Reference Contraction Research Articles

Normal values of scapular muscle activity ratio during arm elevation and the relationship to muscle strength and sex difference.

The normal value of the scapular muscle activity ratio during arm elevation is not clear. To obtain normal values of the scapular muscle activity ratio during arm elevation in healthy individuals. This cross-sectional study enrolled 47 healthy people. The participants performed shoulder flexion and lowered task with 90-degree movements every 2 seconds. Muscle activities of scapular muscles were measured. For normalization, the maximum isometric contraction of the shoulder flexion was measured at 90∘ (reference contraction). The integrated electromyographic value (IEMG) obtained during the task was normalized by IEMG during the reference contraction and the relative IEMG value was calculated. Then, the scapular muscle activity ratio was computed. The relationship between muscle strength and other factors was also investigated. The median values for upper trapezius/serratus anterior and upper trapezius/lower trapezius were often approximately 1, and that for upper trapezius/middle trapezius was often between 1 and 2. The shoulder flexion isometric strength and scapular muscle activity ratio showed significant negative correlations in multiple phases. Normal values for upper trapezius/serratus anterior and upper trapezius/lower trapezius during arm elevation and lowering were generally 1. Low muscle strength may contribute to an abnormal scapular muscle activity balance.

Residual force depression is not related to positive muscle fascicle work during submaximal voluntary dorsiflexion contractions in humans.

Residual force depression (rFD) following active muscle shortening is assumed to correlate most strongly with muscle work, but this has not been tested during voluntary contractions in humans. Using dynamometry, we compared steady-state ankle joint torques (N=16) following tibialis anterior (TA) muscle-tendon unit (MTU) lengthening and shortening to the time-matched torque during submaximal voluntary fixed-end dorsiflexion reference contractions (REF) at a matched MTU length and EMG amplitude. Ultrasound revealed significantly reduced (P<0.001) TA fascicle shortening amplitudes during MTU lengthening without a preload over small and medium amplitudes, respectively, relative to REF. MTU lengthening with a preload over a large amplitude significantly (P<0.001) increased fascicle shortening relative to REF, as well as stretch amplitudes relative to MTU lengthening without a preload (P=0.001). Significant (P=0.028) steady-state fascicle force enhancement relative to REF was observed following MTU lengthening, and was similar among MTU lengthening-hold conditions (3-5%). MTU shortening with and without a preload over small and large amplitudes significantly (P<0.001) increased positive fascicle and MTU work relative to REF, but significant (P=0.006) rFD was observed following MTU shortening with a preload (7-10%) only. rFD was linearly related to positive MTU work [rrm (47)=0.48, P<0.001], but not positive fascicle work [rrm (47)=0.16, P=0.277]. Our findings indicate that MTU lengthening without substantial fascicle stretch enhances steady-state force output, which might arise from less shortening-induced rFD. Our findings also indicate similar rFD following different amounts of positive fascicle/MTU work, which cautions against using work to predict rFD during submaximal voluntary contractions. KEY POINTS: Accurately predicting muscle force is challenging because active muscle shortening depresses force output. The residual force depression (rFD) that exists following active muscle shortening is commonly assumed to correlate strongly and positively with muscle work. We found that tibialis anterior muscle fascicle work and muscle-tendon unit work did not accurately predict rFD during submaximal voluntary dorsiflexion contractions. Fascicle shortening during fixed-end reference contractions also potentially induced rFD of 3-5%, which was similar to the rFD following muscle-tendon unit shortening without a preload. A higher number of active muscle fibres during shortening probably increased rFD, which suggests that motor unit recruitment during shortening might predict rFD.

Axioscapular and neck extensor muscle behavior during isometric shoulder exertions in patients with nonspecific neck pain with and without a scapular downward rotation posture

BackgroundA common observation in persons with neck pain is scapular downward rotation (SDR) with altered muscle behavior. Evidence of changes in axioscapular muscles in neck pain patients remains inconclusive, which may reflect population heterogeneity in previous studies. Research questionAre there differences in behavior of the axioscapular (upper trapezius: UT, lower trapezius: LT and serratus anterior: SA) and neck extensor (NE) muscles during isometric shoulder tasks in patients with neck pain with SDR, patients with no scapular dysfunction and healthy controls? MethodsSixty participants with nonspecific neck pain (30 with SDR and 30 without scapular dysfunction) and 30 controls were recruited. Electromyographic signals were recorded unilaterally from the UT, LT, SA and NE during different isometric shoulder tasks (30° flexion, 30°abduction and 30°external rotation) at 20%, 50% and 100% maximal voluntary contraction (MVC). Activity of UT, LT, SA and NE was normalized with respect to reference contractions. The UT/LT, UT/SA and LT/SA ratios were calculated for each task. ResultsThe neck pain group with SDR had increased UT activity in 30°flexion (20%MVC) and 30°abduction (20% and 50%MVC) compared to the neck pain and control groups without scapular dysfunction (p < 0.05). There were no between group differences in LT and SA activity (p > 0.05). The neck pain groups had greater NE activity in all tasks (p < 0.001). Finally, the neck pain group with SDR had higher UT/LT and UT/SA ratios in a few tasks at low force levels (p ≤ 0.01). SignificanceGreater UT activity and UT/LT and UT/SA ratios during particularly low force isometric shoulder tasks suggest that SDR is associated with altered axioscapular motor control. Greater NE activity in both neck pain groups suggests altered motor control related to neck pain. Changes in the NE and UT behavior should be considered in management of patients with neck pain with observable SDR.

Residual force enhancement is attenuated for quick stretch conditions

It is generally accepted that the magnitude of residual force enhancement is not affected by stretching velocity. However, we recently found that when the stretching velocity was too large, the magnitude of residual force enhancement was attenuated or vanished in cat soleus muscle in situ. Therefore, the purpose of this study was to extend the knowledge by conducting very quick stretches in skinned rabbit psoas and soleus muscle fibers. Skinned psoas (N = 17) or soleus (N = 13) muscle fibers were activated isometrically at an average sarcomere length of 2.7 μm, and then, actively stretched to an average sarcomere length of 3.0 μm in 0.5 or 500 ms, followed by an isometric contraction at that length. In addition, a purely isometric reference contraction was conducted at an average sarcomere length of 3.0 μm to calculate the magnitude of residual force enhancement for both stretch velocities. The magnitude of residual force enhancement was significantly different between 0.5 ms stretch (101.5 ± 5.6% for psoas, and 101.5 ± 2.3% for soleus) and 500 ms stretch (106.2 ± 5.9% for psoas, and 106.8 ± 5.1% for soleus) while the magnitude of residual force enhancement was not significantly different between muscles. In line with our previous finding, the magnitude of residual force enhancement can vary when the stretch velocity is very large. This finding can contribute to clarify the key mechanism for inducing residual force enhancement and to explain contradicting results obtained in previous studies.

Evidence for increased neuromuscular drive following spinal manipulation in individuals with subacromial pain syndrome

BackgroundThoracic spinal manipulation can improve pain and function in individuals with shoulder pain; however, the mechanisms underlying these benefits remain unclear. Here, we evaluated the effects of thoracic spinal manipulation on muscle activity, as alteration in muscle activity is a key impairment for those with shoulder pain. We also evaluated the relationship between changes in muscle activity and clinical outcomes, to characterize the meaningful context of a change in neuromuscular drive. MethodsParticipants with shoulder pain related to subacromial pain syndrome (n = 28) received thoracic manipulation of low amplitude high velocity thrusts to the lower, middle and upper thoracic spine. Electromyographic muscle activity (trapezius-upper, middle, lower; serratus anterior; deltoid; infraspinatus) and shoulder pain (11-point scale) was collected pre and post-manipulation during arm elevation, and normalized to a reference contraction. Clinical benefits were assessed using the Pennsylvania Shoulder Score (Penn) at baseline and 2–3 days post-intervention. FindingsA significant increase in muscle activity was observed during arm ascent (p = 0.002). Using backward stepwise regression analysis, a specific increase in the serratus anterior muscle activity during arm elevation explained improved Penn scores following post-manipulation (p < 0.05). InterpretationThoracic spinal manipulation immediately increases neuromuscular drive. In addition, increased serratus anterior muscle activity, a key muscle for scapular motion, is associated with short-term improvements in shoulder clinical outcomes.

Effect of Active Lengthening and Shortening on Small-Angle X-ray Reflections in Skinned Skeletal Muscle Fibres.

Our purpose was to use small-angle X-ray diffraction to investigate the structural changes within sarcomeres at steady-state isometric contraction following active lengthening and shortening, compared to purely isometric contractions performed at the same final lengths. We examined force, stiffness, and the 1,0 and 1,1 equatorial and M3 and M6 meridional reflections in skinned rabbit psoas bundles, at steady-state isometric contraction following active lengthening to a sarcomere length of 3.0 µm (15.4% initial bundle length at 7.7% bundle length/s), and active shortening to a sarcomere length of 2.6 µm (15.4% bundle length at 7.7% bundle length/s), and during purely isometric reference contractions at the corresponding sarcomere lengths. Compared to the reference contraction, the isometric contraction after active lengthening was associated with an increase in force (i.e., residual force enhancement) and M3 spacing, no change in stiffness and the intensity ratio I1,1/I1,0, and decreased lattice spacing and M3 intensity. Compared to the reference contraction, the isometric contraction after active shortening resulted in decreased force, stiffness, I1,1/I1,0, M3 and M6 spacings, and M3 intensity. This suggests that residual force enhancement is achieved without an increase in the proportion of attached cross-bridges, and that force depression is accompanied by a decrease in the proportion of attached cross-bridges. Furthermore, the steady-state isometric contraction following active lengthening and shortening is accompanied by an increase in cross-bridge dispersion and/or a change in the cross-bridge conformation compared to the reference contractions.

Residual force enhancement in humans: Is there a true non-responder?

When an active muscle is stretched and kept isometrically active, the resulting force is enhanced compared to a purely isometric reference contraction at the same muscle length and activity; a generally accepted muscle property called residual force enhancement (rFE). Interestingly, studies on voluntary muscle action regularly identify a significant number of participants not showing rFE. Therefore, the aim was to unmask possible confounders for this non‐responsive behavior. Ten participants performed maximum voluntary isometric plantarflexion contractions with and without preceding stretch. Contractions were accompanied by the assessment of voluntary activation using the twitch‐interpolation technique. The same test protocol was repeated four additional times with a least on day rest in‐between. Additionally, at the first and fifth sessions, a submaximal tetanic muscle‐stimulation condition was added. At both muscle‐stimulation sessions mean rFE higher 10% (p < 0.028) was found. In contrast, during voluntary muscle action, individual participants showed inconsistent rFE across sessions and only one session (#3) had significant rFE (5%; p = 0.023) in group means. As all participants clearly had rFE in electrical stimulation conditions, structural deficits cannot explain the missing rFE in voluntary muscle action. However, we also did not find variability in voluntary activation levels or muscle activity as the confounding characteristics of “non‐responders.”

The effect of stretch\u2013shortening magnitude and muscle\u2013tendon unit length on performance enhancement in a stretch\u2013shortening cycle

Stretch-induced residual force enhancement (rFE) is associated with increased performance in a stretch–shortening cycle (SSC). Although the influence of different range of motions and muscle–tendon unit lengths has been investigated in pure stretch-hold experiments in vivo, the contribution to a SSC movement in human muscles remains unclear. In two sessions, 25 healthy participants performed isometric reference (ISO), shortening hold (SHO) and SSC contractions on an isokinetic dynamometer. We measured the net knee-joint torque, rotational mechanical work, knee kinematics and fascicle behavior (m. vastus lateralis) of the upper right leg. In session 1 the SHO- and SSC-magnitude was changed respectively (SHO: 50°–20°, 80°–20° and 110°–20°; SSC: 20°–50°–20°, 20°–80°–20° and 20°–110°–20°) and in session 2 the muscle–tendon unit length (SHO: 50°–20°, 80°–50° and 110°–80°; SSC: 20°–50°–20°, 50°–80°–50° and 80°–110°–80°; straight leg = 0°). In both sessions, rotational work was significantly (p < 0.05) increased in the SSC compared to the SHO contractions (in the range of 8.1–17.9%). No significant difference of joint torque was found in the steady-state for all SSC-magnitudes compared to the corresponding SHO contractions in session 1. In session 2, we found only significantly (p < 0.05) less depressed joint torque in the SSC at the longest muscle–tendon unit length compared to the corresponding SHO condition, without any differences in knee kinematics and fascicle behavior. Therefore, the physiological relevance of rFE might be particularly important for movements at greater muscle–tendon unit lengths.

Increased force following muscle stretching and simultaneous fibre shortening: Residual force enhancement or force depression – That is the question?

Residual force enhancement (rFE) describes the increase in isometric force following muscle stretching compared to the corresponding isometric force. Even though rFE is consistently observed in isolated muscle preparations, it is not always observed in human skeletal muscle. This inconsistency might be associated with disociations between length changes in muscle tendon units (MTUs) and fibres. This prompted the question if there is rFE for conditions where the MTU is stretched while fibres shorten. Rabbit tibialis anterior (TA) MTUs (n = 4) were stretched and the isometric forces following stretching were compared to corresponding forces from isometric reference contractions. Unique combinations of stretch speed and activation were used to create conditions of continuous fibre shortening during MTU stretch.Mean force was increased (18 ± 2%) following MTU stretching compared to the isometric reference forces. Without fibre length measurements, this result would be referred to as rFE. However, fibre shortening in the reference contractions was always greater than for the eccentric stretch contractions, suggesting that the observed increase in force might be caused by less residual force depression (rFD) in the stretch tests compared to the reference contractions. However, the work performed by fibre shortening was similar between the reference and the MTU stretch contractions, suggesting that rFD was similar for both experimental conditions. Therefore, we conclude that we observed rFE in the absence of contractile element stretching. However, a lack of knowledge of the molecular mechanisms that distinguish rFE from rFD prevents an unequivocal pronouncement of what caused the enhanced forces after active muscle stretching.

Contribution of Stretch-Induced Force Enhancement to Increased Performance in Maximal Voluntary and Submaximal Artificially Activated Stretch-Shortening Muscle Action.

In everyday muscle action or exercises, a stretch-shortening cycle (SSC) is performed under different levels of intensity. Thereby, compared to a pure shortening contraction, the shortening phase in a SSC shows increased force, work, and power. One mechanism to explain this performance enhancement in the SSC shortening phase is, besides others, referred to the phenomenon of stretch-induced increase in muscle force (known as residual force enhancement; rFE). It is unclear to what extent the intensity of muscle action influences the contribution of rFE to the SSC performance enhancement. Therefore, we examined the knee torque, knee kinematics, m. vastus lateralis fascicle length, and pennation angle changes of 30 healthy adults during isometric, shortening (CON) and stretch-shortening (SSC) conditions of the quadriceps femoris. We conducted maximal voluntary contractions (MVC) and submaximal electrically stimulated contractions at 20%, 35%, and 50% of MVC. Isometric trials were performed at 20° knee flexion (straight leg: 0°), and dynamic trials followed dynamometer-driven ramp profiles of 80°–20° (CON) and 20°–80°–20° (SSC), at an angular velocity set to 60°/s. Joint mechanical work during shortening was significantly (p < 0.05) enhanced by up to 21% for all SSC conditions compared to pure CON contractions at the same intensity. Regarding the steady-state torque after the dynamic phase, we found significant torque depression for all submaximal SSCs compared to the isometric reference contractions. There was no difference in the steady-state torque after the shortening phases between CON and SSC conditions at all submaximal intensities, indicating no stretch-induced rFE that persisted throughout the shortening. In contrast, during MVC efforts, the steady-state torque after SSC was significantly less depressed compared to the steady-state torque after the CON condition (p = 0.034), without significant differences in the m. vastus lateralis fascicle length and pennation angle. From these results, we concluded that the contribution of the potential enhancing factors in SSCs of the m. quadriceps femoris is dependent on the contraction intensity and the type of activation.

Signal transduction pathways of detrusor smooth muscle contraction evoked by prostanoids and isoprostanes in murine urinary bladder

The overactive bladder (OAB) is a clinical condition characterized by symptoms of frequency, urgency with/without incontinence, with a prevalence of 16% in the developed countries. The current pharmacological treatment mainly consists of anticholinergic drugs with several side effects. According to prior experimental results the arachidonic acid (AA) derivate prostanoids and isoprotanes, the latter produced non‐enzymatically during oxidative stress, are not only biomarkers of systemic oxidative stress, but are excreting in the urine and might act directly on the detrusor muscle leading to detrusor overactivity.Our aim was to examine the effects and the signal transduction pathways of prostanoids and isoprotanes in the urinary bladder smooth muscle, and potentially provide theoretical basis for the development of more specific medication of OAB with less adverse effects.Detrusor muscle strips were prepared from wild type (C57BL/6) and knockout mice, deficient for the thromboxane receptor (TP) or the α‐subunits of heterotrimeric G proteins (Gαq/11‐KO, Gα12/13‐KO) without urothelium under dissection microscope. Contraction force was measured by myograph under isometric conditions and normalized to the reference contractions evoked by 124 mM K+.The prostaglandin E2 (PGE2) and prostaglandin F2α (PGF2α), as well as the isoprostane 8‐epi‐PGE2 and 8‐iso‐PGF2α evoked contraction in the urinary bladder strips. The effect of the prostanoids was decreased, and the effect of the isoprostanes was abolished in the strips of TP KO mice, suggesting that the effect of the prostanoids is mediated partially, whereas that of the isoprostanes mainly by the TP. The TP receptor selective agonist U‐46619 evoked dose‐dependent contraction in the bladder strips, and neither the cholinerg antagonist atropine, nor the purinerg P2X‐antagonist pyridoxalphosphate‐6‐azophenyl‐2',4'‐disulfonic acid (PPADS) decreased the responses, indicating that the TP agonist has a direct effect on the detrusor muscle. The responses evoked by U‐46619 were comparable to the effect evoked by cholinergic agonist, carbachol. The contraction responses were decreased in the strips of the Gα12/13‐KO mice. Correspondingly, the responses evoked by the prostanoids and isoprostanes were reduced by the Rho‐kinase (ROCK) inhibitor Y‐27632. In the strips of the Gαq/11‐KO mice, the responses were also decreased and in the presence of Y‐27632 abolished completely.In conclusion, the examined prostanoids and isoprostanes evokes contraction acting directly on the detrusor muscle. These responses are mediated mainly by the TP receptor and are linked to the Gαq/11 and to the Gα12/13‐Rho‐ROCK intracellular signaling pathways in the murine urinary bladder. The Gα12/13‐Rho‐ROCK signaling pathway may provide a novel, more specific pharmacological target with less adverse effects in the treatment of OAB.Support or Funding InformationK‐112964, K‐125174 and NVKP‐16‐1‐2016‐0042 from the Hungarian National Research, Development and Innovation Office and EFOP‐3.6.3‐VEKOP‐16‐2017‐00009.

Pelvic floor and abdominal muscle responses during hypopressive exercises in women with pelvic floor dysfunction.

To measure the neuromuscular activation of the pelvic floor and abdominal muscles concurrently with vaginal closure forces induced during a hypopressive exercise (HE) and to identify the contribution of the HEs sequences (posture and maneuver) in the muscle's activation. A cross-sectional study design was employed. Sixty-six women who had participated in a physical therapy program focused on HEs were recruited. Pelvic floor muscle (PFM) activation was measured using surface electromyography (sEMG) in supine and in the orthostatic position, and vaginal closure force was measured through vaginal dynamometry in supine. Activation of the abdominal, gluteal, and hip adductor muscles was measured using sEMG. Maximum effort voluntary contractions (MVCs) of the PFMs and reference contractions of the abdominal and hip muscles were acquired for normalization purposes. A HE was then performed in a supine position with one leg raised, then in an orthostatic position. During the supine HE, the peak PFM sEMG amplitude was 74.4% to 86.5% (49.6%-109.6%) of MVC, the peak vaginal closure force was between 51.2% and 55.7% (95.5%-382.9%) of MVC, and the muscles of the lateral abdominal wall were activated between 25.4% and 35.3% of the reference contraction. During the orthostatic HE, PFM activation was 61.4% (40.1%-105.6%) of MVC, and the lateral abdominal wall muscles contracted at 22.8% of the reference activation level. The PFMs, abdominal, gluteal, and adductor muscles are activated during the performance of a HE. The activation level of the PFMs and abdominal muscles is likely insufficient to result in strength gains; however, they could have an endurance effect.

The effect of two types of maximal voluntary contraction and two electrode positions in field recordings of forearm extensor muscle activity during hotel room cleaning

Purpose. This study aimed to investigate the effects of using hand grip or resisted wrist extension as the reference contraction, and two electrode positions, on field recordings of forearm extensor muscle activity. Materials and methods. Right forearm extensor muscle activity was recorded using two electrode pairs (over the most prominent part (position 2) and proximal to that (position 1)) during one working day in 13 female hotel housekeepers. Each subject performed the two maximal voluntary contractions (MVCs), and the electrical activity obtained during these (maximal voluntary electrical activity (MVE)) was used for normalization. Each set of recordings was analysed twice, once using hand grip as the MVC and once using resisted wrist extension. Results. Resisted wrist extension showed a higher group mean MVE than hand grip. Position 2 had higher correlation between MVE and force during the MVCs. The workload during cleaning was lower when using resisted wrist extension as reference than when using hand grip (24%MVE vs 46%MVE; p = 0.002 at position 2) for the 90th percentile. The workload (99th percentile) was overestimated in two subjects when using hand grip as reference. Conclusions. Problems associated with poorly activated forearm extensors can be overcome by using resisted wrist extension as reference.

Anaphylatoxin C3a induces vasoconstriction and hypertension mediated by thromboxane A2 in mice

IntroductionSeveral recent reports indicate a marked crosstalk between innate immunity and vasoregulation. Anaphylatoxins C3a and C5a have been reported to induce vasoactive effects, but the mechanism of their actions is obscure. Earlier studies reported increased plasma levels of C3 in cardiovascular diseases, especially pulmonary hypertension. Therefore, complement activation may contribute to changes of the vascular tone and reactivity under pathophysiological conditions. In our present study we investigated the effects of C3a, the activate cleavage product of the most abundant complement protein of the human circulation, on the arterial tone and blood pressure as well as the signaling pathways involved.Materials and methodsThoracic aortic segments were isolated from adult male C57Bl/6 wild type (WT) and knockout (KO) mice deficient in either thromboxane prostanoid receptor (TP KO) or cyclooxygenase 1 (COX1 KO). Isometric tension changes of vascular segments were measured via myography. We studied the effects of C3a by using C3a (63–77) peptide fragment, a specific C3a receptor agonist. Vascular responses were quantitated as percent of reference contraction induced by 124 mM K+. In addition, we performed experiments monitoring blood pressure (BP) changes in vivo. In these experiments zymosan (30 mg/kg, i.v.), a direct complement activator was administered intravenously, after cannulation of carotid artery and jugular vein in anesthetized (pentobarbital, 90 mg/kg i.p.) mice. Blood samples were taken from other groups of WT mice and plasma TXB2 levels were measured 5 minutes after treatment with saline or zymosan.ResultsC3a (63–77) evoked a pronounced vasoconstriction in WT vessels (33±5%), which effect remained unaltered in the absence of endothelium (37±7%). The vessels of COX1 KO mice showed a decreased response (10±2%), and the C3a fragment caused a similarly reduced constriction in TP KO vessels (4±1%). In our in vivo experiments the administration of zymosan led to an increase of BP in WT mice (with an average of 14±5 mmHg), whereas causing a drop of 22±3 mmHg in TP KO mice's BP. In accordance, zymosan caused a two‐fold increase in the plasma level of thromboxane B2 in WT animals.ConclusionsOur experiments showed, that C3a (63–77) causes vasoconstriction in a non‐endothelium‐dependent manner, and this effect is mediated by COX1 and TP. These results propose that C3a‐mediated vasoconstriction is mediated by smooth muscle derived thromboxane A2. In addition, C3a (63–77) induce hypertension, which is also mediated by thromboxane A2. These results altogether indicate that C3a is involved in the regulation of vascular tone and/or reactivity via stimulating thromboxane A2 release.Support or Funding InformationGrant support: Semmelweis Research and Innovation Fund (STIA‐M‐17); Ministry of Human Capacities of Hungary (ÚNKP‐18‐2‐I‐SE‐12, FIKP‐2018 and EFOP‐3.6.3‐VEKOP‐16‐2017‐00009) and the Hungarian NRDIO (OTKA K‐115623, K‐125174 and NVKP_16‐1‐2016‐0042)This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Shortening-induced residual force depression in humans.

When an isometric muscle contraction is immediately preceded by an active shortening contraction, a reduction in steady-state isometric force is observed relative to an isometric reference contraction at the same muscle length and level of activation. This shortening-induced reduction in isometric force, termed "residual force depression" (rFD), has been under investigation for over a half century. Various experimental models have revealed the positive relationship between rFD and the force and displacement performed during shortening, with rFD values ranging from 5 to 39% across various muscle groups, which appears to be due to a stress-induced inhibition of cross-bridge attachments. The current review will discuss the findings of rFD in humans during maximal and submaximal contractions.

On sarcomere length stability during isometric contractions before and after active stretching.

Sarcomere length (SL) instability and SL non-uniformity have been used to explain fundamental properties of skeletal muscles, such as creep, force depression following active muscle shortening and residual force enhancement following active stretching of muscles. Regarding residual force enhancement, it has been argued that active muscle stretching causes SL instability, thereby increasing SL non-uniformity. However, we recently showed that SL non-uniformity is not increased by active muscle stretching, but it remains unclear if SL stability is affected by active stretching. Here, we used single myofibrils of rabbit psoas muscle and measured SL non-uniformity and SL instability during isometric contractions and for isometric contractions following active stretching at average SLs corresponding to the descending limb of the force-length relationship. We defined isometric contractions as contractions during which mean SL remained constant. SL instability was quantified by the rate of change of individual SLs over the course of steady-state isometric force and SL non-uniformity was defined as deviations of SLs from the mean SL at an instant of time. We found that whereas the mean SL remained constant during isometric contraction, by definition, individual SLs did not. SLs were more stable in the force-enhanced, isometric state following active stretching compared with the isometric reference state. We also found that SL instability was not correlated with the rate of change of SL non-uniformity. Also, SL non-uniformity was not different in the isometric and the post-stretch isometric contractions. We conclude that since SL is more stable but similarly non-uniform in the force-enhanced compared with the corresponding isometric reference contraction, it appears unlikely that either SL instability or SL non-uniformity contribute to the residual force enhancement property of skeletal muscle.

Central contributions to torque depression: an antagonist perspective.

Torque depression (TD) is the reduction in steady-state isometric torque following active muscle shortening when compared to an isometric reference contraction at the same muscle length and activation level. Central nervous system excitability differs in the TD state. While torque production about a joint is influenced by both agonist and antagonist muscle activation, investigations of corticospinal excitability have focused on agonist muscle groups. Hence, it is unknown how the TD state affects spinal and supraspinal excitability of an antagonist muscle. Eight participants (~ 24y, three female) performed 14 submaximal dorsiflexion contractions at the intensity needed to maintain a level of integrated electromyographic activity in the soleus equivalent to 15% of that recorded during a maximum plantar flexion contraction. The seven contractions of the TD protocol included a 2s isometric phase at an ankle angle of 140°, a 1s shortening phase at 40°/s, and a 7s isometric phase at an angle of 100°. The seven isometric reference contractions were performed at an ankle angle of 100° for 10s. Motor evoked potentials (MEPs), cervicomedullary motor evoked potentials (CMEPs), and maximal M-waves (Mmax) were recorded from the soleus in both conditions. In the TD compared to isometric reference state, a 13% reduction in dorsiflexor torque was accompanied by 10% lower spinal excitability (normalized CMEP amplitude; CMEP/Mmax), and 17% greater supraspinal excitability (normalized MEP amplitude; MEP/CMEP) for the soleus muscle. These findings demonstrate a neuromechanical coupling following active muscle shortening and indicate that the underlying mechanisms of TD influence antagonist activation during voluntary force production.

Effects of pregnancy on lumbar motion patterns and muscle responses

BACKGROUND CONTEXTThe kinematics of the lumbar region and the activation patterns of the erector spinae muscle have been associated with the genesis of low back pain, which is one of the most common complications associated with pregnancy. Despite the high prevalence of pregnancy-related low back pain, the biomechanical adaptations of the lumbar region during pregnancy remain unknown. PURPOSEThis study analyzes lumbar spine motion and the activation pattern of the lumbar erector spinae muscle in healthy pregnant women. STUDY DESIGNA case-control study. PATIENT SAMPLEThe study involved 34 nulliparous women (control group) and 34 pregnant women in the third trimester (week 36 ± 1). OUTCOME MEASURESWe recorded the parameters of angular displacement of the lumbar spine in the sagittal plane during trunk flexion-extension, and the EMG activity of the erector spinae muscles during flexion, extension, eccentric and concentric contractions, and the myolectrical silence. METHODSThe participants performed several series of trunk flexion-extension movements, which were repeated 2 months postpartum. The position of the lumbar spine was recorded using an electromagnetic motion capture system. EMG activity was recorded by a surface EMG system and expressed as a percentage of a submaximal reference contraction. RESULTSAntepartum measurements showed a decrease (relative to control and postpartum measurements) in lumbar maximum flexion (52.5 ± 10.5° vs 57.3 ± 7.7° and 58.7 ± 8.6°; p < .01), the percentage of lumbar flexion during forward bending (56.4 ± 5.6% vs 59.4 ± 6.8% and 59.7 ± 5.6%; p < .01), and the time keeping maximum levels of lumbar flexion (35.7 ± 6.7% vs 43.8 ± 5.3% and 50.1 ± 3.7%; p < .01). Higher levels of erector spinae activation were observed in pregnant women during forward bending (10.1 ± 4.8% vs 6.3 ± 2.4% and 6.6 ± 2.7%; p < .01) and eccentric contraction (12.1 ± 5.2% vs 9.4 ± 3.1% and 9.1 ± 2.9%; p < .01), as well as a shortened erector spinae myoelectric silence during flexion. ConclusionsPregnant women show adaptations in their patterns of lumbar motion and erector spinae activity during trunk flexion-extension. These changes could be associated with the genesis of pregnancy-related low back pain, by means of biomechanical protection mechanisms against the increase on abdominal mass and ligamentous laxity.

Predictors of residual force enhancement in voluntary contractions of elbow flexors

BackgroundThe steady-state increase in muscle force generating potential following a lengthening contraction is called residual force enhancement (RFE). In this study, we aimed to test for differences in torque, electromyographic activity (EMG), and the associated neuromuscular efficiency (NME) between isometric voluntary contractions of elbow flexors preceded and not preceded by a lengthening contraction. The dependence of such differences on (i) stretch amplitude, (ii) the region of the force–length (FxL) relationship where contraction occurs, and (iii) the individual's ability to produce (negative) work during the stretch was investigated. MethodsSixteen healthy adults participated in the study. Elbow flexor torque, angle, and biceps brachii EMG for purely isometric contractions (reference contractions) and for isometric contractions preceded by active stretches of 20° and 40° were measured at the ascending, plateau, and descending regions of subject-specific FxL curves. All contractions were performed in an isokinetic dynamometer. Two-factor (stretch × FxL region) repeated measures analysis of variance ANOVAs was used to analyze the effect of active stretch on EMG, torque, and NME across conditions. The relationships between mechanical work during stretch—calculated as the torque–angular displacement integral—and the changes in EMG, torque, and NME were analyzed using Pearson correlation. ResultsIn general, torque, EMG, and NME following active stretches differed from the values observed for the purely isometric reference contractions. While although the detailed effects of active stretch on torque and EMG differed between regions of the FxL relationship, NME increased by about 19% for all muscle lengths. Up to 30% of the interindividual variability in torque generating potential change in response to active stretching was accounted for by differences in (negative) work capacity between subjects. ConclusionOur results suggest that (i) RFE contributes to “flatten” the elbow flexor torque–angle relationship, favoring torque production at lengths where the purely isometric torques are reduced substantially, and (ii) RFE contributes to a reduction in energy cost of torque production during isometric contractions for the entire operating range.

Residual Force Enhancement is Attenuated by Shortening in a Magnitude-Dependent Manner

The isometric force attained after active stretch is greater than that attained in a purely isometric contraction at the corresponding muscle length. This property is referred to as residual force enhancement (RFE). Although RFE is considered to contribute to the force/work enhancement in stretch-shortening cycles, it is unclear whether shortening that occurs after active stretch eliminates the influence of RFE. Therefore, we evaluated the influence of shortening on the magnitude of RFE. Skinned fibers of rabbit soleus (N = 45) were used for all experiments. The first test consisted of an isometric reference contraction at an average sarcomere length of 3.0 μm. The second test consisted of a typical way to induce RFE. This was achieved by active stretching of single fibres from an average sarcomere length of 2.7 to 3.0 μm. The third test consisted of a stretch shortening cycle. RFE was induced by active stretch from 3.0 to 3.3μm. Immediately after stretching, fibers were shortened to 3.0 μm (12.5% of the fiber length) in 0.5 ms. Force attained 15 s after the active stretch was used to evaluate the magnitude of RFE. Compared to the isometric reference condition, force was significantly greater in the normal RFE condition (108.4 ± 2.5%) and in the stretch-shortening cycle condition (RFE 103.5 ± 2.4%) (p< 0.001). This difference in the magnitude of RFE disappeared when the magnitude of shortening was decreased from 12.5% to 1% of the fiber length (p = 0.316). In conclusion, RFE is attenuated by shortening following active stretching. This attenuation effect depends on the magnitude of shortening and decreases with decreasing shortening magnitudes. The molecular events underlying these history-dependent force effects remain unknown.