In the single joint posture tested, the decreased serial sarcomere number was manifested by significantly shorter fascicles (10.7cm vs 13.6cm; p<0.0001) without significant differences in sarcomere lengths (3.58μm vs. 3.54μm; p=0.6787) in the paretic compared to the contralateral biceps. No interlimb differences were observed in unimpaired controls, suggesting we observed muscle adaptations associated with stroke rather than natural interlimb variability. This study provides the first direct evidence of the loss of serial sarcomeres in human muscle, observed in a population with neural impairments that lead to disuse and chronically place the affected muscle at a shortened position. This adaptation is consistent with functional consequences (increased passive resistance to elbow extension) that would amplify already problematic, neurally driven motor impairments.SIGNIFICANCE STATEMENTSerial sarcomere number determines a muscle’s length during maximum force production and its available length range for active force generation. Skeletal muscle length adapts to functional demands; for example, animal studies demonstrate that chronically shortened muscles decrease length by losing serial sarcomeres. This phenomenon has never been demonstrated in humans. Integrating multi-scale imaging techniques, including two photon microendoscopy, an innovative advance from traditional, invasive measurement methods at the sarcomere scale, we establish that chronic impairments that place a muscle in a shortened position are associated with the loss of serial sarcomeres in humans. Understanding how muscle adapts following impairment is critical to the design of more effective clinical interventions to mitigate such adaptations and to improve function following motor impairments.

The effects of stretching on muscle morphometry of ovariectomized rats

Introduction: Ageing is responsible for structural alterations, declining of all physiological variables, including range of motion and skeletal muscle function, known as sarcopenia. Objective: The aim of the study was to evaluate the effects of stretching on muscle morphometry in ovariectomized rats. Method: 21 female Wistar rats (12 weeks, 218 ± 22 g) were divided into 4 groups: control (CONTROL, n = 3) intact; ovariectomized and hysterectomized (OH, n = 6); Stretching (STRET, n = 6); ovariectomized and hysterectomized and stretching (OHS, n = 6). The rats were subjected to ovariectomy and hysterectomy. The stretching protocol of the soleus muscle lasted 10 repetitions of 1 minute with 45s interval between each repetition performed 3 times a week for 3 weeks. After 3 weeks, the rats were weighed and the muscles of both hind limbs were removed weighed and analyzed at muscle length; serial sarcomere number; sarcomere length; muscle fiber cross-sectional area (MFCSA) and percentage of connective tissue. Results: The final body weight increased in all groups. The serial sarcomere number of STRET was greater than the OH. The muscle fibers’ cross-sectional area of OHS was higher than CONTROL. Conclusion: It can be concluded that ovariectomy and hysterectomy prevented sarcomerogenesis even when stretching was applied. However, the stretching protocol enhanced muscle trophismof ovariectomized and hysterectomized rats. It might be suggested that longitudinal growth (serial sarcomeres) and radial (ASTFM) are differently regulated by stretching in intact and/or estrogen depleted (ovariectomy and hysterectomy) skeletal muscle.

Microendoscopy reveals positive correlation in multiscale length changes and variable sarcomere lengths across different regions of human muscle

Sarcomere length is a key physiological parameter that affects muscle force output; however, our understanding of the scaling of human muscle from sarcomere to whole muscle is based primarily on cadaveric data. The aims of this study were to explore the in vivo relationship between passive fascicle length and passive sarcomere length at different muscle-tendon unit lengths and determine whether sarcomere and fascicle length relationships are the same in different regions of muscle. A microendoscopy needle probe capable of in vivo sarcomere imaging was inserted into a proximal location of the human tibialis anterior muscle at three different ankle positions [5° dorsiflexion, 5° plantar flexion (PF), and 15° PF] and one distal location at a constant ankle position (5° PF distal). Ultrasound imaging of tibialis anterior fascicles, centered on the location of the needle probe, was performed for each condition to estimate fascicle length. Sarcomere length and fascicle length increased with increasing muscle-tendon unit length, although the correlation between sarcomere length change and muscle fascicle length change was only moderate ( r2 = 0.45). Passive sarcomere length was longer at the distal imaging site than the proximal site ( P = 0.01). When sarcomere number was estimated from sarcomere length and fascicle length, there were fewer sarcomeres in the fibers of distal location than the proximal location ( P = 0.01). These data demonstrate that fascicle length changes are representative of sarcomere length changes, although significant variability in sarcomere length exists within a muscle and sarcomere number per fiber is region-dependent. NEW & NOTEWORTHY Sarcomere and fascicle lengths were measured in vivo from human muscle to examine the relationship between the different scales of organization. Changes in fascicle length were moderately related to sarcomere length changes; however, sarcomere length and number per fiber varied from proximal to distal regions of the muscle. Differences in average sarcomere operating lengths across the muscle suggest potentially different stresses or strains experienced within different regions of muscle.

Effects of GaAs laser and stretching on muscle contusion in rats

ABSTRACT Laser and stretching are used to treat skeletal muscle injuries. This study aimed to evaluate the effects of GaAs laser and stretching in the morphology of the tibialis anterior (TA) muscle after contusion. Thirty-six male rats (349±23g) were divided into six groups (n=6): control group (CG); lesion group (LG); lesion and laser group (LLG); lesion and stretching group (LSG); lesion, laser and stretching group (LLSG); and stretching group (SG). TA was wounded by a contusion apparatus. We used GaAs laser 4.5 J/cm2 dose for 32 s each, beginning 48 h after lesion, for 7 days, once a day. Manual passive stretching was applied by 10 repetitions for 1 minute, initiating on the 8th day, once a day, 3 times a week, during 3 weeks. After 4 weeks, rats were euthanized and we analyzed: muscle weight and length, cross sectional area of muscle fibers (CSAMF), serial sarcomere number (SSN), sarcomere length, and percentage of connective tissue. Comparisons among groups were made by ANOVA and post hoc Tukey tests, with the significance level set at ≤ 0.05. The serial sarcomere number of LLSG was higher than LSG. The sarcomere length of LSG was superior to LLG, LLSG, and SG. SG increased SSN compared to CG, while the percentage of connective tissue of SG decreased in comparison to LLSG. Thus, the sarcomerogenesis of injured muscles was enhanced by laser therapy, stretching, and association of both. The stretching protocol was enough to increase SSN of intact muscles.

Intermittent stretch training of rabbit plantarflexor muscles increases soleus mass and serial sarcomere number

In humans, enhanced joint range of motion is observed after static stretch training and results either from an increased stretch tolerance or from a change in the biomechanical properties of the muscle-tendon unit. We investigated the effects of an intermittent stretch training on muscle biomechanical and structural variables. The left plantarflexors muscles of seven anesthetized New Zealand (NZ) White rabbits were passively and statically stretched three times a week for 4 wk, while the corresponding right muscles were used as nonstretched contralateral controls. Before and after the stretching protocol, passive torque produced by the left plantarflexor muscles as a function of the ankle angle was measured. The left and right plantarflexor muscles were harvested from dead rabbits and used to quantify possible changes in muscle structure. Significant mass and serial sarcomere number increases were observed in the stretched soleus but not in the plantaris or medial gastrocnemius. This difference in adaptation between the plantarflexors is thought to be the result of their different fiber type composition and pennation angles. Neither titin isoform nor collagen amount was modified in the stretched compared with the control soleus muscle. Passive torque developed during ankle dorsiflexion was not modified after the stretch training on average, but was decreased in five of the seven experimental rabbits. Thus, an intermittent stretching program similar to those used in humans can produce a change in the muscle structure of NZ White rabbits, which was associated in some rabbits with a change in the biomechanical properties of the muscle-tendon unit.