The influence of loading intensity on muscle–tendon unit behavior during maximal knee extensor stretch shortening cycle exercise

Chronic Adaptions in Quadriceps Fascicle Mechanics Are Related to Altered Knee Biomechanics After Anterior Cruciate Ligament Reconstruction

Following anterior cruciate ligament reconstruction (ACLR), patients exhibit abnormal walking mechanics and quadriceps dysfunction. Quadriceps dysfunction has been largely attributed to muscle atrophy and weakness. While important, these factors do not capture intrinsic properties of muscle that govern its ability to generate force and withstand load. While fascicle abnormalities after ACLR have been documented in early stages of recovery (<12 mo), long-term effects of ACLR on fascicle mechanics remain unexplored. We evaluated quadriceps fascicle mechanics during walking 3 years post-ACLR and examined the relationship with knee mechanics. Participants included 24 individuals with ACLR and 24 Controls. Linear mixed models compared the ACLR, Contralateral, and Controls limbs for (1) quadriceps strength, (2) fascicle architecture and mechanics, and (3) knee mechanics. No difference in strength or overall fascicle length excursions was found between limbs. The ACLR limb exhibited longer fascicles at heel strike and peak knee extension moment (P < .001-.004), and smaller fascicle angles at heel strike, peak knee extension moment, and overall suppressed fascicle angle excursions (P < .001-.049) relative to the Contralateral and/or Control limb. This indicates an abnormality in fascicle architecture and mechanics following ACLR and suggests abnormalities in contractile function that cannot be explained by muscle weakness and may contribute to long-term gait irregularities.

Biomechanical analysis of distance adjustment in volleyball overhead pass

In volleyball, the overhead pass is important. Coaches usually teach players to push the ball, utilising the legs, especially for a long distance pass. The purpose of this study was to conduct a biomechanical analysis of an overhead pass to different distances. Twelve male elite volleyball players participated. They were told to pass a ball with an overhead pass through a 2.43 m high ring located 3 m, 6 m, and 9 m in front of them. Twenty-four reflective markers were placed on the ball and the subject's body to obtain joint centres, joint angles, and ball position. EMG activities were recorded from four right arm muscles. Triceps brachii and flexor carpi radialis pull and push phase activities differed significantly between 3 m and 6 m, but not between 6 m and 9 m. On the other hand, leg movements were significantly different among 3 m, 6 m, and 9 m, suggesting that leg movements supplement the power given to the ball. Thus, as the distance to a target lengthens in an overhead pass, not only arm movement but also leg movement is required.

Modelling and in vivo evaluation of tendon forces and strain in dynamic rehabilitation exercises: a scoping review

OBJECTIVES

Although exercise is considered the preferred approach for tendinopathies, the actual load that acts on the tendon in loading programmes is usually unknown. The objective of this study was to review the techniques that have been applied in vivo to estimate the forces and strain that act on the human tendon in dynamic exercises used during rehabilitation.

DESIGN

Scoping review.

DATA SOURCES

Embase, PubMed, Web of Science and Google Scholar were searched from database inception to February 2021.

ELIGIBILITY CRITERIA

Cross-sectional studies available in English or Spanish language were included if they focused on evaluating the forces or strain of human tendons in vivo during dynamic exercises. Studies were excluded if they did not evaluate tendon forces or strain; if they evaluated running, walking, jumping, landing or no dynamic exercise at all; and if they were conference proceedings or book chapters.

DATA EXTRACTION AND SYNTHESIS

Data extracted included year of publication, study setting, study population characteristics, technique used and exercises evaluated. The studies were grouped by the types of techniques and the tendon location.

RESULTS

Twenty-one studies were included. Fourteen studies used an indirect methodology based on inverse dynamics, nine of them in the Achilles and five in the patellar tendon. Six studies implemented force transducers for measuring tendon forces in open carpal tunnel release surgery patients. One study applied an optic fibre technique to detect forces in the patellar tendon. Four studies measured strain using ultrasound-based techniques.

CONCLUSIONS

There is a predominant use of inverse dynamics, but force transducers, optic fibre and estimations from strain data are also used. Although these tools may be used to make general estimates of tendon forces and strains, the invasiveness of some methods and the loss of immediacy of others make it difficult to provide immediate feedback to the individuals.

Effects of isometric loading intensity on patellar tendon microvascular response

Acute increases in tendon blood flow and oxygenation after stress (i.e., hyperemic response) can enhance tendon recovery. While loading intensity is a fundamental part of resistance training programs, its effects on tendon's hyperemic response are unknown. This study aimed to compare acute changes in total (total hemoglobin [THb]) and oxygenated hemoglobin (HbO₂ ) concentrations in the patellar tendon after isometric exercise at different intensities. Thirteen participants performed 8 (5 s) isometric knee extensions at 25%, 50%, and 75% maximal load (maximal voluntarily isometric contraction [MVIC]), separated by 20 min recovery, prescribed in randomized and counterbalanced order. Changes in patellar tendon THb, HbO₂ and deoxygenated hemoglobin (HHb) in response to exercise at each intensity were measured using near-infrared spectroscopy. Post-exercise, HbO₂ increased with 50% ( η p 2 $$ {\eta}_p^2 $$  = 0.305, f = 5.26, p < 0.01) and 75% ( η p 2 $$ {\eta}_p^2 $$  = 0.245, f = 4.56, p < 0.01) but not 25% ( η p 2 $$ {\eta}_p^2 $$  = 0.088, f = 1.16, p = 0.339) MVIC, while THb increased in 50% ( η p 2 $$ {\eta}_p^2 $$  = 0.305, f = 5.26, p = 0.01) but not 25% ( η p 2 $$ {\eta}_p^2 $$  = 0.067, f = 0.865, p = 0.51) or 75% ( η p 2 $$ {\eta}_p^2 $$  = 0.126, f = 1.729, p = 0.14) MVIC. Additionally, increasing load from 25% to 50% MVIC resulted in greater THb (f = 2.459, p = 0.43), HbO₂ (f = 3.389, p = 0.13) and HHb (f = 0.320, p = 0.01) post-exercise responses, but no differences were observed between 50% and 75% MVIC (THb: f = 0.748, p = 0.59; HbO₂ : f = 0.825, p = 0.54; HHb: f = 0.713, p = 0.62). Our results suggest there is a loading threshold at ~50% MVIC at which the tendon hyperemic response is fully achieved. Training above this intensity is not expected to provide any additional change to the tendon microvascular response. Therefore, moderate loading seems to be sufficient to fully elicit the patellar tendon hyperemic response that's believed to stimulate tendon healing.

Ground reaction force and electromyograms of lower limb muscles during fast walking

Background

Physically active status is an important contributor to individual health. Walking is regarded as commonly accepted exercise for exercise promotion. Particularly, interval fast walking (FW), consisting of alternating between fast and slow walking speeds, has gained popularity from practical viewpoints. Although previous studies have determined the short- and long-term effects of FW programs on endurance capacity and cardiovascular variables, factors affecting these outcomes have not been clarified. In addition to physiological variables, understanding of mechanical variables and muscle activity during FW would be a help to understand characteristics of FW. In the present study, we compared the ground reaction force (GRF) and lower limb muscle activity between fast walking (FW) and running at equivalent speeds.

Method

Eight healthy men performed slow walking (45% of the maximum walking speed; SW, 3.9 ± 0.2 km/h), FW (85% of the maximum walking speed, 7.4 ± 0.4 km/h), and running at equivalent speeds (Run) for 4 min each. GRF and average muscle activity (aEMG) were evaluated during the contact, braking, and propulsive phases. Muscle activities were determined for seven lower limb muscles: gluteus maximus (GM), biceps femoris (BF), rectus femoris (RF), vastus lateralis (VL), gastrocnemius medialis (MG), soleus (SOL), and tibialis anterior (TA).

Results

The anteroposterior GRF was greater in FW than in Run during the propulsive phase (p < 0.001), whereas the impact load (peak and average vertical GRF) was lower in FW than in Run (p < 0.001). In the braking phase, lower leg muscle aEMGs were higher during Run than during SW and FW (p < 0.001). However, in the propulsive phase, soleus muscle activity was greater during FW than during Run (p < 0.001). aEMG of tibialis anterior was higher during FW than during SW and Run in the contact phase (p < 0.001). No significant difference between FW and Run was observed for HR and RPE.

Conclusion

These results suggest that the average muscle activities of lower limbs (e.g., gluteus maximus, rectus femoris, and soleus) during the contact phase were comparable between FW and running, however, the activity patterns of lower limb muscles differed between FW and running, even at equivalent speeds. During running, muscles were mainly activated in the braking phase related to impact. In contrast, during FW, soleus muscle activity during the propulsive phase was increased. Although cardiopulmonary response was not different between FW and running, exercise using FW might be useful for health promotion among individuals who cannot exercise at high-intensity.

Suppressed quadriceps fascicle behavior is present in the surgical limbs of those with a history of ACL reconstruction

The balance of published data have largely focused on adaptations in muscle and fiber size after anterior cruciate ligament reconstruction (ACLR), failing to account for the dynamic changes in the behavior of the muscles' contractile elements that strongly contribute to force production. To better understand the sources of quadriceps dysfunction, the purpose of our research was to determine if alterations in fascicle behavior are present after ACLR. Unilateral ACLR individuals (9 m/9f; 21 ± 3 yrs; 1.74 ± 0.12 m;71.58 ± 13.31 kg; months from surgery:38 ± 36) and healthy controls (3 m/6f; 23 ± 2 yrs; 1.67 ± 0.10 m; 63.51 ± 10.11 kg) participated. In-vivo vastus lateralis fascicle behavior was recorded using ultrasonography during three maximal isokinetic knee extensions (60°·s^-1). Fascicle length, angle, and shortening velocity were calculated and analyzed from rest to peak torque. Peak knee extension torque was averaged between isokinetic trials (Nm·kg^-1). Group by limb interactions were assessed using separate two-way analyses of variance and were further evaluated by comparing 95% confidence intervals where appropriate. Significant interactions were present for fascicle angle at peak torque (P = 0.01), fascicle length excursion (P = 0.05), fascicle angle excursion (P < 0.01), fascicle shortening velocity (P = 0.05) and strength (P = 0.03). Upon post-hoc evaluation, the surgical limb displayed altered in-vivo fascicle behavior compared to all limbs (P < 0.05) and reduced strength compared to the contralateral and right control limbs (P < 0.05). No other significant interactions were present (P > 0.05). Our data show that those with a history of ACLR have fascicles that are slower, lengthen less and operate with lower angles relative to the axis of force production. Altered fascicle behavior after ACLR may be an important underlying factor to explaining the protracted quadriceps dysfunction.

Neuromuscular adaptations after 12 weeks of light- vs. heavy-load power-oriented resistance training in older adults

This study aimed to determine the specific adaptations provoked by power-oriented resistance training using light (LL-PT, 40% 1-RM) vs. heavy (HL-PT, 80% 1-RM) loads in older adults. Using a randomized within-subject study design, 45 older adults (>65 years) completed an 8-week control period (CTR) followed by 12 weeks of unilateral LL-PT vs. HL-PT on a leg press. The 1-RM, theoretical force at zero velocity (F₀ ), maximal unloaded velocity (V₀ ), and maximal muscle power (P_max ) were determined through a force-velocity relationship test. Isometrically, the rate of force development (RFD) and the corresponding muscle excitation of the knee extensor muscles were assessed. In addition, muscle cross-sectional area (CSA) and architecture of two quadriceps muscles were determined. Changes after CTR, LL-PT and HL-PT were compared using linear mixed models. HL-PT provoked greater improvements in 1-RM and F₀ (effect size (ES) = 0.55-0.68; p < 0.001) than those observed after LL-PT (ES = 0.27-0.47; p ≤ 0.001) (post hoc treatment effect, p ≤ 0.057). By contrast, ES of changes in V₀ was greater in LL-PT compared to HL-PT (ES = 0.71, p < 0.001 vs. ES = 0.39, p < 0.001), but this difference was not statistically significant. Both power training interventions elicited a moderate increase in P_max (ES = 0.65-0.69, p < 0.001). Only LL-PT improved early RFD (ie, ≤100 ms) and muscle excitation (ES = 0.36-0.60, p < 0.05). Increased CSA were noted after both power training programs (ES = 0.13-0.35, p < 0.035), whereas pennation angle increased only after HL-PT (ES = 0.37, p = 0.004). In conclusion, HL-PT seems to be more effective in improving the capability to generate large forces, whereas LL-PT appears to trigger greater gains in movement velocity in older adults. However, both interventions promoted similar increases in muscle power as well as muscle hypertrophy.

High-speed stretch-shortening cycle exercises as a strategy to provide eccentric overload during resistance training

Resistance exercises eliciting eccentric overload (EO) are considered to strongly promote muscular hypertrophy and broad neuromuscular adaptations but typically require specialized equipment. The aims of these experiments were to assess whether EO is achieved during common high-speed stretch-shortening cycle (SSC) exercises (rebound bench press throw [RBPT] and squat jump [SJ]), and to test the effect of the external load on the EO achieved. Twenty-nine under 18 handball players and fifteen physically active males (24.9 ± 3.2 years) took part in the experiments. Testing consisted of a single set of 6 repetitions with light (25%-30% 1-RM), moderate (50% 1-RM), and heavy (70%-75% 1-RM) loads. Eccentric and concentric force near the zero-velocity point (50-200 ms) as well as eccentric-concentric force ratio (EO; %) were calculated. In RBPT, higher EO values were found at 50% 1-RM than 70% 1-RM in the time interval 50 ms before and after the zero-velocity point. Higher EO values were also found at 50% 1-RM than both 30% 1-RM and 70% 1-RM 100 ms before and after the zero-velocity point. For the SJ, higher EO values were found at 50% 1-RM and 75% 1-RM than 25% 1-RM 100 ms before and after the zero-velocity point. Higher EO values were found at 50% 1-RM than 25% 1-RM 200 ms before and after the zero-velocity point. However, the higher EO values in the SJ were found far from the zero-velocity point. High-speed SSC resistance training provides similar EO to other methods and thus should promote muscle hypertrophy and other neuromuscular adaptations.

Reduced knee extensor torque production at low to moderate velocities in postmenopausal women with knee osteoarthritis

This study aimed to determine deficits in knee extensor muscle function through the torque-time and torque-velocity relationships and whether these deficits are associated with reduced functional performance in postmenopausal women with knee osteoarthritis (KOA). A clinical sample of postmenopausal women with established KOA (n = 18, ≥55 years) was compared to an age-matched healthy control sample (CON) (n = 26). The deficits in different parameters of the knee extensor torque-time (maximal isometric torque and rate of torque development) and torque-velocity relationship (maximum muscle power, maximal velocity and torque at 0-500°·s^-1 ) were assessed through a protocol consisting of isometric, isotonic and isokinetic tests. Functional performance was evaluated with sit-to-stand and stair-climbing tasks using a sensor-based technology (ie, time- and power-based outcomes). Postmenopausal women with KOA showed reduced maximal isometric torque (Hedge's g effect size (g) = 1.05, p = 0.001) and rate of torque development (g = 0.77-1.17, all p ≤ 0.02), combined with impaired torque production at slow to moderate velocities (g = 0.92-1.70, p ≤ 0.004), but not at high or maximal velocities (g = 0.16, p > 0.05). KOA were slower (g = 0.81-0.92, p ≤ 0.011) and less powerful (g = 1.11-1.29, p ≤ 0.001) during functional tasks. Additionally, knee extensor deficits were moderately associated with power deficits in stair climbing (r = 0.492-0.659). To conclude, knee extensor muscle weakness was presented in postmenopausal women with KOA, not only as limited maximal and rapid torque development during isometric contractions, but also dynamically at low to moderate velocities. These deficits were related to impaired functional performance. The assessment of knee extensor muscle weakness through the torque-time and torque-velocity relationships might enable individual targets for tailored exercise interventions in KOA.

Acute Physiological Response to Light- and Heavy-load Power-oriented Exercise in Older Adults

This study investigated the acute responses to volume-load-matched heavy-load (80% 1RM) versus light-load (40% 1RM) power-oriented resistance training sessions in well-functioning older adults. Using a randomized cross-over design, 15 volunteers completed each condition on a leg press. Neuromuscular (maximal isometric force and rate of force development) and functional performance (power during sit-to-stand test), lactate, and muscle damage biochemistry (creatine kinase, lactate dehydrogenase and C-reactive protein serum concentration) were assessed pre- and post-exercise. Performance declines were found after heavy-load (Cohen's d effect size (d); maximal isometric force=0.95 d; rate of force development=1.17 d; sit-to-stand power =0.38 d, all p<0.05) and light-load (maximal isometric force=0.45 d; rate of force development=0.9 d; sit-to-stand power=1.17 d, all p<0.05), while lactate concentration increased only after light-load (1.7 d, p=0.001). However, no differences were found between conditions (all p>0.05). Both conditions increased creatine kinase the day after exercise (marginal effect=0.75 d, p<0.001), but no other blood markers increased (all, p>0.05). Irrespective of the load used, power training induced non-clinically significant decreases in sit-to-stand performance, moderate declines in maximal isometric force, but pronounced decreases in the rate of force development. Furthermore, the metabolic stress and muscle damage were minor; both sessions were generally well tolerated by well-functioning older adults without previous experience in resistance training.

Ultrasound imaging to assess skeletal muscle architecture during movements: a systematic review of methods, reliability, and challenges

B-mode ultrasound is often used to quantify muscle architecture during movements. Our objectives were to 1) systematically review the reliability of fascicle length (FL) and pennation angles (PA) measured using ultrasound during movements involving voluntary contractions; 2) systematically review the methods used in studies reporting reliability, discuss associated challenges, and provide recommendations to improve the reliability and validity of dynamic ultrasound measurements; and 3) provide an overview of computational approaches for quantifying fascicle architecture, their validity, agreement with manual quantification of fascicle architecture, and advantages and drawbacks. Three databases were searched until June 2019. Studies among healthy human individuals aged 17-85 yr that investigated the reliability of FL or PA in lower-extremity muscles during isoinertial movements and that were written in English were included. Thirty studies (n = 340 participants) were included for reliability analyses. Between-session reliability as measured by coefficient of multiple correlations (CMC), and coefficient of variation (CV) was FL CMC: 0.89-0.96; CV: 8.3% and PA CMC: 0.87-0.90; CV: 4.5-9.6%. Within-session reliability was FL CMC: 0.82-0.99; CV: 0.0-6.7% and PA CMC: 0.91; CV: 0.0-15.0%. Manual analysis reliability was FL CMC: 0.89-0.96; CV: 0.0-15.9%; PA CMC: 0.84-0.90; and CV: 2.0-9.8%. Computational analysis FL CMC was 0.82-0.99, and PA CV was 14.0-15.0%. Eighteen computational approaches were identified, and these generally showed high agreement with manual analysis and high validity compared with phantoms or synthetic images. B-mode ultrasound is a reliable method to quantify fascicle architecture during movement. Additionally, computational approaches can provide a reliable and valid estimation of fascicle architecture.

High-density electromyographic assessment of stretch reflex activity during drop jumps from varying drop heights

This study investigated how drop heights and their associated drop jump performance relate to stretch reflex modulations. Eleven male subjects performed ten drop jumps from each of three individually predetermined drop heights. These were the drop height resulting in maximal performance (OPT), as well as 10 cm below (LOW) and above (HIGH) maximal performance. To quantify drop jump performance the reactive strength index, derived from force plate measures, was used. High-density surface EMG provided both stretch reflex response timing and size, as well as novel insight into the associated motor unit recruitment via muscle fiber conduction velocity estimations. These measures were examined in the vastus lateralis (VL), soleus (SOL) and gastrocnemius medialis (GM). Drop jump performance improved by 9% (p < 0.001) from LOW to OPT and decreased by 5% (p = 0.008) from OPT to HIGH. Despite decreasing performance, stretch reflex responses were largest at HIGH. Stretch reflex responses timing did not change; staying within the short (SOL, <60 ms) and medium (VL, GM; 60-85 ms) latency response time-frames. Motor unit recruitment appeared to change across drop heights only for VL, whereas activation intensity only changed for SOL. These results indicate that during drop jumps above OPT neuromuscular modifications result in VL no longer being maximally recruited.

Influence of forearm orientation on biceps brachii tendon mechanics and elbow flexor force steadiness

Achilles tendon mechanics influence plantar flexion force steadiness (FS) and balance. In the upper limb, elbow flexor FS is greater in supinated and neutral forearm orientations compared to pronated, with contributions of tendon mechanics remaining unknown in position-dependent FS. This study investigated whether distal biceps brachii (BB) tendon mechanics across supinated, neutral and pronated forearm orientations influence position-dependent FS of the elbow flexors. Eleven males (23 ± 3 years) performed submaximal isometric elbow flexion tasks at low (5, 10% maximal voluntary contraction (MVC)) and high (25, 50, 75% MVC) force levels in supinated, neutral and pronated forearm orientations. Distal BB tendon elongation and CSA were recorded on ultrasound to calculate mechanics of tendon stress, strain and stiffness. Relationships between FS, calculated as coefficient of variation (CV) of force, and tendon mechanics were evaluated with multiple regressions. Supinated and neutral were ∼50% stronger and ∼60% steadier than pronated (p < 0.05). Tendon stress was ∼52% greater in supinated and neutral compared to pronated, tendon strain was ∼36% greater in neutral than pronated (p < 0.05), while tendon stiffness (267.4 ± 78.9 N/mm) did not differ across orientations (p > 0.05). At low forces, CV of force was predicted by MVC (r²: 0.52) in supinated, and MVC and stress in neutral and pronated (r²: 0.65-0.81). At high force levels, CV of force was predicted by MVC and stress in supinated (r²: 0.49), and MVC in neutral (r²: 0.53). Absolute strength and tendon mechanics influence the ability of the BB tendon to distribute forces, and thus are key factors in position-dependent FS.

Comparative analysis of quadriceps and hamstrings strength in knee osteoarthritis before and after total knee arthroplasty: a cross-sectional study

Objective

Compare the maximal isokinetic muscle strength of knee extensor and flexor muscles between patients with knee osteoarthritis and patients submitted to total knee arthroplasty.

Methods

Volunteers were divided into five groups (n = 20): Control; Ahlbäck I and II; Ahlbäck IV; six months after total knee arthroplasty; 12 months after total knee arthroplasty. An isokinetic knee strength evaluation was conducted for the quadriceps and hamstrings at 60°/s.

Results

Significant differences in the peak torque of the quadriceps and hamstrings were found among the groups (p < 0.001). The Ahlbäck IV, six-month, and 12-month postoperative groups demonstrated lower values when compared to the Control and Ahlbäck I and II groups. When percentage values were compared to the Control group, mean differences ranged from 7% to 41%.

Conclusion

Patients with healthy knees or early stage osteoarthritis have higher quadriceps and hamstrings strengths than those with a more advanced stage of the disease, even after knee replacement. These findings suggest that the traditional rehabilitation programs do not recover strength to levels observed in individuals without knee osteoarthritis.

The Influence of External Load on Quadriceps Muscle and Tendon Dynamics during Jumping

Tendons possess both viscous (rate-dependent) and elastic (rate-independent) properties that determine tendon function. During high-speed movements external loading increases both the magnitude (FT) and rate (RFDT) of tendon loading.

PURPOSE

The influence of external loading on muscle and tendon dynamics during maximal vertical jumping was explored.

METHODS

Ten resistance-trained men performed parallel-depth, countermovement vertical jumps with and without additional load (0%, 30%, 60%, and 90% of maximum squat lift strength), while joint kinetics and kinematics, quadriceps tendon length (LT) and patellar tendon FT and RFDT were estimated using integrated ultrasound, motion analysis and force platform data and muscle tendon modelling.

RESULTS

Estimated FT and RFDT, but not peak LT, increased with external loading. Temporal comparisons between 0% and 90% loads revealed that FT was greater with 90% loading throughout the majority of the movement (11%-81% and 87%-95% movement duration). However, RFDT was greater with 90% load only during the early movement initiation phase (8%-15% movement duration) but was greater in the 0% load condition later in the eccentric phase (27%-38% movement duration). LT was longer during the early movement (12%-23% movement duration) but shorter in the late eccentric and early concentric phases (48%-55% movement duration) with 90% load.

DISCUSSION

External loading positively influenced peak FT and RFDT but tendon strain appeared unaffected, suggesting no additive effect of external loading on patellar tendon lengthening during human jumping. Temporal analysis revealed that external loading resulted in a large initial RFDT that may have caused dynamic stiffening of the tendon and attenuated tendon strain throughout the movement. These results suggest that external loading influences tendon lengthening in both a load- and movement-dependent manner.

COMPARISON OF FUNCTIONAL ACTIVITIES ON STRUCTURAL CHANGES OF THE INFERIOR PATELLAR POLE

Background

It is well known that eccentric and concentric exercise produce varied amounts of stress on the connective tissues. Diagnostic ultrasound has been used to measure these structural changes by observing fascicle length, angle, and thickness; however, there is a lack of evidence comparing the structural changes as it relates to eccentric, concentric, and stretching protocols.

Purpose

The purpose of this study was to compare the acute effects of static stretching, eccentric, concentric, and a combination of eccentric/concentric exercises on structural changes of the muscle tendon unit at the inferior patellar pole utilizing the diagnostic ultrasound.

Study Design

A repeated measures 2 × 4 within factorial study design with repeated measures on both factors was used to determine the differences in patellar tendon thickness within and between groups.

Methods

Forty-seven healthy subjects were screened for any lower extremity deficits or orthopaedic pathology. Forty-four (N=44) subjects completed all four protocols; the attrition was due to injuries to the lower extremity, occurring unrelated to the study. A baseline measurement of the anterior inferior patellar tendon was performed with the diagnostic ultrasound prior to each participant completing one of the four interventions per week over a four-week period. Interventions completed by each participant included static stretching, concentric, eccentric, and combined concentric and eccentric exercises. Immediately following each intervention, a post-intervention inferior patellar tendon measurement was recorded using the diagnostic ultrasound.

Results

Significant differences in anterior to posterior tendon thickness of the inferior patellar tendon were observed between pre (4.983 ± 0.041mm) and post (5.198 ± 0.055mm) measurements (p<0.0005) for the main effect of time. However, no differences in tendon thickness were noted comparing each intervention to one another (p=0.351).

Conclusion

Differences in tendon thickness were noted acutely for pre- to post measurements across all interventions. Further research is needed to determine if differences in tendon thickness exist with a longer duration of exercise over time and with different types of intervention.

Intrasession and Intersession Reliability of Quadriceps' and Hamstrings' Electromyography During a Standardized Hurdle Jump Test With Single Leg Landing

The objective of this study was to develop a standardized test to determine quadriceps and hamstrings muscle activation in a position emulating a noncontact anterior cruciate ligament injury. We assessed the intrasession and intersession reliability of surface electromyography (EMG) of the dominant leg after single-leg landing from a standardized hurdle jump. Eighteen subjects (10 males, 8 females) participated in 4 repeated sessions. During each session, individuals performed 3 successful jumps over a hurdle set to 75% of their maximal countermovement jump height and landed on their dominant leg. A jump was only considered successful if the individual could maintain the landing position for longer than 2 seconds after initial ground contact. In one of the 4 sessions, subjects were tested again after a 4-minute rest. The activation of the vastus lateralis (VL), vastus medialis (VM), and biceps femoris (BF), were examined by quantifying the root mean squared (RMS) EMG for 2 seconds immediately after the initial contact. Data from all 3 successful jumps were used to generate intraclass correlation coefficients (ICC), which were then used to determine intrasession and intersession reliability of surface EMG for each muscle. Intrasession reliability was excellent with ICC values of 0.96, 0.94, and 0.93 for the VL, VM, and BF, respectively. Additionally, intersession ICCs were 0.92 (VL), 0.95 (VM), and 0.94 (BF). The standardized hurdle jump with single-leg landing seemed to be a reliable technique for measuring muscle activation for 3 muscles that contribute to knee stabilization.

The neuromotor effects of transverse friction massage

BACKGROUND

Transverse friction massage (TFM), as an often used technique by therapists, is known for its effect in reducing the pain and loosing the scar tissues. Nevertheless, its effects on neuromotor driving mechanism including the electromechanical delay (EMD), force transmission and excitation-contraction (EC) coupling which could be used as markers of stiffness changes, has not been computed using ultrafast ultrasound (US) when combined with external sensors.

AIM

Hence, the aim of this study was to find out produced neuromotor changes associated to stiffness when TFM was applied over Quadriceps femoris (QF) tendon in healthy subjcets.

METHODS

Fourteen healthy males and fifteen age-gender matched controls were recruited. Surface EMG (sEMG), ultrafast US and Force sensors were synchronized and signals were analyzed to depict the time delays corresponding to EC coupling, force transmission, EMD, torque and rate of force development (RFD).

RESULTS

TFM has been found to increase the time corresponding to EC coupling and EMD, whilst, reducing the time belonging to force transmission during the voluntary muscle contractions.

CONCLUSIONS

A detection of the increased time of EC coupling from muscle itself would suggest that TFM applied over the tendon shows an influence on changing the neuro-motor driving mechanism possibly via afferent pathways and therefore decreasing the active muscle stiffness. On the other hand, detection of decreased time belonging to force transmission during voluntary contraction would suggest that TFM increases the stiffness of tendon, caused by faster force transmission along non-contractile elements. Torque and RFD have not been influenced by TFM.

Faster Movement Speed Results in Greater Tendon Strain during the Loaded Squat Exercise

INTRODUCTION

Tendon dynamics influence movement performance and provide the stimulus for long-term tendon adaptation. As tendon strain increases with load magnitude and decreases with loading rate, changes in movement speed during exercise should influence tendon strain.

METHODS

Ten resistance-trained men [squat one repetition maximum (1RM) to body mass ratio: 1.65 ± 0.12] performed parallel-depth back squat lifts with 60% of 1RM load at three different speeds: slow fixed-tempo (TS: 2-s eccentric, 1-s pause, 2-s concentric), volitional-speed without a pause (VS) and maximum-speed jump (JS). In each condition joint kinetics, quadriceps tendon length (LT), patellar tendon force (FT), and rate of force development (RFDT) were estimated using integrated ultrasonography, motion-capture, and force platform recordings.

RESULTS

Peak LT, FT, and RFDT were greater in JS than TS (p < 0.05), however no differences were observed between VS and TS. Thus, moving at faster speeds resulted in both greater tendon stress and strain despite an increased RFDT, as would be predicted of an elastic, but not a viscous, structure. Temporal comparisons showed that LT was greater in TS than JS during the early eccentric phase (10-14% movement duration) where peak RFDT occurred, demonstrating that the tendon's viscous properties predominated during initial eccentric loading. However, during the concentric phase (61-70 and 76-83% movement duration) differing FT and similar RFDT between conditions allowed for the tendon's elastic properties to predominate such that peak tendon strain was greater in JS than TS.

CONCLUSIONS

Based on our current understanding, there may be an additional mechanical stimulus for tendon adaptation when performing large range-of-motion isoinertial exercises at faster movement speeds.

Inhomogeneous Quadriceps Femoris Hypertrophy in Response to Strength and Power Training

INTRODUCTION

Previous studies have reported inhomogeneous changes in quadriceps femoris (QF) cross-sectional area (CSA) in response to strength training. It is assumed that these differential changes in muscle shape influence the muscle's functional capacity during high-force and high-power movements. The purpose of the current study was to compare intermuscular and intramuscular QF adaptations to high-load strength training and fast-speed power training.

METHODS

Thirty-six non-strength-trained men were randomly assigned to four groups and completed 8 wk of parallel-depth heavy squat-lift training (HS-P), parallel-depth jump squat training (JS-P), volitional-depth jump squat training (JS-V), or no training (C). Quadriceps femoris, vastus lateralis (VL), intermedius (VI), medialis (VM), and rectus femoris (RF) CSA were measured in distal-, mid-, and proximal-thigh regions using extended field-of-view ultrasonography and compared using a 3 × 2 mixed-model MANOVA with Bonferroni post hoc tests (P < 0.05).

RESULTS

Parallel-depth heavy squat-lift training and JS-P elicited similar changes in mid-CSA(QF) as well as summed CSA of the QF, VL, VI, and VM. Cross-sectional area of the VL (CSA(VL)) and CSA(VI) increased in both HS-P and JS-P at mid-thigh, but only JS-P significantly increased CSA proximally, and only HS-P significantly increased CSA distally. Cross-sectional area of the VM (CSA(VM)) increased in HS-P and JS-P distally, but only HS-P increased at mid-thigh. No hypertrophy was observed in RF at any location and no significant differences were observed between JS-P and JS-V. Parallel-depth heavy squat-lift training elicited greater proximal hypertrophy in each of the vasti muscles, whereas only JS-P elicited distal VL and VI hypertrophy.

CONCLUSIONS

These observed inhomogeneous changes in CSA may alter the thigh's moment of inertia and moment arms of muscle "compartments," and the influence of elastic component force transmission on the muscular force expression. Such selective hypertrophy is speculated to be biomechanically beneficial to high-force or high-power movements used in training.

Vastus lateralis maximum force-generating potential occurs at optimal fascicle length regardless of activation level

PURPOSE

Despite the fact that everyday movements are hardly ever performed with muscles contracting maximally, our understanding of the force-length relationship is mostly based on in vitro studies using maximal activation. In this study, the in vivo submaximal and maximal force-length relationships of vastus-lateralis were investigated. Force-length relationships were obtained based on maximal and submaximal levels of force and, also, on EMG activation.

METHODS

Nine subjects performed isometric knee extensor contractions at ten knee angles (80°-170°). Knee extensor torque, and vastus-lateralis EMG and fascicle lengths were acquired simultaneously. Fascicle lengths and knee angles at peak force occurrence were compared across maximal and submaximal conditions.

RESULTS

The submaximal force-fascicle length relationships depend crucially on the approach used: in the force-based approach, peak forces are constrained to occur at the same MTU length and, because of series elasticity, occur at longer fascicle lengths for decreasing force levels. In contrast, in the activation-based approach, peak force occurrence is not constrained to a given muscle length for submaximal contractions and occurs at similar fascicle lengths but shorter MTU lengths (more extended knee angles) as force decreases.

CONCLUSIONS

Our results support the hypothesis that vastus-lateralis fascicle length for maximal force production is about constant for maximal and submaximal levels of activation, presumably taking advantage of optimal myofilament overlap at that fascicle length. This result implies that optimal vastus-lateralis lengths occur at different knee angles for different levels of activation, which is in stark contrast to findings in the literature in which submaximal force-fascicle length relationships were based on force rather than activation.

Regional Surface Electromyography of the Vastus Lateralis During Strength and Power Exercises

Neuromuscular activation during and chronic adaptation from exercise are innately linked and both can vary along a muscle's length. During high-force and high-speed exercise, intramuscular hypertrophy follows set patterns that provide the greatest biomechanical advantages. However, it is unknown if muscle activity as recorded by surface electromyography (sEMG) will follow these patterns. The purpose of the present study was to compare vastus lateralis intramuscular sEMG during the heavy squat (HS) and unloaded jump squat (JS) exercises. Ten subjects performed HS with 80% of maximum load and unloaded JS to parallel-depth, while intramuscular peak sEMG and mean sEMG were measured at 33% (proximal), 50% (middle), and 67% (distal) thigh length. Muscle activity was compared between regions and exercises using a 3 × 2 repeated measures ANOVA with Bonferoni post hoc corrections. Peak sEMG was greater proximally in JS than HS (p = 0.033), but similar in the middle and distal regions (p = 0.521, 0.594, respectively), whereas mean sEMG was similar between all regions (p = 0.150-0.979). In addition, a main effect was found in which peak and mean sEMG were greater proximally than the middle and distal regions (p = 0.001, 0.006). Muscle activity measured using sEMG displayed dissimilar patterns to previously observed regional hypertrophy. Specifically, although previous research found greater proximal hypertrophy in JS than HS, in the present study peak sEMG was greater in HS than JS. Furthermore, distally where HS elicited greater hypertrophy than JS, no differences in sEMG were present. Thus, regional sEMG appears not to be a viable tool for predicting differences in regional hypertrophy, most likely due to technological constraints and intramuscular differences in muscle structure.

Factors of force potentiation induced by stretch-shortening cycle in plantarflexors

Muscle force is potentiated by countermovement; this phenomenon is called stretch-shortening cycle (SSC) effect. In this study, we examined the factors strongly related to SSC effect in vivo, focusing on tendon elongation, preactivation, and residual force enhancement. Twelve healthy men participated in this study. Ankle joint angle was passively moved by a dynamometer, with a range of motion from 15° dorsiflexion (DF) to 15° plantarflexion (PF). Muscle contraction was evoked by electrical stimulation, with stimulation timing adjusted to elicit three types of contraction: (1) concentric contraction without preliminary contraction (CON), (2) concentric contraction after preliminary eccentric contraction (ECC), and (3) concentric contraction after preliminary isometric contraction (ISO). Joint torque was recorded at DF5°, PF0°, and PF5°, respectively. SSC effect was calculated as the ratio of joint torque obtained in ECC or ISO with respect to that obtained in CON at the aforementioned three joint angles. SSC effect was prominent in the first half of movement in both ECC (DF5°, 329.3 ± 101.2%; PF0°, 159.2 ± 29.4%; PF5°, 125.5 ± 20.8%) and ISO (DF5°, 276.4 ± 87.0%; PF0°, 134.5 ± 24.5%; PF5°, 106.8 ± 18.0%) conditions. SSC effect was significantly larger in ECC than in ISO at all joint angles (P < 0.001). Even without preliminary eccentric contraction (i.e., ISO condition), SSC effect was clearly large, indicating that a significant part of SSC effect is derived from preactivation. However, the active lengthening-induced force potentiation mechanism (residual force enhancement) also contributes to SSC effect.

Detection of the electromechanical delay and its components during voluntary isometric contraction of the quadriceps femoris muscle

Electromechanical delay (EMD) was described as a time elapsed between first trigger and force output. Various results have been reported based on the measurement method with observed inconsistent results when the trigger is elicited by voluntary contraction. However, mechanomyographic (MMG) sensor placed far away on the skin from the contracting muscle was used to detect muscle fiber motion and excitation-contraction (EC) coupling which may give unreliable results. On this basis, the purpose of this study was to detect EMD during active muscle contraction whilst introducing an ultrafast ultrasound (US) method to detect muscle fiber motion from a certain depth of the muscle. Time delays between onsets of EMG-MMG, EMG-US, MMG-FORCE, US-FORCE, and EMG-FORCE were calculated as 20.5 ± 4.73, 28.63 ± 6.31, 19.21 ± 6.79, 30.52 ± 8.85, and 49.73 ± 6.99 ms, respectively. Intrarater correlation coefficient (ICC) was higher than MMG when ultrafast US was used for detecton of the Δt EMG-US and Δt US-FORCE, ICC values of 0.75 and 0.70, respectively. Synchronization of the ultrafast ultrasound with EMG and FORCE sensors can reveal reliable and clinically useful results related to the EMD and its components when muscle is voluntarily contracted. With ultrafast US, we detect onset from the certain depth of the muscle excluding the tissues above the muscle acting as a low-pass filter which can lead to inaccurate time detection about the onset of the contracting muscle fibers. With this non-invasive technique, understanding of the muscle dynamics can be facilitated.