Cardiovascular responses during arm exercise and orthostatic challenge in individuals with paraplegia

In this study the cardiorespiratory responses during arm crank ergometry (ACE) performed at two submaximal intensities (30% and 50% of heart rate reserve) and moderate orthostatic challenge were investigated in individuals with paraplegia (PARA). The effect of concurrent electrical stimulation (ES)-induced leg muscle contractions on the responses to ACE during orthostatic challenge was also investigated. Eight PARA (T5–T12) and eight able-bodied (AB) individuals participated in this study, however only seven subjects from each group completed all tests and were used in subsequent data analyses. Oxygen uptake ( ), heart rate (f _c), stroke volume (SV) and cardiac output ( ) were assessed during (1) ACE alone, (2) ACE and lower body negative pressure (ACE+LBNP), and, in PARA only, (3) ACE+LBNP with ES (ACE+LBNP+ES). In both PARA and AB, ACE+LBNP decreased SV (by 13–18% and 20–23%, respectively) and increased f _c (by 13–15% and 16%, respectively) compared to ACE alone. The decrease in SV was greater in AB than in PARA (significant group × trial interaction; both ACE intensities pooled), but there was no difference in the magnitude of increase in f _c between groups. ES-induced leg muscle contractions increased SV (up to 16%) but did not change or . The smaller reduction in SV from ACE to ACE+LBNP in PARA may indicate a mechanism by which adequate central blood volume can be maintained in the face of orthostatic challenge, despite the absence of supraspinal control below the spinal cord lesion. With ES-induced leg muscle contractions, the decrease in SV, which occurred during ACE+LBNP, was reversed via reactivation of the lower limb muscle pump and augmented venous return. Electronic Publication     

Mechanomyographic responses to concentric isokinetic muscle contractions

The purpose of this investigation was to examine the effects of velocity of contraction on the mechanomyographic (MMG) responses to maximal concentric isokinetic leg extension movements. Eight adult males [mean (SD) age, 22.3 (1.3) years] performed maximal leg extensions on a calibrated Cybex 6000 dynamometer at velocities of 60, 120, 180, 240, 300, and 360°?·?s^?1. MMG responses were detected by a piezoelectric recording device placed over the vastus lateralis muscle. Intraclass reliability correlations ranged from R?=?0.84 to 0.97 and from 0.90 to 0.99 for peak torque and MMG amplitude values, respectively, with no significant differences (P?>?0.05) between the mean values for test versus retest at any contraction velocity. There were significant differences (P??1 [115 (37) Nm]. The mean MMG amplitude at 60°?·?s^?1 [61 (67) mV] was significantly less (P??1 [452 (451) mV]. These results indicate a velocity-related dissociation between MMG amplitude and peak torque. Furthermore, it was hypothesized that the increases in MMG amplitude were due to velocity-related decreases in muscle stiffness which allowed for greater muscle fiber oscillations.     

Cardiac responses to maximal anisotonic isometric contractions during handgrip and leg extension

A group of 14-healthy men performed anisotonic isometric contractions (AIC), for 60 s, at an intensity of 100% maximal voluntary contraction force (MVC) during handgrip (HG) and leg extension (LE). Heart rate (f _c), stroke volume index (SVI) and cardiac output index (Q_cI) were measured during the last 10 s of both AIC by an impedance reography method. Force (F) exerted by the subjects was recorded continuously and reported as a relative force (F _r) (% MVC). The F generated during MVC was greater for LE than for HG (502.I N compared to 374.6 N, P < 0.001). The rate of decrease in F _r was significantly slower for LE than HG for the first 25 s of the exercise (phase 1 of AIC). The F _r developed by the subjects at the end of AIC was 40% MVC for both LE and HG. The increase in f _c was greater for LE (63 beats · min^–1) than for HG (52 beats · min^–1), P < 0.01. The SVI decreased significantly from the resting level by 17.0 ml · m^–2 and by 18.2 ml · m^–2 for LE and HG, respectively. The Q_cI increased insignificantly for HG by 0.091 · min^–1 · m^–2 andsignificantly forLE by 0.561 · min^–1 · m^–2 (P < 0.001). It was concluded that although both AIC caused a significant decrease in SVI, greater increases in f _c and Q_c were observed for LE than for HG. The greater f _c and Q_c reported during LE was probably related to the greater relative force exerted by LE during phase 1 of AIC. It seems, therefore that central command might have dominated for phase 1 of AIC but that the muscle reflex also contributed significantly to the control of the cardiac response to the high intensity AIC.     

Cardiovascular responses to orthostatic tests after a 42-day head-down bed-rest

Cardiovascular responses to orthostatic tests were studied before and after a prolonged 42 day-head-down bed-rest (HDBR;?6°) experiment simulating a long duration space flight. Seven men participating in the experiment underwent stand tests (10?min) and lower body negative pressure (LBNP) tests (5?min at ?25, ?35, ?45?mmHg). Heart rate variability and spontaneous baroreflex response slope (SBS) were analysed to assess autonomic nervous system responses. Changes in plasma volume (PV) were assessed at the end of HDBR. At the end of HDBR, four subjects could not complete the stand tests and one could not complete the LBNP test. A higher stressed heart rate with standing (+44% before and +57% after HDBR) and LBNP exposure (+19% before and +34% after HDBR) were observed. A decrease in blood pressure (BP) reflecting a reduced vasomotor response was only observed with standing (mean BP +21% before and ?8% after HDBR); LBNP was less sensitive probably because it was performed 6?h after the stand test. The PV decreased by 10.6%. A decline in spectrum total power reflecting a reduced variance of RR-interval, a decrease in parasympathetic activity and an increase in sympathetic one were observed at the end of HDBR. The reduced parasympathetic indicator and SBS would suggest that the vagal nerve component of the cardiovascular control had been diminished. Except for a lower BP when standing after HDBR, no significant difference was observed between finishers and non-finishers. Autonomic nervous system changes including reduced vasomotor responses constituted important contributors to the orthostatic intolerance observed here and after space flights. Some autonomic and PV changes seemed to be opposite to those observed with training and would suggest a role of reduced physical activity in cardiovascular changes induced by HDBR.     

Cardiovascular responses of cardiac transplant patients to arm and leg exercise

This investigation compares the cardiovascular responses of normal (n=10) and cardiac transplant (n=14) subjects to peak arm and leg exercise. It also tests the hypothesis that the higher heart rate (f _c) in normal subjects during light (30 W) submaximal arm versus leg exercise is due to cardiac innervation. In cardiac transplant patients, power output, oxygen consumption ,f _c and rate pressure product were 54%, 28%, 7%, and 8% lower during peak arm than leg exercise, respectively. In normal subjects, power output, ,f _c and rate pressure product were 61%, 33%, 8%, and 11% lower during peak arm than leg exercise, respectively. In cardiac transplant patients there was no significant difference in absolutef _c during submaximal arm and leg exercise. In normal subjects, absolutef _c during arm and leg exercise was [mean (SD)] 97 (4) beats · min^–1 and 92 (4) beats · min^–1, respectively (P=0.07). Plasma noradrenaline was increased more during arm than leg exercise in both cardiac transplant and normal subjects. Maximal leg testing is useful when determining the capacity of cardiac transplant patients to perform arm work. The higher absolutef _c reported by other investigators for normal subjects during submaximal arm versus leg exercise may be mediated by cardiac innervation.     

Cardiovascular responses to orthostatic stress in healthy altitude dwellers, and altitude residents with chronic mountain sickness

High altitude (HA) dwellers have an exceptionally high tolerance to orthostatic stress, and this may partly be related to their high packed cell and blood volumes. However, it is not known whether their orthostatic tolerance would be changed after relief of the altitude-related hypoxia. Furthermore, orthostatic tolerance is known also to be influenced by the efficiency of the control of peripheral vascular resistance and by the effectiveness of cerebral autoregulation and these have not been reported in HA dwellers. In this study we examined plasma volume, orthostatic tolerance and peripheral vascular and cerebrovascular responses to orthostatic stress in HA dwellers, including some with chronic mountain sickness (CMS) in whom packed cell and blood volumes are particularly large. Eleven HA control subjects and 11 CMS patients underwent orthostatic stress testing, comprising head-up tilting with lower body suction, at their resident altitude (4338 m) and at sea level. Blood pressure (Portapres), heart rate (ECG), brachial and middle cerebral artery blood velocities (Doppler) were recorded during the test. Plasma volumes were found to be similar in both groups and at both locations. Packed cell and blood volumes were higher in CMS patients than controls. All subjects had very good orthostatic tolerances at both locations, compared to previously published data in lowland dwellers. In CMS patients responses of forearm vascular resistance to the orthostatic stress, at sea level, were smaller than controls (P < 0.05). Cerebral blood velocity was less in CMS than in controls (P < 0.01) and, at sea level, it decreased more than the controls in response to head-up tilting (P < 0.02). Cerebral autoregulation, assessed from the relationship between cerebral pressure and velocity, was also impaired in CMS patients compared to HA controls, when examined at sea level (P < 0.02). These results have shown that the good orthostatic tolerance seen in high altitude dwellers at altitude is also seen at sea level. There was no difference in orthostatic tolerance between CMS patients, with their exceptionally large blood volumes, and the HA controls. This may be because peripheral vascular and cerebrovascular responses (at least at sea level) are impaired in the CMS patients relative to HA controls. Thus, the advantage of the large blood volume may be offset by the smaller vascular responses.     

Cardiovascular and respiratory responses to passive leg cycle exercise in people with spinal cord injuries

The purpose of this study was to determine the effect of passive leg cycle exercise (PLE) on cardiovascular and respiratory responses in people with spinal cord injuries (PSCI). Eight PSCI with lesions from T8 to L1 and five control subjects (CS) performed PLE at pedalling frequencies of 20 or 40 rpm for 7 min at room temperature of about 25°C. We measured, at rest and during PLE, the pulmonary ventilation (V_E), oxygen uptake (VO₂), cardiac output (Q), stroke volume (SV), heart rate (HR) and arterial blood pressure, as well as the skin blood flow (SBF) in the lower limb after PLE. An increase in pedalling frequency promoted an increase in V_E and VO₂ in both groups. Compared with the CS, the PSCI showed significantly smaller increases in VO₂ (P < 0.05). The Q_c was significantly elevated during PLE at 20 and 40 rpm in CS, and at 40 rpm in PSCI (P < 0.05). In CS, it resulted from increases in both SV and HR, whereas in PSCI, it was contributed to by a greater increase in SV without a rise in HR. In CS, the increase in pedalling frequency promoted the increases in SV and HR and consequently in Q_c In PSCI, however, the values remained constant irrespective of pedalling frequency. The arterial blood pressure and SBF in the lower limbs were unchanged by PLE in both groups. These results would suggest that passive leg exercise promotes venous return from the paralyzed lower limbs in PSCI.     

Cardiovascular responses to sustained maximal isometric contractions of the finger flexors

Summary This study investigated cardiovascular responses to 2 min sustained submaximal (20% MVC) and maximal (100% MVC) voluntary isometric contractions of the finger flexors in healthy young women. Cardiovascular variables investigated were: heart rate (f _c), mean arterial pressure ( _a), and stroke volume (SV). Doppler echocardiography was used to estimate SV from measures of aortic diameter (AD) and time-velocity integrals. Preliminary studies indicated that AD did not change significantly after 2 min sustained 100% MVC. Therefore, pre-exercise AD values were used to calculate SV before, during and after exercise. During the 2-min 100% MVC period, f _c and _aincreased significantly during the first 30 s of contraction. f _c then remained constant during the remainder of the 2-min contraction period, while _acontinued to rise. SV did not change significantly during the 100% MVC task but increased significantly during recovery from sustained 100% MVC. The data suggest that the magnitude of cardiovascular responses to isometric exercise is dependent on the specific task performed, and that there is a different pattern of response for f _c, _a, and SV during 20% and 100% MVC tasks. Unlike f _c and _a, SV did not change significantly during isometric exercise, but increased significantly after sustained 100% MVC.     

Cardiovascular and autonomic responses to passive arm or leg movement in men and women

European Journal of Applied Physiology - Women display an attenuated mechanoreflex during leg movement; however, sex differences in the response to arm movement are unknown. Men...     

Relationship between work and electromyographic activity during repeated leg muscle contractions in orienteers

Summary Contraction work (CW) was recorded for each of 200 repetitive isokinetic plantar flexions (1.05 rad · s^–1) and knee extensions (1.57 rad · s^–1) in 14 elite male orienteers. Simultaneous recordings of integrated electromyograms (iEMG) were obtained from the 3 parts of triceps surae and from 3 superficial portions of quadriceps femoris. CW in both muscle groups decreased significantly during the first 30 contractions (the fatigue phase), followed by a steady state level. The relative steady state level was higher for the plantar flexors (70±17%) than for the knee extensors (56±12%). For quadriceps a significant increase in iEMG occurred during the first 10 contractions followed by a decrease, whereas the iEMG of the plantar flexors showed a gradual decrease to the steady state level, which was similar for the two muscle groups (71–72%). The chosen expression of output/input balance (CW/iEMG) was constant throughout the plantarflexion test but decreased during the initial 20 knee extensions down to 82%. Thus, the fatigue phase of the knee extensions appeared to be divided into two; the first part had decreases in both CW and CW/iEMG and the second part with a decrease in CW alone. In contrast the plantar flexors only showed the characteristics of the second part.     

Frontal plane leg alignment and muscular activity during maximum eccentric contractions in individuals with and without patellofemoral pain syndrome

PURPOSE: The role of frontal plane tibiofemoral alignment in subjects with patellofemoral pain syndrome (PFPS) is controversial and rarely discussed in the literature. As well, little research has been done on the effects of the hamstrings muscles on PFPS. The aim of the current study was to determine whether, in individuals with PFPS, frontal plane tibiofemoral alignment or muscular activity of the index knee's crossing muscles is altered during maximum eccentric leg press exercise. METHODS: This cross-sectional study involved 19 patients with PFPS and 19 control subjects who were matched according to gender, age, and physical activity. During eccentric leg press action, frontal plane tibiofemoral alignment was assessed with a motion analysis system based on skin markers. Simultaneously, surface-electromyography was used to assess the activity levels of the relevant knee crossing muscles. To assess the activity under functional conditions, a leg press with a footplate having variable stability was used for barefoot testing. RESULTS: The PFPS subjects did not have significantly different frontal plane leg alignment compared to controls. On electromyography (EMG), PFPS patients had significantly lower levels of hamstring activity during eccentric leg exercise. The differences between the two groups (%; absolute differences normalized EMG) ranged from 20% (semitendinosus; stable footplate; p=0.017) to 21% (biceps femoris; unstable footplate; p=0.019) and 32% (semitendinosus; unstable footplate; p=0.002). CONCLUSIONS: PFPS is not linked to altered frontal plane leg alignment during eccentric leg pressing. However, PFPS is associated with eccentric under-activation of the hamstrings, which may be a compensatory strategy that maintains patellofemoral joint pressure within bearable levels.     

Cardiovascular responses in Type A children during a cognitive challenge

The few studies which have examined stress-induced cardiovascular reactivity among Type A children have had equivocal results. In the present study, 41 extreme Type A and 46 extreme Type B children were monitored for heart rate and blood pressure during a challenging cognitive task under clear or ambiguous performance standards. Significant heart-rate and blood-pressure responses were observed, but no effects could be attributed to the behavior pattern or performance standards. Type A's were more self-involved, generally attributing their performance to effort. Under ambiguous performance standards, they perceived themselves more negatively and attributed performance to luck and task difficulty. These results do not support the exaggerated cardiovascular response hypothesis but do suggest that Type A children look toward external cues in the evaluation of both themselves and their performance, particularly when the external standards for performance are ambiguous.Selected data described in this paper were presented as part of a poster session at the American Psychological Association Annual Convention, August 26–30, 1983, Anaheim, California. This work was supported by Grant R01 HL 19877 from the National Heart, Lung, and Blood Institute.     

Correlation analysis of muscle spindle responses to single motor unit contractions.

1. Cross-correlation techniques have been used to study the responses of muscle spindle afferents from the soleus muscle of the cat to twitch contractions of single motor units. 2. Cross-correlograms (post-stimulus time histograms) were used to give the frequency of occurrence of a receptor spike at various times following the initiation of a motor unit contraction together with a display of the average twitch tension wave form. 3. The cross-correlograms revealed that the contraction of a single motor unit can be an effective stimulus to a spindle receptor and may induce afferent firing pattern alterations similar to those observed with whole muscle contraction. 4. The cross-correlograms also revealed quantitative differences in the response of a receptor to contraction of different motor units and to contraction of the same motor unit at different lengths. These differences reflect subtle changes in receptor deformation developed by the twitch of a motor unit under different conditions and by the twitches of different motor units. The results are consistent with anatomical data on the number and distribution of motor units and receptor organs in cat soleus. 5. These findings emphasize that rather than simply acting as generalized force or length sensors for the muscle as a whole, each receptor's spike train carries information about the state of a particular set of motor units.     

Cardiovascular responses to an active coping challenge as predictors of blood pressure patterns 10 to 15 years later.

To assess the long-term predictive importance of high cardiovascular reactivity in relation to subsequent blood pressure, 51 men from a pool of 204 men originally tested at age 18 to 22 years were recruited for blood pressure assessment 10 to 15 years later. Initial testing uniformly involved monitoring of systolic pressure, diastolic pressure, and heart rate during a reaction time task involving threat of shock. In 30 of the 51 men who participated at follow-up, initial testing had also included separate visits to obtain relaxation-only baseline levels of the cardiovascular indices. At follow-up, in addition to clinic-type stethoscopic determinations, blood pressure and heart rate were assessed during work and social and leisure activities via ambulatory monitoring. Men with higher levels of systolic pressure during the task showed higher stethoscopic and ambulatory systolic pressure at follow-up. Likewise, men with higher levels of diastolic pressure during the task showed higher diastolic levels at follow-up. In the 30 men with both good task and baseline data from initial testing, those with high heart rate reactivity (task minus baseline) showed higher systolic, diastolic, and heart rate levels at follow-up than low heart rate reactors, even though their baseline blood pressures had not differed at initial testing. Similarly, men with high systolic reactivity showed higher diastolic pressure at follow-up than low systolic reactors. Multiple regression analyses also demonstrated that systolic, diastolic, and heart rate reactivity improve prediction of follow-up blood pressure when added to models incorporating the standard risk factors, baseline blood pressure, and parental history of hypertension.     

Cutaneous vasomotor adjustments during arm-cranking in individuals with paraplegia

Skin blood flow (SKBF) was evaluated during arm-cranking exercise in able-bodied control subjects (AB; n=6) and in individuals with low- (LP; T10–T12 lesions; n=6) and high-level paraplegia (HP; T5–T9 lesions; n=6), using laser Doppler flowmetry (LDF). During moderate exercise SKBF decreased to [mean (SD)] 82 (15)% of the pre-exercise resting level in AB, whereas it increased to 158 (52)% in LP and to 112 (51)% in HP (the LP:AB difference, P < 0.05). During intense exercise SKBF increased to 366 (180)% of the resting level in AB, whereas it increased only moderately [147 (68)%] in both paraplegic groups (the paraplegic:AB difference, P < 0.05). The paraplegics developed a higher esophageal and leg skin temperature, which was attributed to the lack of active vasodilation and evaporative cooling over the legs. The results indicate that individuals with paraplegia suffer from impaired cutaneous vasoconstriction at the onset of arm exercise, and possess only a limited vasodilatory capability in the paralyzed regions. During intense exercise, thermoregulation depends critically on active cutaneous vasodilation and skin cooling. Accepted: 25 August 2000     

Cardiovascular responses to human calf muscle stretch during varying levels of muscle metaboreflex activation

The purpose of the present study was to investigate the cardiovascular responses to muscle metaboreflex- and concurrent muscle stretch-induced mechanoreflex activation. Eight subjects (7 males, 1 female) performed 90 s of isometric calf plantarflexion at 0, 30, 50 and 70% of maximum voluntary contraction. During exercise and for 3.5 min postexercise, circulatory occlusion (PECO) was ensured by inflation of a thigh cuff. After 90 s of PECO the calf muscle was stretched for 60 s (Stretch). Heart rate (HR; assessed from ECG), blood pressure (BP; Finapres) and phase of respiratory cycle were recorded. Exercise increased diastolic BP (DBP) from rest by 1+/-0.8, 14+/-2.5, 29+/-3.9 and 35+/-3.6 mmHg, during the 0, 30, 50 and 70% conditions, respectively (ANOVA rest versus exercise, P<0.05). During PECO DBP remained elevated, by 2+/-0.4, 8+/-0.3, 12+/-0.3 and 13+/-0.9 mmHg, respectively. Stretch produced a further increase in DBP that was not different between conditions (3+/-1.4, 2+/-0.8, 3+/-1.0 and 3+/-0.9 mmHg, for the 0, 30, 50 and 70%, respectively). HR increased during exercise but returned to baseline during PECO. HR increased at Stretch onset in all conditions. No EMG was detected from the gastrocnemius and soleus during Stretch. Our data show that the cardiovascular responses to human calf Stretch are independent of the level of concurrent muscle metaboreflex activation.     

Effects of increased muscle perfusion pressure on responses to dynamic leg exercise in man

Summary Ventilatory, cardiovascular and metabolic functions and work performance were studied in men performing incremental-load dynamic leg exercise until exhaustion.Part I: Responses to supine exercise were investigated in 8 subjects during exposure of the lower body to subatmospheric pressure at –6.67 kPa (–50 mm Hg) (Lower Body Negative Pressure, LBNP). Due to curtailment of stroke volume, cardiac output was reduced by LBNP over a wide range of work intensities, including heavy loads: ventilation, oxygen uptake and blood lactate concentrations increased with work load, but at lower rates than in the control condition.Part II: In 9 subjects, work performance was compared in three conditions: supine exercise with and without LBNP, and upright exercise. Performance in supine exercise was enhanced by LBNP, and was further improved in upright exercise. In supine exercise, the LBNP-induced reduction in blood lactate and enhancement of work performance are attributed to a more efficient muscle blood flow resulting from increased local perfusion pressure. This strongly suggests that the primary limitation of work performance was set by the peripheral circulation in working muscles rather than by cardiac performance. A similar mechanism may, in part, explain why work performance in dynamic leg exercise was greater in the upright than in the supine posture. It is also concluded that supine leg exercise during LBNP is a useful model of upright exercise, with regard to the central circulation and the circulation in working muscles.     

Cardiovascular responses at the onset of passive leg cycle exercise in paraplegics with spinal cord injury

The purpose of this study was to examine the cardiovascular responses at the onset of passive leg cycle exercise (PLCE) in paraplegics with spinal cord injury (PSCI) to investigate the increase in venous return from the paralyzed lower limbs during PLCE. Six male PSCI having lesions at levels ranging from T8 to L1 and five male able-bodied subjects (ABS) participated in this study. The subjects performed PLCE at pedalling frequencies of 40 rpm for 6 min. Cardiac output (Q˙ _c), stroke volume (SV) and heart rate (f _c) were measured before and during PLCE. In the steady state (4th and 5th min) of PLCE, both PSCI and ABS showed a significant increase in Q˙ _c. At the onset of PLCE, however, clear differences in the cardiovascular response were found between PSCI and ABS. The ABS showed a rapid and marked increase in f _c and consequently Q˙ _c within 20 s of the onset of PLCE. On the other hand, in PSCI, the Q˙ _c increased more slowly, compared with that in ABS, because of a smaller increase in f _c and a delayed increase in SV. The observed delay in the increases of Q˙ _c and SV at the onset of PLCE in PSCI was presumably due to the absence of afferent reflexes from the lower limbs, and to the additional time needed for venous return to arrive at the heart from the passively moved muscles. Accepted: 23 September 1999     

Effects of muscle shortening on the responses of cat tendon organs to unfused contractions

1. The discharges from individual Golgi tendon organs of peroneus tertius and brevis muscles were recorded in anesthetized cats. Responses to unfused isometric contractions of single motor units and combinations of motor units were compared with responses to contractions eliciting muscle shortening (i.e., shortening contractions). 2. In 75% of the examined instances, the effect of muscle shortening during unfused contractions was a slight decrease in tendon organ activation, in keeping with the reduction of contractile tension recorded at the muscle tendon. In other instances there was either no change in tendon organ response or, in less than 10% of instances, a slight increase For two motor units eliciting similar activation of a given tendon organ under isometric conditions, the effect of shortening contraction was not necessarily the same. 3. The reductions observed in tendon organ discharges upon muscle shortening were less than proportional to the reductions of contractile tension and difficult to correlate with the properties of motor units, as determined under isometric conditions. The present observations suggest three main reasons for this lack of relation. 4. The first reason depended on the properties of motor units, in that the relation between length changes and tension changes was not the same for all units. Two motor units developing similar isometric tensions did not necessarily produce the same degree of muscle shortening. Some units produced relatively significant shortening without much loss of tension. 5. Second, the dynamic sensitivity of tendon organs is known to exert a major influence on their responses to isometric unfused contractions, accounting for 1:1 driving of discharge during tension oscillations and high frequency bursts upon abrupt increase of tension. Although less tension was produced and the rate of tension development was slower in shortening contractions, similar manifestations of the dynamic sensitivity of tendon organs were observed. In such cases, the responses of tendon organs were the same whether or not the muscle shortened during contraction. 6. Third, when several motor units were stimulated in combination, the unloading influences of in-parallel units were facilitated by muscle shortening so that unloading effects, which were hardly visible under isometric conditions became evident during shortening contractions.