Observing an expert's action swapped with an observer's face increases corticospinal excitability during combined action observation and motor imagery.

This study aimed to examine whether observing an expert's action swapped with an observer's face increases corticospinal excitability during combined action observation and motor imagery (AOMI). Twelve young males performed motor imagery of motor tasks with different difficulties while observing the actions of an expert performer and an expert performer with a swapped face. Motor tasks included bilateral wrist dorsiflexion (EASY) and unilateral two-ball rotating motions (DIFF). During the AOMI of EASY and DIFF, single-pulse transcranial magnetic stimulation was delivered to the left primary motor cortex, and motor-evoked potentials (MEPs) were obtained from the extensor carpi ulnaris and first dorsal interosseous muscles of the right upper limb, respectively. Visual analogue scale (VAS) assessed the subjective similarity of the expert performer with the swapped face in the EASY and DIFF to the participants themselves. The MEP amplitude in DIFF was larger in the observation of the expert performer with the swapped face than that of the expert performer (P = 0.012); however, the corresponding difference was not observed in EASY (P = 1.000). The relative change in the MEP amplitude from observing the action of the expert performer to that of the expert performer with the swapped face was positively correlated with VAS only in DIFF (r = 0.644, P = 0.024). These results indicate that observing the action of an expert performer with the observer's face enhances corticospinal excitability during AOMI, depending on the task difficulty and subjective similarity between the expert performer being observed and the observer.

Validity of spatiotemporal and ground reaction force estimates during resisted sprinting with a motorized loading device.

We aimed to examine the validity of estimating spatiotemporal and ground reaction force (GRF) parameters during resisted sprinting using a robotic loading device (1080 Sprint). Twelve male athletes (age: 20.9 ± 2.2 years; height: 174.6 ± 4.2 cm; weight: 69.4 ± 6.1 kg; means ± SDs) performed maximal resisted sprinting with three different loads using the device. The step frequency and length and step-averaged velocity, anteroposterior GRF (Fap ), and the ratio of Fap to resultant GRF (RF) were estimated using the velocity and towing force data measured using the device. Simultaneously, the corresponding values were measured using a 50-m force plate system. The proportional and fixed biases of the estimated values against those measured using the force plate system were determined using ordinary least product (OLP) regression analysis. Proportional and fixed biases were observed for most variables. However, the proportional bias was small or negligible except for the step frequency. Conversely, the fixed bias was small for step-averaged velocity (0.11 m/s) and step length (0.04 m), whereas it was large for step frequency (0.54 step/s), Fap (16N), and RF (2.22%). For all variables except step frequency, the prediction intervals in the OLP regression dramatically decreased when the corresponding values were smoothed using a two-step moving average. These results indicate that by using the velocity and force data recorded in the loading device, most of the spatiotemporal and GRF variables during resisted sprinting can be estimated with some correction of the fixed bias and data smoothing using the two-step moving average.

Associations of muscle volume of individual human plantar intrinsic foot muscles with morphological profiles of the foot.

Human plantar intrinsic foot muscles consist of 10 muscles that originate and insert within the sole of the foot. It is known that the anatomical cross-sectional area (ACSA) and muscle thickness of two plantar intrinsic foot muscles, the flexor hallucis brevis (FHB) and abductor hallucis (ABH), associate with morphological parameters of the foot, such as total and truncated foot length and navicular height. However, it is unclear how the size for each of the plantar intrinsic foot muscles associates with various morphological profiles of the foot. This study aimed to elucidate this subject. By using magnetic resonance imaging (MRI), serial images of the right foot were obtained in 13 young adult men without foot deformities. From the obtained MR images, ACSA for each of the individual plantar intrinsic foot muscles was analyzed along the foot length, and then its muscle volume (MV) was calculated. The analyzed muscles were the abductor digiti minimi (ABDM), ABH, adductor hallucis oblique head (ADDH-OH), adductor hallucis transverse head (ADDH-TH), flexor digitorum brevis (FDB), FHB, and quadratus plantae (QP). Furthermore, MV of the whole plantar intrinsic foot muscle (WHOLE) was defined as the total MVs of all the analyzed muscles. As morphological parameters, total foot length, truncated foot length, forefoot width, ball circumference, instep circumference, navicular height, great toe eversion angle, and little toe inversion angle were measured using a laser three-dimensional foot scanner in standing and sitting conditions. In addition, navicular drop (ND) and normalized truncated navicular height (NTNH) were also calculated as medial longitudinal arch (MLA) height indices. The MV of WHOLE was significantly associated with the forefoot width, ball circumference, and instep circumference (r = 0.647-0.711, p = 0.006-0.013). Positive correlations were found between the forefoot width and MV of FHB, FDB, and QP (r = 0.564-0.653, p = 0.015-0.045), between the ball circumference and MV of QP (r = 0.559, p = 0.047), between the instep circumference and MV of FHB (r = 0.609, p = 0.027), and between the little toe inversion angle and MV of QP (r = 0.570, p = 0.042). The MVs of ABH, ABDM, and ADDH-OH were not significantly correlated with any morphological parameters of the foot. Similarly, no significant correlations were found between MV of each muscle and either of the MLA height indices (ND and NTNH). Thus, the current results indicate that forefoot width and circumferential parameters (instep and ball circumference), not MLA height, associate with the size of the whole plantar intrinsic foot muscles, especially those specialized in toe flexion (FHB, FDB, and QP).

Projected frontal area and its components during front crawl depend on lung volume.

We examined the influence of lung volume on the vertical body position, trunk inclination, and projected frontal area (PFA) during swimming and the inter-relationships among these factors. Twelve highly trained male swimmers performed a 15 m front crawl with sustained maximal inspiration (INSP), maximal expiration (EXP), and intermediate (MID) at a target velocity of 1.20 m·s-1 . Using our developed digital human model, which allows inverse kinematics calculations by fitting individual body shapes measured with a three-dimensional photonic image scanner to individually measured underwater motion capture data, vertical center of mass (CoM) position, trunk inclination, and PFA were calculated for each complete stroke cycle. In particular, the PFA was calculated by automatic processing of a series of parallel frontal images obtained from a reconstructed digital human model. The vertical CoM position was higher with a larger lung-volume level (p < 0.01). The trunk inclination was smaller in INSP and MID than in EXP (p < 0.01). PFA was smaller with a larger lung-volume level (p < 0.01). Additionally, there was a significant interaction of vertical CoM position and trunk inclination with PFA (p = 0.006). There was a negative association between PFA and vertical CoM position, and a positive association between PFA and trunk inclination less than the moderate vertical CoM position (each p < 0.05). These results obtained using our methodology indicate that PFA decreases with increasing lung volume due to an increase in vertical CoM position, and additionally due to a decrease in trunk inclination at low-to-moderate lung-volume levels.

Reconstruction of net force fluctuations from surface EMGs of multiple muscles in steady isometric plantarflexion.

The purposes of this study were to clarify if force fluctuations during steady multi-muscle contractions have a temporal correlation with a low-frequency component of rectified surface EMG (rEMG) in the involved muscles and collection of that component across muscles allows for the reconstruction of force fluctuations across a wide range of contraction intensities. Healthy young men (n = 15) exerted steady isometric plantarflexion force at 5-60% of maximal force. Surface EMG was recorded from the medial and lateral gastrocnemii, soleus, peroneus longus, abductor hallucis, and tibialis anterior muscles. The cross-correlation function (CCF) between plantarflexion force fluctuations and low-pass filtered rEMG in each muscle was calculated for 8 s. To reconstruct force fluctuations from rEMGs, the product of rEMG and an identified constant factor were summed across muscles with time-lag compensation for electro-mechanical delay. A distinct peak of the CCF was found between plantarflexion force fluctuations and rEMG in most cases except for the tibialis anterior. The CCF peak was greatest in the medial gastrocnemius and soleus. Reconstructed force from rEMGs was temporally correlated with measured force fluctuations across contraction intensities (average CCF peak: r = 0.65). The results indicate that individual surface rEMG has a low-frequency component that is temporally correlated with net force fluctuations during steady multi-muscle contractions and contributes to the reconstruction of force fluctuations across a wide range of contraction intensities. It suggests a potential applicability of individual surface EMGs for identifying the contributing muscles to controlling or disturbing isometric steady force in multi-muscle contractions.

Does Intra-abdominal Pressure Have a Causal Effect on Muscle Strength of Hip and Knee Joints?

ABSTRACT: Tayashiki, K, Kanehisa, H, and Miyamoto, N. Does intra-abdominal pressure have a causal effect on muscle strength of hip and knee joints? J Strength Cond Res 35(1): 41-46, 2021-It remains unclear whether intra-abdominal pressure (IAP) has a causal effect on lower-limb muscle strength. This study aimed to clarify whether or not changes in IAP, induced by changing breathing state, influence muscle strength of hip and knee extensor and flexor. Eighteen healthy males (age: 22.0 ± 2.2 years, height: 1.71 ± 0.03 m, and body mass: 68.1 ± 6.1 kg) performed maximal voluntary isometric contractions (MVICs) of hip and knee extensor and flexor during breath-hold at full inspiration (inspiratory condition) or expiration (expiratory condition), or during normal breath-hold (normal condition). Intra-abdominal pressure was obtained by a pressure transducer placed in the rectum and determined at the time at which the developed torque reached to the maximum. The IAP during each MVIC was significantly greater in inspiratory condition than in expiratory condition (p < 0.05). The maximal torque of hip extensor was significantly greater in inspiratory condition than in expiratory condition (p < 0.05). By contrast, the maximal torque of each of hip flexor, knee extensor, and knee flexor was not different among the 3 breath-hold conditions. The IAP was significantly correlated with the maximal torque of hip extensor in each breath-hold condition. The current results suggest that a sufficient increase in IAP has a causal effect to specifically improve muscle strength of hip extensor.

Ground reaction force across the transition during sprint acceleration.

Abrupt changes in kinematics during sprint acceleration called transitions have previously been observed. This study aimed to examine whether ground reaction force (GRF) variables during sprint acceleration also show specific features of the transitions. Twenty-one male sprinters performed 60-m sprints, during which GRF data were recorded. Step-to-step spatiotemporal and GRF variables were approximated using an exponential function and three straight lines. Moreover, statistical parametric mapping (SPM) was used to test changes in GRF curves across the transitions. For running speed, the exponential approximation resulted in smaller root-mean-square (RMS) of residuals. For the other variables, however, RMS of residuals was smaller when the three lines approximation was adopted. Breakpoints around the 5th and 15th steps were detected using effective vertical impulse during the braking phase with the three lines approximation. Across the breakpoints, SPM showed significant differences in the antero-posterior GRF curves at the next step after the first breakpoint and at the second breakpoint. Moreover, the second braking phase of the antero-posterior GRF appeared at the next step after the first breakpoint, and the corresponding first propulsive phase disappeared at the second breakpoint. Consequently, changes in GRF variables during sprint acceleration are likely accompanied by specific alterations. The breakpoints around the 5th and 15th steps found in an effective vertical impulse during the braking phase can be a criterion indicating transitions in GRF variables during sprint acceleration. The transitions are characterized by an appearance and disappearance of the second braking and first propulsive phases, respectively, of the antero-posterior GRF.

Relationship between Trunk Muscularity and Club Head Speed in Male Golfers.

This study examined how the volume of trunk muscles and its bilateral asymmetry are related to club head speed in golfers. Fourteen right-handed male golfers performed five driver shots, and the club head speed for each trial was calculated from a three-dimensional reflective marker position of the club head immediately before impact. The volume of each side of the rectus abdominis, erector spinae, psoas major, quadratus lumborum, lateral abdominal wall muscle, and multifidus was determined using magnetic resonance imaging. For each muscle, the ratio of the larger to smaller side in muscle volume was calculated to assess bilateral asymmetry. The club head speed correlated positively with the volume of each side of the rectus abdominis and erector spinae, left quadratus lumborum, and the asymmetric ratio of the psoas major (r=0.595-0.747), but negatively with the asymmetric ratio of the quadratus lumborum (r=-0.641). Multiple regression analysis revealed that the right erector spinae volume and the asymmetric ratio of the psoas major were significant contributors for the club head speed (R2=0.797). These results indicate that the variation in the club head speed can be strongly explained by the absolute volume and bilateral asymmetry of specific trunk muscles.

COL5A1 rs12722 polymorphism is not associated with passive muscle stiffness and sports-related muscle injury in Japanese athletes.

BACKGROUND: Poor joint flexibility has been repeatedly proposed as a risk factor for muscle injury. The C-to-T polymorphism (rs12722) in the 3'-untranslated region of the collagen type V α1 chain gene (COL5A1) is reportedly associated with joint flexibility. Flexibility of a normal joint is largely determined by passive muscle stiffness, which is influenced by intramuscular collagenous connective tissues including type V collagen. The present study aimed to test the hypothesis that the COL5A1 rs12722 polymorphism influences joint flexibility via passive muscle stiffness, and is accordingly associated with the incidence of muscle injury. METHODS: In Study 1, we examined whether the rs12722 polymorphism is associated with joint flexibility and passive muscle stiffness in 363 healthy young adults. Joint flexibility was evaluated by passive straight-leg-raise and sit-and-reach tests, and passive muscle stiffness was measured using ultrasound shear wave elastography. In Study 2, the association of the rs12722 polymorphism with sports-related muscle injury was assessed in 1559 Japanese athletes. Muscle injury history and severity were assessed by a questionnaire. In both Study 1 and Study 2, the rs12722 C-to-T polymorphism in the COL5A1 was determined using the TaqMan SNP Genotyping Assay. RESULTS: Study 1 revealed that the rs12722 polymorphism had no significant effect on range of motion in passive straight-leg-raise and sit-and-reach tests. Furthermore, there was no significant difference in passive muscle stiffness of the hamstring among the rs12722 genotypes. In Study 2, rs12722 genotype frequencies did not differ between the muscle injury and no muscle injury groups. Moreover, no association was observed between rs12722 polymorphism and severity of muscle injury. CONCLUSIONS: The present study does not support the view that COL5A1 rs12722 polymorphism has a role as a risk factor for sports-related muscle injury, or that it is a determinant for passive muscle stiffness in a Japanese population.

Effects of inspiratory muscle strength and inspiratory resistance on neck inspiratory muscle activation during controlled inspirations.

NEW FINDINGS: What is the central question of this study? What factors influence the onset and magnitude of activation of the neck inspiratory muscles during inspiration? What is the main finding and its importance? Recruitment of the sternocleidomastoid and scalene muscles during inspiration, measured by means of surface EMG, was strongly correlated with maximal inspiratory pressure. This result indicates that muscle recruitment depends on the capacity of an individual to generate inspiratory pressure. Surface measurements of neck inspiratory muscle EMG activity might complement tests currently used for the screening of respiratory-related disease. ABSTRACT: The aims of the present study were as follows: (i) to examine the relationship between the onset of recruitment of the neck inspiratory muscles and inspiratory muscle strength; and (ii) to clarify the effect of inspiratory resistance on neck inspiratory muscle activation during inspiration at specific flow rates and to specific lung volumes. Inspiratory muscle strength, as indicated by maximal inspiratory pressure (MIP), and peak inspiratory flow rate (PFR) were measured in healthy participants. Subsequently, participants inspired at target inspiratory flow rates between 20 and 100% of PFR as closely as possible, with and without artificial inspiratory resistance. Electromyographic activity (EMGRMS ) of the sternocleidomastoid and scalene muscles was measured from surface electrodes at each target flow rate for each 10% increment of forced vital capacity (FVC) between 20 and 50% of FVC. Recruitment onset for each muscle was determined from %PFR-EMGRMS curves at each lung volume (%FVC). Finally, linear regression analyses were performed for MIP and recruitment onset for each muscle at each %FVC. Recruitment onset during inspiration without inspiratory resistance was strongly correlated with MIP (r > 0.60, P < 0.040). Specifically, a lower MIP was associated with earlier muscle recruitment (i.e. recruitment at a lower flow rate), especially for the sternocleidomastoid muscle (r > 0.75, P < 0.005). Recruitment of both neck inspiratory muscles at a given flow rate was also earlier when inspiratory resistance was added (P = 0.002). These results indicate that the recruitment and activation of the neck inspiratory muscles depends on both inspiratory muscle strength and inspiratory resistance.

External mechanical work done during the acceleration stage of maximal sprint running and its association with running performance.

This study aimed to elucidate how external mechanical work done during maximal acceleration sprint running changes with increasing running velocity and is associated with running performance. In twelve young males, work done at each step over 50 m from the start was calculated from mechanical energy changes in horizontal anterior-posterior and vertical directions and was divided into braking (-Wkap and -Wv, respectively) and propulsive (+Wkap and +Wv, respectively) phases. The maximal running velocity (Vmax) appeared at 35.87±7.76 m and the time required to run 50 m (T50 m) was 7.11±0.54 s. At 80% Vmax or higher, +Wkap largely decreased and -Wkap abruptly increased. The change in the difference between +Wkap and |-Wkap| (ΔWkap) at every step was relatively small at 70% Vmax or lower. Total work done over 50 m was 82.4±7.5 J kg-1 for +Wkap, 36.2±4.4 J kg-1 for |-Wkap|, 14.3±1.9 J kg-1 for +Wv, and 10.4±1.2 J kg-1 for |-Wv|. The total ΔWkap over 50 m was more strongly correlated with T50 m (r=-0.946, P<0.0001) than the corresponding associations for the other work variables. These results indicate that in maximal sprint running over 50 m, work done during the propulsive phase in the horizontal anterior-posterior direction accounts for the majority of the total external work done during the acceleration stage, and maximizing it while suppressing work done during the braking phase is essential to achieve a high running performance.

Suspended Push-up Training Augments Size of not only Upper Limb but also Abdominal Muscles.

We investigated the effects of sling-based, suspended push-up training on muscle size and function of upper limb and abdominal muscles. Eight men conducted suspended push-ups to failure 3 sets/session, 3 sessions/week, for 8 weeks. The maximum number of push-ups during training gradually and significantly increased from the first to last training session (+92%), suggesting improved muscle endurance. After the training, muscle thickness of the elbow extensors (+16%) and flexors (+3%), as well as abdominal muscles (rectus abdominis: RA,+27%; external oblique: EO,+14%) significantly increased. No changes occurred in maximum isometric strength of elbow extension or flexion, nor in 1-repetition maximum bench press. In a follow-up experiment, electromyograms (EMGs) of RA, EO and internal oblique (IO) during suspended push-ups to failure were measured and normalized to those during maximum voluntary contraction of each muscle (% EMGmvc) in six men. EMG significantly increased when reaching failure in all muscles (RA: 46-88%, EO: 32-50%, IO: 19-52%, start-end), and was particularly high in RA. These results suggest that suspended push-up training can augment size of not only upper limb but also abdominal muscles, likely attributable to high muscle activities during exercise; however, this does not necessarily improve maximum strength after training thus warrants careful interpretation/application.

Causal effect of intra-abdominal pressure on maximal voluntary isometric hip extension torque.

PURPOSE: Intra-abdominal pressure (IAP) has been recently shown to be associated specifically with maximal voluntary isometric contraction (MVC) torque of hip extension, although the causal relationship remains unclear. The present study aimed to elucidate whether IAP has a causal effect on hip extension MVC torque. METHODS: IAP during hip extension MVC was changed by controlling the lung volume (i.e., depth of inspiration). Twelve healthy males conducted MVCs of hip extension during breath-hold at full inspiration (inspiratory condition) or expiration (expiratory condition), or during normal breath-hold (normal condition). IAP during MVCs was measured a pressure transducer placed in the rectum. RESULTS: The IAP during hip extension MVC was significantly higher in inspiratory condition (132.0 ± 46.1 mmHg) than in the other two conditions and also higher in normal condition (104.6 ± 35.9 mmHg) than in expiratory condition (77.0 ± 39.1 mmHg). The hip extension MVC torque was significantly higher in inspiratory condition (297.7 ± 82.7 N m) than in expiratory condition (266.4 ± 84.5 N m). In each condition, the hip extension MVC torque correlated with IAP during the MVC task. CONCLUSION: The current results suggest that IAP has a positive causal effect on hip extension MVC torque and that a sufficient increase in IAP directly leads to an enhancement of hip extension MVC torque.

Vertical Impulse as a Determinant of Combination of Step Length and Frequency During Sprinting.

This study aimed to clarify the influence of vertical impulse on the magnitude of step length (SL) and frequency (SF) and their ratio during the entire acceleration phase of maximal sprinting. Thirty-nine male soccer players performed 60-m sprints, during which step-to-step ground reaction forces were recorded over a 50-m distance. The mean values of spatiotemporal variables and vertical and anteroposterior impulses for each set of four steps during the acceleration phase until the 28th step were computed to examine relationships among variables in seven sections. When controlling for the influence of running speed, stature and corresponding duration of braking or propulsion, vertical impulses during the propulsive phase at the 1st-4th step section and those during the braking phases in the sections from the 5th-8th to the 25th-28th step were positively correlated with SL and SL/SF ratio and negatively correlated with SF, whereas the anteroposterior impulses were not correlated with SL or SF. In conclusion, the current results demonstrate that vertical impulse during the propulsive phase in the initial acceleration stage and that during the braking phase in the middle and later acceleration stages are the most likely determinants of the combination of SL and SF during sprinting.

Age-Related Differences in Spatiotemporal Variables and Ground Reaction Forces During Sprinting in Boys.

PURPOSE: We aimed to elucidate age-related differences in spatiotemporal and ground reaction force variables during sprinting in boys over a broad range of chronological ages. METHODS: Ground reaction force signals during 50-m sprinting were recorded in 99 boys aged 6.5-15.4 years. Step-to-step spatiotemporal variables and mean forces were then calculated. RESULTS: There was a slower rate of development in sprinting performance in the age span from 8.8 to 12.1 years compared with younger and older boys. During that age span, mean propulsive force was almost constant, and step frequency for older boys was lower regardless of sprinting phase. During the ages younger than 8.8 years and older than 12.1 years, sprint performance rapidly increased with increasing mean propulsive forces during the middle acceleration and maximal speed phases and during the initial acceleration phase. CONCLUSION: There was a stage of temporal slower development of sprinting ability from age 8.8 to 12.1 years, being characterized by unchanged propulsive force and decreased step frequency. Moreover, increasing propulsive forces during the middle acceleration and maximal speed phases and during the initial acceleration phase are probably responsible for the rapid development of sprinting ability before and after the period of temporal slower development of sprinting ability.

Step-to-step spatiotemporal variables and ground reaction forces of intra-individual fastest sprinting in a single session.

We aimed to investigate the step-to-step spatiotemporal variables and ground reaction forces during the acceleration phase for characterising intra-individual fastest sprinting within a single session. Step-to-step spatiotemporal variables and ground reaction forces produced by 15 male athletes were measured over a 50-m distance during repeated (three to five) 60-m sprints using a long force platform system. Differences in measured variables between the fastest and slowest trials were examined at each step until the 22nd step using a magnitude-based inferences approach. There were possibly-most likely higher running speed and step frequency (2nd to 22nd steps) and shorter support time (all steps) in the fastest trial than in the slowest trial. Moreover, for the fastest trial there were likely-very likely greater mean propulsive force during the initial four steps and possibly-very likely larger mean net anterior-posterior force until the 17th step. The current results demonstrate that better sprinting performance within a single session is probably achieved by 1) a high step frequency (except the initial step) with short support time at all steps, 2) exerting a greater mean propulsive force during initial acceleration, and 3) producing a greater mean net anterior-posterior force during initial and middle acceleration.

Association of Sprint Performance With Ground Reaction Forces During Acceleration and Maximal Speed Phases in a Single Sprint.

We aimed to clarify the mechanical determinants of sprinting performance during acceleration and maximal speed phases of a single sprint, using ground reaction forces (GRFs). While 18 male athletes performed a 60-m sprint, GRF was measured at every step over a 50-m distance from the start. Variables during the entire acceleration phase were approximated with a fourth-order polynomial. Subsequently, accelerations at 55%, 65%, 75%, 85%, and 95% of maximal speed, and running speed during the maximal speed phase were determined as sprinting performance variables. Ground reaction impulses and mean GRFs during the acceleration and maximal speed phases were selected as independent variables. Stepwise multiple regression analysis selected propulsive and braking impulses as contributors to acceleration at 55%-95% (ß > 0.72) and 75%-95% (ß > 0.18), respectively, of maximal speed. Moreover, mean vertical force was a contributor to maximal running speed (ß = 0.48). The current results demonstrate that exerting a large propulsive force during the entire acceleration phase, suppressing braking force when approaching maximal speed, and producing a large vertical force during the maximal speed phase are essential for achieving greater acceleration and maintaining higher maximal speed, respectively.

Acute effect of static stretching on passive stiffness of the human gastrocnemius fascicle measured by ultrasound shear wave elastography.

PURPOSE: Passive muscle stiffness and muscle architecture at a given joint angle, as well as slack angle of the muscle have been shown to change after an acute bout of stretching. However, it remains unclear whether passive muscle stiffness at a given fascicle length is reduced after stretching. We aimed to elucidate the acute effect of static stretching on the passive fascicle stiffness using ultrasound shear wave elastography. METHODS: Shear modulus, fascicle length, and slack angle of the medial gastrocnemius (MG) as well as passive plantar flexion torque during passive dorsiflexion were measured before and after a 5-min static stretching in 14 healthy males. RESULTS: After stretching, passive torques were significantly reduced at >50% of range of motion (ROM). Shear modulus at a given fascicle length was significantly reduced at >80% of the change in fascicle length during passive dorsiflexion. Slack angle of MG was observed at the middle part of ROM and significantly shifted toward more dorsiflexed position after stretching. CONCLUSION: The present study showed the significant effectiveness of static stretching on the passive fascicle stiffness. Furthermore, the present results suggest that both the shift in slack angle and the reduction in passive fascicle stiffness contribute to produce the change in passive torque-joint angle relationship during passive dorsiflexion. Notably, the contribution of the reduced passive fascicle stiffness to the decrease in passive torque is substantial over the latter part of ROM.

Unintended activity in homologous muscle during intended unilateral contractions increases with greater task difficulty.

PURPOSE: The present study aimed to examine (1) the effect of task difficulty on unintended muscle activation (UIMA) levels in contralateral homologous muscle, (2) the difference between young and old adults in degree of UIMA with respect to task difficulty, and (3) temporal correlations between intended and contralateral unintended muscle activity at low frequency during unilateral intended force-matching tasks. METHODS: Twelve young (21.8 ± 2.4 years) and twelve old (69.9 ± 5.3 years) adult men performed steady isometric abductions with the left index finger at 20-80% of maximal voluntary contraction force. Two task difficulties were set by adjusting the spacing between two bars centered about the target force used for visual feedback on a monitor. The amplitude of surface electromyogram (aEMG) for both hands was calculated and normalized with respect to the maximal value. To determine if oscillations between intended and unintended muscle activities were correlated, cross-correlation function (CCF) of rectified EMG for both hands at low frequency was calculated for samples deemed adequate. RESULTS: The unintended aEMG (right hand) had significant main effects in task difficulty, age, and target force (all P < 0.05) without any interactions. Distinct significant peaks in CCF (0.38 on average, P < 0.05) with small time lags were present between rectified EMGs of intended and unintended muscles in 14 of the 17 samples. CONCLUSIONS: The current results indicate that UIMA increases with greater task difficulty regardless of age, and temporal correlations exist between intended and contralateral unintended muscle activities at low frequency.

Neck inspiratory muscle activation patterns during well-controlled inspiration.

PURPOSE: Surprisingly, the activation characteristics of the neck inspiratory muscles as a function of key inspiratory mechanical parameters have yet to be demonstrated experimentally under well-controlled conditions. This study aimed to elucidate the muscle activation patterns of the neck inspiratory muscles by strictly controlling flow rate and lung volume. METHODS: Thirteen healthy subjects matched their inspiratory flow rate at approximately 20-100% of peak flow rate (PFR) as steady as possible during inspiration. Amplitude of surface electromyogram (EMG) of the sternocleidomastoid (SCM) and scalene were calculated for every increase in %PFR over a duration corresponding to an increase in lung volume by 10% of forced vital capacity (FVC), as well as for every 5% increment of FVC over a point corresponding to an increase in flow rate by 20%PFR to determine the %PFR-EMG and %FVC-EMG relations, respectively. RESULTS: Regression analyses showed that EMGs of the neck inspiratory muscles exponentially increased with increase in %PFR and their associated variables which reflect recruitment onset when increasing flow rate increased with increasing %FVC. In %FVC-EMG relation, a linear regression analysis showed positive slope at all %PFR and positive y-intercept at 80% PFR. CONCLUSIONS: The main new finding is that the neck inspiratory muscle activities increase with flow rate as well as lung volume. The positive y-intercept of the %FVC-EMG relation at higher %PFR indicates that the neck inspiratory muscles are always activated even when lung volume level is low, implying that SCM is not necessarily an "accessory" muscle as described in previous observations.