Effects of low-load blood flow restriction training in healthy adult tendons: A systematic review and meta-analysis.

OBJECTIVE: To systematically review the effects of low-load blood flow restriction training (LL-BFR) on healthy adult tendons. DESIGN: A systematic review with meta-analysis. LITERATURE SEARCH: Six electronic databases were searched by two researchers. STUDY SELECTION CRITERIA: Clinical trials comparing the effects of LL-BFR to high-load resistance training (HL-RT) or low-load resistance training (LL-RT) in healthy adult tendons. DATA SYNTHESIS: Two reviewers selected the eligible clinical trials, and one reviewer exported the data. Two reviewers evaluated the study quality and risk of bias using the PEDro scale and the ROB2 scale. We performed meta-analysis where appropriate using a random-effects model. We rated the quality of evidence using GRADE. RESULTS: Six studies were eligible. We analyzed tendon cross-sectional area (CSA) and tendon stiffness as the outcomes. Across all comparisons, there was low-to moderate-quality evidence of a difference between LL-BFR and LL-RT immediately after exercise. There was high-quality evidence of no difference between LL-BFR and HL-RT in the long term. CONCLUSION: The effects of LL-BFR on the tendons depends on the time and dose of the intervention. LL-BFR could be useful to increase the CSA of the tendons in a similar or superior way to HL-RT after 8 weeks of intervention.

Distal hamstrings tendons mechanical properties at rest and contraction using free-hand 3-D ultrasonography.

Tendon properties impact human locomotion, influencing sports performance, and injury prevention. Hamstrings play a crucial role in sprinting, particularly the biceps femoris long head (BFlh), which is prone to frequent injuries. It remains uncertain if BFlh exhibits distinct mechanical properties compared to other hamstring muscles. This study utilized free-hand three-dimensional ultrasound to assess morphological and mechanical properties of distal hamstrings tendons in 15 men. Scans were taken in prone position, with hip and knee extended, at rest and during 20%, 40%, 60%, and 80% of maximal voluntary isometric contraction of the knee flexors. Tendon length, volume, cross-sectional area (CSA), and anteroposterior (AP) and mediolateral (ML) widths were quantified at three locations. Longitudinal and transverse deformations, stiffness, strain, and stress were estimated. The ST had the greatest tendon strain and the lowest stiffness as well as the highest CSA and AP and ML width strain compared to other tendons. Biceps femoris short head (BFsh) exhibited the least strain, AP and ML deformation. Further, BFlh displayed the highest stiffness and stress, and BFsh had the lowest stress. Additionally, deformation varied by region, with the proximal site showing generally the lowest CSA strain. Distal tendon mechanical properties differed among the hamstring muscles during isometric knee flexions. In contrast to other bi-articular hamstrings, the BFlh high stiffness and stress may result in greater energy absorption by its muscle fascicles, rather than the distal tendon, during late swing in sprinting. This could partly account for the increased incidence of hamstring injuries in this muscle.

Comparison of In Vivo Stiffness of Tendons Commonly Used for Anterior Cruciate Ligament Reconstruction - A Shear Wave Elastography Study.

RATIONALE AND OBJECTIVES: There are currently no studies investigating the in vivo stiffness of the most commonly used autografts for anterior cruciate ligament reconstruction (ACLR) using Shear wave elastography (SWE). We hypothesize that there are differences regarding the elastic properties between the three tendons commonly used for ACLR and that they are influenced by patient-related factors. MATERIALS AND METHODS: 80 healthy subjects (25 females, 55 males, age: 25.33 ± 4.76 years, BMI: 23.76 ± 3.14 kg/m2, 40 semiprofessional athletes, athlete group [AG], age: 25.51 [19-29]; 40 healthy controls, control group [CG], age: 25.50 [20-29]) were recruited as participants. In addition to patient reported outcome scores, every participant underwent a standardized multimodal ultrasound protocol consisting of B-mode-ultrasound (B-US), Color Doppler-ultrasound (CD-US) and a SWE examination of the bilateral quadriceps tendon (QT), patellar tendon (PT) and semitendinosus tendon (ST). RESULTS: The highest shear wave velocity (SWV) were observed in ST (4.88 (4.35-5.52) m/s, ST vs QT, p = 0.005; ST vs PT, p < 0.001) followed by QT (4.61 (4.13-5.26) m/s, QT vs PT, p < 0.001) and PT (3.73 (3.30-4.68) m/s). Median QT, PT and ST stiffness was significantly higher in AG compared to CG. Male subjects tend to have stiffer QT and PT than female subjects. Positive correlation with SWV was obtained for age and activity level. CONCLUSION: There are significant differences regarding in vivo tendon stiffness between the most frequently used autograft tendon options for ACLR. The quantitative information obtained by SWE could be of particular interest for graft choice for ACLR.

Attenuation of muscle spindle firing with artificially increased series compliance during stretch of relaxed muscle.

Muscle spindles relay vital mechanosensory information for movement and posture, but muscle spindle feedback is coupled to skeletal motion by a compliant tendon. Little is known about the effects of tendon compliance on muscle spindle feedback during movement, and the complex firing of muscle spindles makes these effects difficult to predict. Our goal was to investigate changes in muscle spindle firing using added series elastic elements (SEEs) to mimic a more compliant tendon, and to characterize the accompanying changes in firing with respect to muscle-tendon unit (MTU) and muscle fascicle displacements (recorded via sonomicrometry). Sinusoidal, ramp-and-hold and triangular stretches were analysed to examine potential changes in muscle spindle instantaneous firing rates (IFRs) in locomotor- and perturbation-like stretches as well as serial history dependence. Added SEEs effectively reduced overall MTU stiffness and generally reduced muscle spindle firing rates, but the effect differed across stretch types. During sinusoidal stretches, peak and mean firing rates were not reduced and IFR was best-correlated with fascicle velocity. During ramp stretches, SEEs reduced the initial burst, dynamic and static responses of the spindle. Notably, IFR was negatively related to fascicle displacement during the hold phase. During triangular stretches, SEEs reduced the mean IFR during the first and second stretches, affecting the serial history dependence of mean IFR. Overall, these results demonstrate that tendon compliance may attenuate muscle spindle feedback during movement, but these changes cannot be fully explained by reduced muscle fascicle length or velocity, or MTU force.

Biomechanical assessment of gastrocnemii and Achilles tendon using MyotonPRO: in vivo measurements, and preliminary in situ measurements using formalin-fixed tissues.

PURPOSE: This study aims to evaluate the reliability and validity of using MyotonPRO to quantify the mechanical properties of the muscle-tendon unit through in vivo measurements and preliminary in situ measurements using formalin-fixed tissues. MATERIALS AND METHODS: The mechanical properties of gastrocnemii and the Achilles tendon of 12 healthy adults (six males and six females, 34.9 ± 5.8 years) were examined for in vivo test twice within a day and once post-24 hours using MyotonPRO, while nine human cadavers (formalin-fixed, 3 males and 6 females, 89.9 ± 5.1 years) were assessed for preliminary in situ test with identical time schedule to evaluate the within-day and inter-day reliability and validity. RESULTS: In vivo tests had very high within-day (ICC: 0.96-0.99) and inter-day reliability (ICC: 0.83-0.96), while in situ tests (formalin-fixed tissues) showed high within-day (ICC: 0.87-0.99) and inter-day reliability (ICC: 0.76-0.98) for the results of tone and stiffness. There was no significant difference in the stiffness of the free part of the Achilles tendon between in vivo and in situ conditions. The stiffness of the lateral gastrocnemius (r = 0.55, p = 0.018), proximal part of the Achilles tendon (r = 0.56, p = 0.015), and free part of the Achilles tendon (r = 0.47, p = 0.048) before removing the skin was significantly correlated with that after removing the skin condition. CONCLUSIONS: The findings of the current study suggest that MyotonPRO is reliable and valid for evaluating tendon stiffness both in vivo and in situ (formalin-fixed tissues).

Effects of tendon elastic energy and electromyographic activity pattern on jumping height and pre-stretch augmentation during jumps with different pre-stretch intensity.

The present study aimed to investigate the effects of tendon elastic energy and electromyographic activity patterns (ratio of pre-landing to concentric: mEMG PLA/CON; ratio of eccentric to concentric; mEMG ECC/CON) on jump performance. Twenty-nine males performed five kinds of unilateral jumps using only ankle joint (no-countermovement jump: noCMJ; countermovement jump: CMJ; drop jumps at 10, 20 and 30 cm drop height: DJ10, DJ20 and DJ30). Jumping height, pre-stretch augmentation and electromyographic activity of the plantar flexor muscles were measured. The elastic energy of the Achilles tendon was measured during isometric contractions. Relative tendon elastic energy (to body mass) was highly correlated with jumping heights of CMJ, DJ10 and DJ20 but not with noCMJ and DJ30, whereas that was significantly correlated with pre-stretch augmentation in CMJ, but not with three DJs. The mEMG PLA/CON was significantly correlated with the pre-stretch augmentation of DJ20 and DJ30, but not with DJ10, whereas the mEMG ECC/CON was significantly correlated with the pre-stretch augmentation of DJ20 and DJ30, but not with CMJ and DJ10. These results suggested that jumping exercises with low pre-stretch intensity benefited from tendon elastic energy, but those with high pre-stretch intensity benefited from electromyographic activity patterns.

Determinants of joint stiffness and jumping height during drop jump.

The present study aimed to examine the effects of muscle-tendon mechanical properties and electromyographic activity on joint stiffness and jumping height and to explore the determinants of joint stiffness and jumping height. Twenty-nine males performed unilateral drop jumps at three drop heights (10, 20, and 30 cm) using only the ankle joint on the sledge apparatus. Ankle joint stiffness, jumping height, and electromyographic activity of the plantar flexor muscles were measured during drop jumps. Active muscle stiffness of the medial gastrocnemius muscle was calculated according to changes in the estimated muscle force and fascicle length during fast stretching at five different angular velocities (100, 200, 300, 500, and 600 deg s-1 ) after submaximal isometric contractions. Tendon stiffness and elastic energy were measured during ramp and ballistic contractions. Active muscle stiffness was significantly correlated with joint stiffness, except for a few conditions. Tendon stiffness measured during ramp and ballistic contractions was not significantly correlated with joint stiffness. The ratios of electromyographic activity before landing and during the eccentric phase to that during the concentric phase were significantly correlated with joint stiffness. In addition, jumping heights at 10 and 20 cm (except for 30 cm) drop heights were strongly associated with the tendon elastic energy, whereas no other measured variables showed significant correlations with jumping heights. These results suggested that (1) joint stiffness is determined by active muscle stiffness and electromyographic activity patterns during jumping, and (2) jumping height is determined by tendon elastic energy.

Man vs. machine: surgeon vs. elastography assessment of the quality of the rotator cuff.

Background: The most common complication of arthroscopic rotator cuff repair is retear, which is more common in larger tears and older patients. We hypothesized that the quality of the torn tendon is important in protecting against retear. Surgeons have traditionally assessed the quality of repaired tendons with a four-point Likert scale. Shear Wave Elastography Ultrasound (SWEUS) is a recent technological advancement that can quantify soft-tissue stiffness. This study aimed to determine how closely a surgeon's intraoperative ranking of tissue quality during rotator cuff repair correlated to postoperative supraspinatus tendon stiffness measured by SWEUS. Methods: This was a prospective case series on 50 patients undergoing arthroscopic rotator cuff repair, involving SWEUS measurements of each patient's supraspinatus tendon at 8 days, 6 weeks, 12 weeks, 6 months, and 12 months. The intraoperative surgeon score of tissue quality for each patient was ranked on a four-point Likert scale. Each patient's scores were compared to postoperative SWEUS velocity measurements of the supraspinatus tendon postrepair. Results: The SWEUS determined stiffness of supraspinatus tendons at their repaired insertion site postrepair increased by 22% from 6.3 ± 0.2 m/s to 7.7 ± 0.3 m/s over 12 months as the tendons healed (P = .0001). Supraspinatus tendon stiffness was greater in patients with smaller tears (r = -0.50, P = .001) and of younger age (r = -0.58, P = .00001). Surgeons also consistently rated younger patients (rs = -0.49, P = .0001) and smaller tears (rs = -0.56, P = .00001) as having superior intraoperative tendon quality. The correlations between SWEUS velocity and surgeon tissue quality rankings were modest at best and strongest at 12 weeks (rs = 0.27, P = .04). There were modest associations between SWEUS tendon stiffness and surgeon tendon mobility rankings at 6 weeks (rs = 0.26, P = .04) and repair quality rankings at 12 months (rs = 0.36, P = .02). Conclusions: These data support the finding that machines (SWEUS) are better at assessing torn rotator cuff tendon quality and whether that tendon will heal after repair than the 'person' performing the surgery. Supraspinatus tendons lose stiffness as they get older and when the tear is larger, likely explaining why retear post-cuff repair is more common with advanced age and larger tears.

PIEZO1 gain-of-function gene variant is associated with elevated tendon stiffness in humans.

Prolonged periods of increased physical demands can elicit anabolic tendon adaptations that increase stiffness and mechanical resilience or conversely can lead to pathological processes that deteriorate tendon structural quality with ensuing pain and potential rupture. Although the mechanisms by which tendon mechanical loads regulate tissue adaptation are largely unknown, the ion channel PIEZO1 has been implicated in tendon mechanotransduction, with human carriers of the PIEZO1 gain-of-function variant E756del displaying improved dynamic vertical jump performance compared with noncarriers. Here, we sought to examine whether increased tendon stiffness in humans could explain this increased performance. We assessed tendon morphological and mechanical properties with ultrasound-based techniques in 77 participants of Middle- and West-African descent, and we measured their vertical jumping performance to assess potential functional consequences in the context of high tendon strain-rate loading. Carrying the E756del gene variant (n = 30) was associated with 46.3 ± 68.3% (P = 0.002) and 45.6 ± 69.2% (P < 0.001) higher patellar tendon stiffness and Young's modulus compared with noncarrying controls, respectively. Although these tissue level measures strongly corroborate the initial postulate that PIEZO1 plays an integral part in regulating tendon material properties and stiffness in humans, we found no detectable correlation between tendon stiffness and jumping performance in the tested population that comprised individuals of highly diverse physical fitness level, dexterity, and jumping ability.NEW & NOTEWORTHY The E756del gene variant causes overactivity of the mechanosensitive membrane channel PIEZO1 and is suspected to upregulate tendon collagen cross linking. In human carriers of E756del, we found increased patellar tendon stiffness but similar tendon lengths and cross-sectional areas, directly supporting the premise that PIEZO1 regulates human tendon stiffness at the level of tissue material properties.

Attenuation of muscle spindle firing with artificially increased series compliance during stretch of relaxed muscle.

Muscle spindles relay vital mechanosensory information for movement and posture, but muscle spindle feedback is coupled to skeletal motion by a compliant tendon. Little is known about the effects of tendon compliance on muscle spindle feedback during movement, and the complex firing of muscle spindles make these effects difficult to predict. Our goal was to investigate changes in muscle spindle firing using added series elastic elements (SEEs) to mimic a more compliant tendon, and to characterize the accompanying changes in firing with respect to muscle-tendon unit (MTU) and muscle fascicle displacements (recorded via sonomicrometry). Sinusoidal, ramp-hold-release, and triangular stretches were analyzed to examine potential changes in muscle spindle instantaneous firing rates (IFRs) in locomotor- and perturbation-like stretches as well as history dependence. Added SEEs effectively reduced overall MTU stiffness and generally reduced muscle spindle firing rates, but the effect differed across stretch types. During sinusoidal stretches, peak firing rates were reduced and IFR was strongly correlated with fascicle velocity. During ramp stretches, SEEs reduced the dynamic and static responses of the spindle during lengthening but had no effect on initial bursts at the onset of stretch. Notably, IFR was negatively related to fascicle displacement during the hold phase. During triangular stretches, SEEs reduced the mean IFR during the first and second stretches, affecting the history dependence of mean IFR. Overall, these results demonstrate that tendon compliance may attenuate muscle spindle feedback during movement, but these changes cannot be fully explained by reduced muscle fascicle length and velocity.

Patellar tendon elastic properties derived from in vivo loading and kinematics.

Patellar complications frequently limit the success of total knee arthroplasty. In addition to the musculoskeletal forces themselves, patellar tendon elastic properties are essential for driving patellar loading. Elastic properties reported in the literature exhibit high variability and appear to differ according to the methodologies used. Specifically in total knee arthroplasty patients, only limited knowledge exists on in vivo elastic properties and their corresponding loads. For the first time, we report stiffness, Young's modulus, and forces of the patellar tendon, derived from four patients with telemetric total knee arthroplasties using a combined imaging and measurement approach. To achieve this, synchronous in vivo telemetric assessment of tibio-femoral contact forces and fluoroscopic assessment of knee kinematics, along with full body motion capture and ground reaction forces, fed musculoskeletal multi-body models to quantify patellar tendon loading and elongation. Mechanical patellar tendon properties were calculated during a squat and a sit-stand-sit activity, with resulting tendon stiffness and Young's modulus ranging from 511 to 1166 N/mm and 259 to 504 MPa, respectively. During these activities, the patellar tendon force reached peak values between 1.31 and 2.79 bodyweight, reaching levels of just âˆ¼0.5 bodyweight below the tibio-femoral forces. The results of this study provide valuable input data for mechanical simulations of the patellar tendon and the whole resurfaced knee.

Contractile properties are less affected at long than short muscle length after eccentric exercise.

PURPOSE: The aim of the present study was to investigate whether the electrically evoked muscle responses are differently affected over time by the knee joint angle after an exercise-induced muscle damage (EIMD). We hypothesized that low-frequency-evoked responses would be less affected at long than short muscle length, and that mechanisms located within the muscle and tendinous tissues would be involved. METHODS: Fifteen males performed 45 min loaded downhill walking (DW) exercise. Maximal voluntary contraction torque (MVC), optimal angle for torque production, voluntary activation level (VAL), twitch, doublet at 10 and 100 Hz (Db10 and Db100, respectively), rate of torque development (RTD), post-activation potentiation (PAP), muscle shear elastic modulus (µ) and aponeurosis stiffness were assessed before, after, and 4, 24, 48, 72 and 168 h after the exercise at a knee angle of 40°, 90° and 120° (0°: full extension). RESULTS: MVC, VAL and Db100 were similarly decreased across joint angles after the DW and optimal angle was not affected. Twitch, Db10, Db10/Db100, PAP and RTD were less affected and muscle µ more increased at long than short muscle lengths (p < 0.05), especially during the first 24 h after the DW exercise. CONCLUSION: Low-frequency-evoked responses were more preserved at long than short muscle length the first 24 h after the DW exercise, suggesting that joint angle should be taken into account to assess muscular alterations after EIMD. This length-dependence could be associated to the higher sensitivity to Ca2+ and the higher increase in muscle stiffness at long than short muscle length.

Adolescent boys who participate in sports exhibit similar ramp torque control with young men despite differences in strength and tendon characteristics.

PURPOSE: The goal of this paper was to determine if sports participation influences torque control differently for adolescent boys and young men during a slow ramp task. METHODS: Twenty-one adolescent boys (11 athletes) and 31 young men (16 athletes) performed a slow ramp increase in plantar flexion torque from 0 to maximum. We quantified torque control as the coefficient of variation (CV) of torque during the ramp and quantified the Achilles tendon mechanical properties using ultrasonography. RESULTS: Relative to adolescent boys, young men were taller, heavier, stronger, and had a longer and stiffer Achilles tendon. However, these characteristics were not different between athletes and non-athletes in adolescent boys. For the CV of torque, there was a significant interaction with sports participation, indicating that only adolescent boys who were non-athletes had greater variability than young men. The CV of torque of all participants was predicted from the maximum torque and torque oscillations from 1 to 2 Hz, whereas the CV of torque for adolescent boys was predicted only from torque oscillations from 1 to 2 Hz. CONCLUSION: These findings suggested that adolescent boys who participate in sports exhibited lower torque variability during a slow ramp task, which was not explained by differences in Achilles tendon properties or strength.

Role of Knee and Ankle Extensors' Muscle-Tendon Properties in Dynamic Balance Recovery from a Simulated Slip.

Participants exposed to a simulated slip with forward loss of balance (FLB) develop large lower limb joint moments which may be a limiting factor for those whose muscle-tendon units' (MTUs) properties are deteriorated. Whether the age-related decline in these properties limits participants' capacity to recover their balance following a slip with FLB remains unclear. We combined isokinetic dynamometry, ultrasound and EMG to understand how knee extensor and ankle plantarflexor muscle strength and power, rate of moment development, electromechanical delay, and tendon stiffness affected the balance of young (25.3 ± 3.9 years) and older adults (62.8 ± 7.1 years) when recovering from a single slip with FLB triggered whilst walking on a split-belt instrumented treadmill. Except for the patellar tendon's stiffness, knee extensor and ankle plantarflexor electromechanical delays, older adults' MTUs properties were deteriorated compared to those of young participants (p < 0.05). We found no significant relationship between age or the MTUs properties of participants and balance recovery. These findings provide additional support that neither maximal nor explosive strength training are likely to be successful in preventing a fall for healthy older adults, and that other type of interventions, such as task-specific training that has already proved efficacious in reducing the risk of falling, should be developed.

Reliability and Accuracy of a Time-Efficient Method for the Assessment of Achilles Tendon Mechanical Properties by Ultrasonography.

The assessment of the force-length relationship under mechanical loading is widely used to evaluate the mechanical properties of tendons and to gain information about their adaptation, function, and injury. This study aimed to provide a time-efficient ultrasound method for assessing Achilles tendon mechanical properties. On two days, eleven healthy young non-active adults performed eight maximal voluntary isometric ankle plantarflexion contractions on a dynamometer with simultaneous ultrasonographic recording. Maximal tendon elongation was assessed by digitizing ultrasound images at rest and at maximal tendon force. Achilles tendon stiffness index was calculated from the ratio of tendon force-to-strain. No within- and between-day differences were detected between the proposed method and manual frame by frame tracking in Achilles tendon maximal force, maximal elongation, maximal strain, and stiffness index. The overall coefficient of variation between trials ranged from 3.4% to 10.3% and average difference in tendon tracking between methods was less than 0.6% strain. Furthermore, an additional assessment demonstrated significant differences between elite athletes, healthy young, and older adults in Achilles tendon force and stiffness index. Hence, the analysis has the potential to reliably and accurately monitor changes in Achilles tendon mechanical properties due to aging and altered mechanical loading in a time-efficient manner.

Sprint start performance: the potential influence of triceps surae electromechanical delay.

In the sprint start, a defined sequence of distinct response delays occurs before the athlete produces a movement response. Excitation of lower limb muscles occurs prior to force production against the blocks, culminating in a movement response. The time delay between muscle excitation and movement, electromechanical delay (EMD), is considered to influence sprint start response time (SSRT). This study examined the delay in sprint start performance from EMD of the triceps surae muscle and examined whether certain sprinters gain an advantage in SSRT. Nineteen experienced sprinters performed sprint starts from blocks, with SSRT measured by an International Association of Athletics Federations (IAAF)-approved starting block system. EMD times were detected during a heel-lift experiment. Using revised SSRT limits, based on concerns over the validity of the IAAF 100 ms false start limit, EMD produced a significant moderate correlation with SSRT (r = 0.572, p = 0.011). Regression analysis determined that together, EMD and signal processing time (the delay between the auditory signal and muscle excitation) accounted for 37% of the variance in SSRT. Initial results suggest EMD is part of the response time process and that certain athletes may gain a performance advantage due to reduced EMD.

Immediate Effect of Whole Body Vibration on Knee Extensor Tendon Stiffness in Hemiparetic Stroke Patients.

Background and Objectives: Whole body vibration is widely used to enhance muscle performance, but evidence of its effects on the tendon stiffness of the knee extensor tendon in stroke remains inconclusive. Our study was aimed to determine the difference in patellar and quadriceps tendon stiffness between hemiparetic and unaffected limbs in stroke patients and to investigate the immediate effect of whole body vibration on tendon stiffness. Materials and Methods: The patellar and quadriceps tendon stiffness of first-ever hemiplegic stroke patients was evaluated with elastography to compare the differences between hemiparetic and unaffected limbs. After one 20 min session of whole body vibration exercise in the standing position, tendon stiffness was again measured to evaluate the immediate effects of whole body vibration on tendon stiffness. Results: The results showed no significant differences in the tendon stiffness of the patellar and quadriceps tendons between hemiparetic and unaffected limbs. However, significant associations were found between the tendon stiffness of the patellar and quadriceps tendons and knee extensor spasticity on the hemiparetic side (ρ = 0.62; p = 0.044). There were no significant changes in tendon stiffness after a single session of whole body vibration. Conclusions: In conclusion, knee extensor tendon stiffness in hemiparetic limbs is positively correlated to the degree of knee extensor spasticity in stroke patients. However, a single session of whole body vibration does not alter tendon stiffness.

The energy of muscle contraction. IV. Greater mass of larger muscles decreases contraction efficiency.

While skeletal muscle mass has been shown to decrease mass-specific mechanical work per cycle, it is not yet known how muscle mass alters contraction efficiency. In this study, we examined the effect of muscle mass on mass-specific metabolic cost and efficiency during cyclic contractions in simulated muscles of different sizes. We additionally explored how tendon and its stiffness alters the effects of muscle mass on mass-specific work, mass-specific metabolic cost and efficiency across different muscle sizes. To examine contraction efficiency, we estimated the metabolic cost of the cycles using established cost models. We found that for motor contractions in which the muscle was primarily active during shortening, greater muscle mass resulted in lower contraction efficiency, primarily due to lower mass-specific mechanical work per cycle. The addition of a tendon in series with the mass-enhanced muscle model improved the mass-specific work and efficiency per cycle with greater mass for motor contractions, particularly with a shorter excitation duty cycle, despite higher predicted metabolic cost. The results of this study indicate that muscle mass is an important determinant of whole muscle contraction efficiency.

The Influence of Growth, Maturation and Resistance Training on Muscle-Tendon and Neuromuscular Adaptations: A Narrative Review.

The purpose of this article is to provide an overview of the growth, maturation and resistance training-related changes in muscle-tendon and neuromuscular mechanisms in youth, and the subsequent effect on performance. Sprinting, jumping, kicking, and throwing are common movements in sport that have been shown to develop naturally with age, with improvements in performance being attributed to growth and maturity-related changes in neuromuscular mechanisms. These changes include moderate to very large increases in muscle physiological cross-sectional area (CSA), muscle volume and thickness, tendon CSA and stiffness, fascicle length, muscle activation, pre-activation, stretch reflex control accompanied by large reductions in electro-mechanical delay and co-contraction. Furthermore, a limited number of training studies examining neuromuscular changes following four to 20 weeks of resistance training have reported trivial to moderate differences in tendon stiffness, muscle CSA, muscle thickness, and motor unit activation accompanied by reductions in electromechanical delay (EMD) in pre-pubertal children. However, the interaction of maturity- and training-related neuromuscular adaptions remains unclear. An understanding of how different neuromuscular mechanisms adapt in response to growth, maturation and training is important in order to optimise training responsiveness in youth populations. Additionally, the impact that these muscle-tendon and neuromuscular changes have on force producing capabilities underpinning performance is unclear.

Effect of Knee Angle and Quadriceps Muscle Force on Shear-Wave Elastography Measurements at the Patellar Tendon.

Shear-wave elastography (SWE) is a non-invasive imaging technique that provides estimates of tissue stiffness via shear-wave speed measurements. No standardized protocol currently exists for SWE of the patellar tendon, which may be influenced by knee angle and quadriceps muscle force. In this study, the reliability of SWE in cadaveric patellar tendons was examined at three knee angles (0°, 30° and 60°) and three quadriceps muscle forces (0, 50 and 100 N). Shear-wave speed was significantly higher at a knee angle of 60° than at 0° or 30° (increases of 7% and 9%, respectively), and when the quadriceps muscle force was greater than or equal to 50 N (increase of 15%). SWE of the patellar tendon displayed excellent repeatability regardless of knee angle as long as no quadriceps force was generated (intra-class correlation coefficient ≥0.91). This research illustrates the importance of controlling knee angle and quadriceps force for consistency and comparison of SWE results.