The force–velocity relationship of the human soleus muscle during submaximal voluntary lengthening actions

In experiments on isolated animal muscle, the force produced during active lengthening contractions can be up to twice the isometric force, whereas in human experiments lengthening force shows only modest, if any, increase in force. The presence of synergist and antagonist muscle activation associated with human experiments in situ may partly account for the difference between animal and human studies. Therefore, this study aimed to quantify the force–velocity relationship of the human soleus muscle and assess the likelihood that co-activation of antagonist muscles was responsible for the inhibition of torque during submaximal voluntary plantar flexor efforts. Seven subjects performed submaximal voluntary lengthening, shortening(at angular, velocities of +5, –5, +15, –15 and +30, and –30° s^–1) and isometric plantar flexor efforts against an ankle torque motor. Angle-specific (90°) measures of plantar flexor torque plus surface and intramuscular electromyography from soleus, medial gastrocnemius and tibialis anterior were made. The level of activation (30% of maximal voluntary isometric effort) was maintained by providing direct visual feedback of the soleus electromyogram to the subject. In an attempt to isolate the contribution of soleus to the resultant plantar flexion torque, activation of the synergist and antagonist muscles were minimised by: (1) flexing the knee of the test limb, thereby minimising the activation of gastrocnemius, and (2) applying an anaesthetic block to the common peroneal nerve to eliminate activation of the primary antagonist muscle, tibialis anterior and the synergist muscles, peroneus longus and peroneus brevis. Plantar flexion torque decreased significantly (P<0.05) after blocking the common peroneal nerve which was likely due to abolishing activation of the peroneal muscles which are synergists for plantar flexion. When normalised to the corresponding isometric value, the force–velocity relationship between pre- and post-block conditions was not different. In both conditions, plantar flexion torques during shortening actions were significantly less than the isometric torque and decreased at faster velocities. During lengthening actions, however, plantar flexion torques were not significantly different from isometric regardless of angular velocity. It was concluded that the apparent inhibition of lengthening torques during voluntary activation is not due to co-activation of antagonist muscles. Results are presented as mean (SEM).     

Evidence for reduced efficacy of the Ia-pathway during shortening plantar flexions with increasing effort

To determine whether the soleus (SOL) H-reflex is modulated during shortening contractions in a manner that has been observed for isometric contractions, SOL H-reflexes and M-waves were elicited via percutaneous electrical stimulation to the tibial nerve at an intensity that evoked an H-reflex at 50% of its maximum in 11 healthy subjects. Paired electrical stimuli were delivered as the ankle angle passed through 90° at an interval of 400 ms while the subject performed shortening contractions at levels of plantar flexion torque ranging between 2 and 30% of that during a maximal voluntary contraction (MVC). H-reflexes were also recorded during the performance of isomeric contractions of plantar flexors at similar levels of plantar flexion torque and at the same joint angle (muscle length) in an additional five healthy subjects. Correlations were examined between the peak-to-peak amplitude of the first H-reflexes, M-waves and plantar flexion torques in both protocols. It was revealed that no significant correlation was found between the SOL H-reflex and increasing plantar flexion torque during shortening contractions (ρ = −0.07, P = 0.15), while a strong positive correlation was observed for the isometric conditions (ρ = 0.99, P < 0.01). No significant change was observed in the SOL M-wave for either contraction type. Furthermore, the H-reflexes elicited via paired stimuli with the same background activity in voluntary shortening contractions showed almost identical amplitudes, suggesting that the level of homosynaptic post-activation depression did not change in response to the varying levels of activation in voluntary shortening contractions. Therefore, the lack of increase in the H-reflex during shortening contractions at increasing intensities is possibly due to a centrally regulated increase in presynaptic inhibition. Such a downward modulation of the reflex suggests that Ia-excitatory input onto the SOL motoneurone pool needs to be reduced during the performance of shortening contractions.     

Force–time history effects in voluntary contractions of human tibialis anterior

When an isometrically activated muscle is stretched or shortened the isometric steady-state force after the length change is increased (residual force enhancement) or decreased (force depression), respectively compared to a purely isometric contraction. This behavior has been observed consistently from the single sarcomere to the whole muscle level. However, the results for voluntary contractions in vivo are controversial and there are no studies for maximal voluntary contractions of medium sized muscles like the human ankle dorsiflexors. We investigated the effect of active muscle stretching and shortening for in vivo human tibialis anterior (n = 12) for maximal voluntary contractions for two magnitudes of stretching (15° and 30°) and two speeds of contraction (10°/s and 45°/s). Torques during stretches were higher compared to the purely isometric reference contractions and peak torques occurred prior to the end of the stretch. During the stretch, muscular activity decreased after peak torque had been reached for the high speed stretch experiments. In the steady-state phase following stretch, torque was increased for all experimental conditions but not for all time periods following stretch. The amount of residual force enhancement was independent of amplitude and speed of stretch. In the steady-state phase following shortening, torques were decreased compared to the isometric reference contractions and force depression was increased with increasing speeds of shortening.     

Electrically evoked eccentric and concentric torque–velocity relationships in human knee extensor muscles

The torque–velocity relationship, obtained during in situ conditions in humans, demonstrates a levelling‐off of eccentric torque output at the isometric torque level, at least for knee extensor actions. In contrast, the in vitro force–velocity relationship for animal muscle preparations is characterized by a sharp rise in eccentric force from isometric maximum. A force‐regulating ‘protective’ mechanism has been suggested during maximal voluntary high‐tension eccentric muscle actions. To investigate this phenomenon, maximal voluntary and three different levels of submaximal, electrically induced torques were compared during isometric and low velocity (10, 20 and 30° s^–1) isokinetic eccentric and concentric knee extensor actions in 10 healthy, moderately trained subjects. Eccentric torque was higher than isometric during electrically evoked, but not during maximal voluntary muscle actions. In contrast, concentric torque was significantly lower than isometric for both maximal voluntary and submaximal, electrically evoked conditions. Comparisons of normalized torques (isometric value under each condition set to 100%) demonstrated that the maximal voluntary eccentric torque had to be increased by 20%, and the isometric by 10% in order for the maximal voluntary torque–velocity curve to coincide with the electrically stimulated submaximal ones. These results support the notion that a tension‐regulating mechanism is present primarily during eccentric maximal voluntary knee extensor actions.     

Electromyogram patterns during plantarflexions at various angular velocities and knee angles in human triceps surae muscles

To investigate the influence of the various knee angles and ankle angular velocities on synergistic muscle activities, the surface electromyograms (EMG) were recorded from the triceps surae muscles, i.e. lateral gastrocnemius (LG), medial gastrocnemius (MG) and soleus (SOL) muscles. Six healthy young men performed ankle plantarflexions at three ankle angular velocities of 6, 30 and 60°?·?s^?1 and three knee angles of 0, 30 and 60° (0° equalling full extension) under constant load (5% and 10% maximal voluntary contraction). At the fully-extended knee angle (0°), peak values of integrated EMG (peak iEMG) during ankle plantarflexions were significantly increased (P?P??1) the peak iEMG were significantly increased (P?P?相似文献   

Vastus lateralis surface and single motor unit electromyography during shortening,lengthening and isometric contractions corrected for mode‐dependent differences in force‐generating capacity

Aim: Knee extensor neuromuscular activity, rectified surface electromyography (rsEMG) and single motor unit EMG was investigated during isometric (60° knee angle), shortening and lengthening contractions (50–70°, 10° s^?1) corrected for force–velocity‐related differences in force‐generating capacity. However, during dynamic contractions additional factors such as shortening‐induced force losses and lengthening‐induced force gains may also affect force capacity and thereby neuromuscular activity. Therefore, even after correction for force–velocity‐related differences in force capacity we expected neuromuscular activity to be higher and lower during shortening and lengthening, respectively, compared to isometric contractions. Methods: rsEMG of the three superficial muscle heads was obtained in a first session [10 and 50% maximal voluntary contraction (MVC)] and additionally EMG of (46) vastus lateralis motor units was recorded during a second session (4–76% MVC). Using superimposed electrical stimulation, force‐generating capacity for shortening and lengthening contractions was found to be 0.96 and 1.16 times isometric (Iso) force capacity respectively. Therefore, neuromuscular activity during submaximal shortening and lengthening was compared with isometric contractions of respectively 1.04Iso (=1/0.96) and 0.86Iso (=1/1.16). rsEMG and discharge rates were normalized to isometric values. Results: rsEMG behaviour was similar (P > 0.05) during both sessions. Shortening rsEMG (1.30 ± 0.11) and discharge rate (1.22 ± 0.13) were higher (P < 0.05) than 1.04Iso values (1.05 ± 0.05 and 1.03 ± 0.04 respectively), but lengthening rsEMG (1.05 ± 0.12) and discharge rate (0.90 ± 0.08) were not lower (P > 0.05) than 0.86Iso values (0.76 ± 0.04 and 0.91 ± 0.07 respectively). Conclusion: When force–velocity‐related differences in force capacity were taken into account, neuromuscular activity was not lower during lengthening but was still higher during shortening compared with isometric contractions.     

Effect of contraction history on torque deficits by stretches of active rat skeletal muscles.

Effects of contraction history on torque deficits by stretches of active skeletal muscles were examined. After three contractions using maximal and submaximal activation (80 and 20 Hz) at an ankle position of 40 degrees (i.e., long muscle length) and with maximal activation at 120 degrees (i.e., short muscle length), the isometric and stretch torques (15 stretches) of rat plantar flexor muscles (bout 1) were measured. Controls were unconditioned. Stretches (i.e., ankle rotation from 90 degrees to 40 degrees, velocity: 50 degrees. s-1) were imposed on maximal isometric contractions at 90 degrees (i.e., preloaded stretches). All groups performed a second bout following 2 hours of rest after bout 1. After maximal contractions at long muscle length, preload torque at 90 degrees and stretch torque at 40 degrees for stretch 1 of bout 1 were 25% and 18% lower than the other groups. However, for all groups, bout 1 ended and bout 2 began and ended with similar isometric and stretch torques. Stretches early in bout 2, with preloads similar to stretches in bout 1, had greater stretch torques resulting in larger torque deficits. Torque deficits, possibly caused by damage to muscle structures and excitation-contraction uncoupling, were not prevented by a history of isometric contractions. Different contraction histories can result in similar isometric torques but different stretch torques.     

Electrically evoked eccentric and concentric torque-velocity relationships in human knee extensor muscles

The torque-velocity relationship, obtained during in situ conditions in humans, demonstrates a levelling-off of eccentric torque output at the isometric torque level, at least for knee extensor actions. In contrast, the in vitro force-velocity relationship for animal muscle preparations is characterized by a sharp rise in eccentric force from isometric maximum. A force-regulating 'protective' mechanism has been suggested during maximal voluntary high-tension eccentric muscle actions. To investigate this phenomenon, maximal voluntary and three different levels of submaximal, electrically induced torques were compared during isometric and low velocity (10, 20 and 30 degrees s-1) isokinetic eccentric and concentric knee extensor actions in 10 healthy, moderately trained subjects. Eccentric torque was higher than isometric during electrically evoked, but not during maximal voluntary muscle actions. In contrast, concentric torque was significantly lower than isometric for both maximal voluntary and submaximal, electrically evoked conditions. Comparisons of normalized torques (isometric value under each condition set to 100%) demonstrated that the maximal voluntary eccentric torque had to be increased by 20%, and the isometric by 10% in order for the maximal voluntary torque-velocity curve to coincide with the electrically stimulated submaximal ones. These results support the notion that a tension-regulating mechanism is present primarily during eccentric maximal voluntary knee extensor actions.     

Normalized lengthening peak torque is associated with temporal twitch characteristics in elderly women but not young women

Aim: To determine if greater normalized torque during maximal effort lengthening actions in elderly women compared with young women is related to age‐associated adjustments in neural activation and/or contractile function. Methods: The right knee extensors of 14 young women (21–30 years) and 12 elderly women (65–78 years) were assessed for isometric, shortening and lengthening peak torque, electromyography (EMG) activity, and isometric twitch contractile properties. Knee extensor contractile tissue volume was determined using magnetic resonance imaging. Normalized torque was determined as peak torque per unit of knee extensor contractile tissue volume. Results: Normalized torque during the isometric and shortening actions was similar between age groups (P > 0.05); however, lengthening normalized torque was significantly higher for the elderly women (P < 0.05). In the young women, a significant relationship existed between normalized torque and EMG for all muscle actions (P < 0.05), while no association was found between normalized torque and temporal twitch characteristics for any muscle action (P > 0.05). In the elderly women, a significant relationship existed between normalized torque and EMG for the isometric and shortening muscle actions (P < 0.05), but not for lengthening normalized torque and EMG (P > 0.05). Furthermore, no association existed between isometric and shortening normalized torque, and temporal twitch characteristics in the elderly women (P > 0.05); however, a significant relationship existed between lengthening normalized torque, and the rate of relaxation and contraction duration (P < 0.05). Conclusions: The greater capacity to develop lengthening peak torque relative to contractile tissue volume in the elderly women appeared to be associated with age‐related adjustments in the temporal twitch characteristics rather than neural activation.     

Force enhancement during and following muscle stretch of maximal voluntarily activated human quadriceps femoris

Force enhancement during and following muscle stretch has been observed for electrically and voluntarily activated human muscle. However, especially for voluntary contractions, the latter observation has only been made for adductor pollicis and the ankle joint muscles, but not for large muscles like quadriceps femoris. Therefore, the aim of this study was to investigate the effects of active muscle stretch on force production for maximal voluntary contractions of in vivo human quadriceps femoris (n = 15). Peak torques during and torques at the end of stretch, torques following stretch, and passive torques following muscle deactivation were compared to the isometric torques at corresponding muscle length. In addition, muscle activation of rectus femoris, vastus medialis and vastus lateralis was obtained using surface EMG. Stretches with different amplitudes (15, 25 and 35° at a velocity of 60° s⁻¹) were performed on the plateau region and the descending limb of the force–length relation in a random order. Data analysis showed four main results: (1) peak torques did not occur at the end of the stretch, but torques at the end of the stretch exceeded the corresponding isometric torque; (2) there was no significant force enhancement following muscle stretch, but a small significant passive force enhancement persisted for all stretch conditions; (3) forces during and following stretch were independent of stretch amplitude; (4) muscle activation during and following muscle stretch was significantly reduced. In conclusion, although our results showed passive force enhancement, we could not provide direct evidence that there is active force enhancement in voluntarily activated human quadriceps femoris.     

Study of human muscle contraction using electrically evoked twitch responses during passive shortening and lengthening movements

Summary Electrically evoked twitch properties of the human plantarflexor muscles were measured with the muscles at a constant length (static) and during passive shortening and lengthening. A Kin-Com dynamometer system was used to passively move the ankle joint at 0.52 rad s⁻¹ (30‡ s⁻¹), as well as to record the twitch responses which were elicited by supramaximal electric shocks applied over the tibial nerve in the popliteal fossa. In the lengthening and shortening conditions, twitches were evoked by triggering the shocks so that the twitch response occurred at a similar angular position for all three conditions. The lengthening twitch peak torque was about twice as large as that recorded for the shortening condition. There was, however, no statistical difference in the twitch time course in these three testing conditions. This twofold increase in the peak twitch torque during lengthening, compared to shortening, is much greater than the torque increase reported during eccentric, as compared to concentric maximal voluntary contractions. These findings suggest that a deactivation process of the contractile system occurs during muscle shortening, while in contrast, during passive lengthening a potentiation mechanism is acting, and that both these mechanisms are independent of volitional muscle activation. The present study is the first to demonstrate the possibility of electrically evoked contractions of human muscles during passive lengthening and shortening. We believe that the use of such evoked contractions may be promising for the study of contractile behaviour of human skeletal muscles during eccentric and concentric conditions. This work was supported by a grant from l'Institut de Recherche en Santé et en Sécurite du Travail du Québec (IRSST)     

Muscle activation and the isokinetic torque-velocity relationship of the human triceps surae

Summary The influence of muscle activation and the time allowed for torque generation on the angle-specific torque-velocity relationship of the triceps surae was studied during plantar flexion using supramaximal electrical stimulation and a release technique on six male subjects [mean (SD) age 25 (4) years]. Torque-velocity data were obtained under different levels of constant muscle activation by varying the stimulus frequency and the time allowed for isometric torque generation prior to release and isokinetic shortening. To eliminate the effects of the frequency response on absolute torque the isokinetic data were normalized to the maximum isometric torque values at 0.44 rad. There were no significant differences in the normalized torques generated at any angular velocity using stimulus frequencies of 20, 50 or 80 Hz. When the muscle was stimulated at 50 Hz the torques obtained after a 400 ms and 1 s pre-release isometric contraction did not differ significantly. However, with no pre-release contraction significantly less torque was generated at all angular velocities beyond 1.05 rad · s^–1 when compared with either the 200, 400 ms or 1 s condition. With a 200 ms pre-release contraction significantly less torque was generated at angular velocities beyond 1.05 rad · s^–1 when compared with the 400 ms or 1 s conditions. It would seem that the major factor governing the shape of the torque-velocity curve at a constant level of muscle activation is the time allowed for torque generation.     

Fatiguing handgrip exercise alters maximal force-generating capacity of plantar-flexors

Exercise-induced fatigue causes changes within the central nervous system that decrease force production capacity in fatigued muscles. The impact on unrelated, non-exercised muscle performance is still unclear. The primary aim of this study was to examine the impact of a bilateral forearm muscle contraction on the motor function of the distal and unrelated ankle plantar-flexor muscles. The secondary aim was to compare the impact of maximal and submaximal forearm contractions on the non-fatigued ankle plantar-flexor muscles. Maximal voluntary contractions (MVC) of the forearm and ankle plantar-flexor muscles as well as voluntary activation (VA) and twitch torque of the ankle plantar-flexor muscles were assessed pre-fatigue and throughout a 10-min recovery period. Maximal (100 % MVC) and submaximal (30 % MVC) sustained isometric handgrip contractions caused a decreased handgrip MVC (to 49.3 ± 15.4 and 45.4 ± 11.4 % of the initial MVC for maximal and submaximal contraction, respectively) that remained throughout the 10-min recovery period. The fatigue protocols also caused a decreased ankle plantar-flexor MVC (to 77 ± 8.3 and 92.4 ± 6.2 % of pre-fatigue MVC for maximal and submaximal contraction, respectively) and VA (to 84.3 ± 15.7 and 97.7 ± 16.1 % of pre-fatigue VA for maximal and submaximal contraction, respectively). These results suggest central fatigue created by the fatiguing handgrip contraction translated to the performance of the non-exercised ankle muscles. Our results also show that the maximal fatigue protocol affected ankle plantar-flexor MVC and VA more severely than the submaximal protocol, highlighting the task-specificity of neuromuscular fatigue.     

Fascicle–tendon behavior of the gastrocnemius and soleus muscles during ankle bending exercise at different movement frequencies

The present study investigated the effect of movement frequencies on the behavior of fascicles and tendons of synergistic muscles. Seven male subjects performed ankle bending (calf-raise) exercises at four movement frequencies (1.33, 1.67, 1.84, and 2.00 Hz), performed with an identical range of ankle joint motion. The fascicle and tendon behavior of medial gastrocnemius (MG) and soleus (SOL) was measured by ultrasonography while kinematic and kinetic parameters of the ankle were recorded. The torque of ankle joint was larger at higher exercise frequencies. The length change of muscle decreased and that of tendon increased at higher frequencies both for MG and for SOL, with no significant inter-muscle differences in the relative changes of muscle or tendon lengths to that of MTU. Changes of pennation angles and electromyographic activities as a function of movement frequency were also comparable for MG and SOL. These results suggest that under a stretch–shortening cycle action, the muscle–tendon interaction is altered by the movement frequency toward greater use of tendon elastic energy to provide greater MTU power at a higher frequency. Results also suggest that the movement frequency dependence of fascicle and tendon behavior is comparable between MG and SOL.     

Reflex and non-reflex torque responses to stretch of the human knee extensors

Reflex responses to unexpected stretches are well documented for selected muscles in both animal and human. Moreover, investigations of their possible functional significance have revealed that stretch reflexes can contribute substantially to the overall stiffness of a joint. In the lower extremity only the muscles spanning the human ankle joint have been investigated in the past. This study implemented a unique hydraulic actuator to study the contributions of the knee extensor stretch reflex to the overall knee joint torque. The quadriceps muscles were stretched at various background torques, produced either voluntarily or by electrical stimulation, and thus the purely reflex mediated torque could be calculated. The stretch had a velocity of 67°/s and an amplitude of 20°. A reflex response as measured by electromyography (EMG) was observed in all knee extensors at latencies of 26 – 36 ms. Both phasic and tonic EMG stretch responses increased with increasing background torques. Lines of best fit produced correlation coefficients of 0.59 – 0.78. This study is the first to examine the reflex contribution of the knee extensors to the total torque at background torques of 0 – 90% MVC. The contribution of the reflex mediated torque is initially low and peaked at background torques of 20 – 40% MVC. In terms of the total torque the reflex contributed 16 – 52% across all levels of background torque. It is concluded that during medium background torque levels such as those obtained during walking, the stretch reflex of the quadriceps muscle group contributes substantially to the total torque around the knee joint.     

Time of day effects on isometric and isokinetic torque developed during elbow flexion in humans

 The aim of this study was, firstly, to confirm or refute the existence of circadian rhythms during several velocities of concentric action of the elbow flexor muscles and, secondly, to compare the characteristics of these circadian rhythms with those obtained during isometric actions. Eight volunteer subjects participated in this study. The circadian rhythms were obtained from six test sessions (TS) carried out at different times of day over 6 days with only one TS a day. During each TS, oral temperature and the torque of the muscle action were measured. The subjects made, on an isokinetic ergometer, two maximal isokinetic concentric elbow flexions at five angular velocities (60, 120, 180, 240 and 300° · s⁻¹) and at an angle of 60°. Torque-angular velocity relationships, which characterised the functioning of the muscle during concentric and isometric actions, were established for the different times of day. The values of the torque recorded at each of the angular velocities presented a clear circadian rhythm. After normalisation of the torque values, no significant differences were observed among the computed characteristics of the circadian rhythms obtained at different angular velocities. Since the circadian rhythms during isometric and concentric torque were the same, the characteristics of the circadian rhythms of the musculo-skeletal system can be studied using either type of muscle action. The results indicated that torque and temperature varied concomitantly during the day. Thus, the recording of body temperature allows one to estimate the times of occurrence of maximal and minimal values in the circadian rhythm of muscle torque. Accepted: 10 October 2000     

Interdependence of torque,joint angle,angular velocity and muscle action during human multi-joint leg extension

Purpose

Force and torque production of human muscles depends upon their lengths and contraction velocity. However, these factors are widely assumed to be independent of each other and the few studies that dealt with interactions of torque, angle and angular velocity are based on isolated single-joint movements. Thus, the purpose of this study was to determine force/torque–angle and force/torque–angular velocity properties for multi-joint leg extensions.

Methods

Human leg extension was investigated (n = 18) on a motor-driven leg press dynamometer while measuring external reaction forces at the feet. Extensor torque in the knee joint was calculated using inverse dynamics. Isometric contractions were performed at eight joint angle configurations of the lower limb corresponding to increments of 10° at the knee from 30 to 100° of knee flexion. Concentric and eccentric contractions were performed over the same range of motion at mean angular velocities of the knee from 30 to 240° s^?1.

Results

For contractions of increasing velocity, optimum knee angle shifted from 52 ± 7 to 64 ± 4° knee flexion. Furthermore, the curvature of the concentric force/torque–angular velocity relations varied with joint angles and maximum angular velocities increased from 866 ± 79 to 1,238 ± 132° s^?1 for 90–50° knee flexion. Normalised eccentric forces/torques ranged from 0.85 ± 0.12 to 1.32 ± 0.16 of their isometric reference, only showing significant increases above isometric and an effect of angular velocity for joint angles greater than optimum knee angle.

Conclusions

The findings reveal that force/torque production during multi-joint leg extension depends on the combined effects of angle and angular velocity. This finding should be accounted for in modelling and optimisation of human movement.     

The influence of muscle length on the fatigue-related reduction in joint range of motion of the human dorsiflexors

The fatigue-related reduction in joint range of motion (ROM) during dynamic contraction tasks may be related to muscle length-dependent alterations in torque and contractile kinetics, but this has not been systematically explored previously. Twelve young men performed a repetitive voluntary muscle shortening contraction task of the dorsiflexors at a contraction load of 30% of maximum voluntary isometric contraction (MVC) torque, until total 40° ROM had decreased by 50% at task failure (POST) to 20° ROM. At both a short (5° dorsiflexion) and long muscle length (35° plantar flexion joint angle relative to a 0° neutral ankle joint position), voluntary activation, MVC torque, and evoked tibialis anterior contractile properties of a 52.8 Hz high-frequency isometric tetanus [peak evoked torque, maximum rate of torque development (MRTD), maximum rate of relaxation (MRR)] were evaluated at baseline (PRE), at POST, and up to 10 min of recovery. At POST, we measured similar fatigue-related reductions in torque (voluntary and evoked) and slowing of contractile kinetics (MRTD and MRR) at both the short and long muscle lengths. Thus, the fatigue-related reduction in ROM could not be explained by length-dependent fatigue. Although torque (voluntary and evoked) at both muscle lengths was depressed and remained blunted throughout the recovery period, this was not related to the rapid recovery of ROM at 0.5 min after task failure. The reduction in ROM, however, was strongly related to the reduction in joint angular velocity (R ²  = 0.80) during the fatiguing task, although additional factors cannot yet be overlooked.     

Effects of a 5-h hilly running on ankle plantar and dorsal flexor force and fatigability

This study aimed to examine the effects of a 5-h hilly run on ankle plantar (PF) and dorsal flexor (DF) force and fatigability. It was hypothesised that DF fatigue/fatigability would be greater than PF fatigue/fatigability. Eight male trail long distance runners (42.5 ± 5.9 years) were tested for ankle PF and DF maximal voluntary isokinetic contraction strength and fatigue resistance tests (percent decrement score), maximal voluntary and electrically evoked isometric contraction strength before and after the run. Maximal EMG root mean square (RMS(max)) and mean power frequency (MPF) values of the tibialis anterior (TA), gastrocnemius lateralis (GL) and soleus (SOL) EMG activity were calculated. The peak torque of the potentiated high- and low-frequency doublets and the ratio of paired stimulation peak torques at 10 Hz over 100 Hz (Db10:100) were analysed for PF. Maximal voluntary isometric contraction strength of PF decreased from pre- to post-run (-17.0 ± 6.2%; P < 0.05), but no significant decrease was evident for DF (-7.9 ± 6.2%). Maximal voluntary isokinetic contraction strength and fatigue resistance remained unchanged for both PF and DF. RMS(max) SOL during maximal voluntary isometric contraction and RMS(max) TA during maximal voluntary isokinetic contraction were decreased (P < 0.05) after the run. For MPF, a significant decrease for TA (P < 0.05) was found and the ratio Db10:100 decreased for PF (-6.5 ± 6.0%; P < 0.05). In conclusion, significant isometric strength loss was only detected for PF after a 5-h hilly run and was partly due to low-frequency fatigue. This study contradicted the hypothesis that neuromuscular alterations due to prolonged hilly running are predominant for DF.     

Vestibular influences on human postural control in combinations of pitch and roll planes reveal differences in spatiotemporal processing

The present study examined the influence of bilateral peripheral vestibular loss (BVL) in humans on postural responses to multidirectional surface rotations in the pitch and roll planes. Specifically, we examined the effects of vestibular loss on the directional sensitivity, timing, and amplitude of early stretch, balance correcting, and stabilizing reactions in postural leg and trunk muscles as well as changes in ankle torque and trunk angular velocity following multidirectional rotational perturbations of the support surface. Fourteen normal healthy adults and five BVL patients stood on a dual axis rotating platform which rotated 7.5° at 50°/s through eight different directions of pitch and roll combinations separated by 45°. Directions were randomized within a series of 44 perturbation trials which were presented first with eyes open, followed by a second series of trials with eyes closed. Vestibular loss did not influence the range of activation or direction of maximum sensitivity for balance correcting responses (120–220 ms). Response onsets at approximately 120 ms were normal in tibialis anterior (TA), soleus (SOL), paraspinals (PARAS), or quadriceps muscles. Only SOL muscle activity demonstrated a 38- to 45-ms delay for combinations of forward (toe-down) and roll perturbations in BVL patients. The amplitude of balance correcting responses in leg muscles between 120 and 220 ms was, with one exception, severely reduced in BVL patients for eyes open and eyes closed conditions. SOL responses were decreased bilaterally for toe-up and toe-down perturbations, but more significantly reduced in the downhill (load-bearing) leg for combined roll and pitch perturbations. TA was significantly reduced bilaterally for toe-up perturbations, and in the downhill leg for backward roll perturbations. Forward perturbations, however, elicited significantly larger TA activity in BVL between 120 and 220 ms compared to normals, which would act to further destabilize the body. As a result of these changes in response amplitudes, BVL patients had reduced balance correcting ankle torque between 160 and 260 ms and increased torque between 280 and 380 ms compared to normals. There were no differences in the orientation of the resultant ankle torque vectors between BVL and normals, both of which were oriented primarily along the pitch plane. For combinations of backward (toe-up) and roll perturbations BVL patients had larger balance correcting and stabilizing reactions (between 350 and 700 ms) in PARAS than normals and these corresponded to excessive trunk pitch and roll velocities. During roll perturbations, trunk velocities in BVL subjects after 200 ms were directed along directions different from those of normals. Furthermore, roll instabilities appeared later than those of pitch particularly for backward roll perturbations. The results of the study show that combinations of roll and pitch surface rotations yield important spatiotemporal information, especially with respect to trunk response strategies changed by BVL which are not revealed by pitch plane perturbations alone. Our results indicate that vestibular influences are earlier for the pitch plane and are directed to leg muscles, whereas roll control is later and focused on trunk muscles. Electronic Publication