Force depression in human quadriceps femoris following voluntary shortening contractions.

The purpose of this study was to investigate whether the isometric muscle force, redeveloped following maximal-effort voluntary shortening contractions in human skeletal muscle, is smaller than the purely isometric muscle force at the corresponding length. Isometric knee extensor moments, surface electromyographic (EMG) signals of quadriceps femoris, and interpolated twitch moments (ITMs) were measured while 10 subjects performed purely isometric knee extensor contractions at a 60 degrees knee angle and isometric knee extensor contractions at a 60 degrees knee angle preceded by maximal-effort voluntary shortening of the quadriceps muscles. It was found that the knee extensor moments were significantly decreased for the isometric-shortening-isometric contractions compared with the isometric contractions for the group as a whole, whereas the corresponding EMG and ITM values were the same. This study is the first to demonstrate force depression following muscle shortening for voluntary contractions. We concluded that force depression following muscle shortening is an actual property of skeletal muscle rather than a stimulation artifact and that force depression during voluntary contraction is not accompanied by systematic changes in muscle activation as evaluated by EMG and ITM.     

Observations on force enhancement in submaximal voluntary contractions of human adductor pollicis muscle.

It has been observed consistently and is well accepted that the steady-state isometric force after active muscle stretch is greater than the corresponding isometric force for electrically stimulated muscles and maximal voluntary contractions. However, this so-called force enhancement has not been studied for submaximal voluntary efforts; therefore, it is not known whether this property affects everyday movements. The purpose of this study was to determine whether there was force enhancement during submaximal voluntary contractions. Human adductor pollicis muscles (n = 17) were studied using a custom-built dynamometer, and both force and activation were measured while muscle activation and force were controlled at a level of 30% of maximal voluntary contraction. The steady-state isometric force and activation after active stretch were compared with the corresponding values obtained during isometric reference contractions. There was consistent and reliable force enhancement in 8 of the 17 subjects, whereas there was no force enhancement in the remaining subjects. Subjects with force enhancement had greater postactivation potentiation and a smaller resistance to fatigue in the adductor pollicis. We conclude from these results that force enhancement exists during submaximal voluntary contractions in a subset of the populations and suggest that it may affect everyday voluntary movements in this subset. On the basis of follow-up testing, it appears that force enhancement during voluntary contractions is linked to potentiation and fatigue resistance and therefore possibly to the fiber-type distribution in the adductor pollicis muscle.     

Shortening-induced depression of voluntary force in unfatigued and fatigued human adductor pollicis muscle.

The goals of this study were to investigate adductor pollicis muscle (n = 7) force depression after maximal electrically stimulated and voluntarily activated isovelocity (19 and 306 degrees /s) shortening contractions and the effects of fatigue. After shortening contractions, redeveloped isometric force was significantly (P < 0.05) depressed relative to isometric force obtained without preceding shortening. For voluntarily and electrically stimulated contractions, relative force deficits respectively were (means +/- SE) 25.0 +/- 3.5 and 26.6 +/- 1.9% (19 degrees /s), 7.8 +/- 2.2 and 11.5 +/- 0.6% (306 degrees /s), and 23.9 +/- 4.4 and 31.6 +/- 4.7% (19 degrees /s fatigued). The relative force deficit was significantly smaller after fast compared with slow shortening contractions, whereas activation manner and fatigue did not significantly affect the deficit. It was concluded that in unfatigued and fatigued muscle the velocity-dependent relative force deficit was similar with maximal voluntary activation and electrical stimulation. These findings have important implications for experimental studies of force-velocity relationships. Moreover, if not accounted for in muscle models, they will contribute to differences observed between the predicted and the actually measured performance during in vivo locomotion.     

Catchlike property in human adductor pollicis muscle

The "catchlike" property is defined as the dramatic force increase in skeletal muscles when a single pulse is added at the onset of a sub-tetanic low-frequency stimulation train. This property has been observed in single motor units, whole animal and human muscles. It is an inherent property of muscle fibres and is not related to an increase in motor unit recruitment. Despite an abundance of observations, its origin remains unclear. The aim of this study was to induce the catchlike property in human adductor pollicis and identify its possible origin. Thumb adduction forces were measured using ulnar nerve electrical stimulation at 10Hz for reference trains (RTs) with one extra pulse 8ms after the first stimulation pulse for the experimental trains (ETs). Tests were performed at two muscle length and three stimulation levels and muscle stiffness and potentiation were quantified for all test conditions. The ETs showed higher forces and greater rates of force increase than the RTs. In addition, force increase was more pronounced at short compared to long muscle length, but no differences were found in force increase for the three stimulation levels. Furthermore, potentiation and stiffness were similar across all experimental conditions. Together, these results suggest that the increase in force associated with the catchlike property is neither caused by an increased proportion of attached cross-bridges nor potentiation of the muscle, but appears to be muscle length dependent and present in both slow and fast motor units.     

Effects of speed and distance of muscle shortening on force depression during voluntary contractions

The purpose of this study was to determine the influence of speed and distance of muscle shortening on the amount of force depression for voluntary contractions. Two experimental tests were performed. In the first test, subjects performed isometric knee extensor contractions following muscle shortening produced by isokinetic knee extensions over the range 25-50 degrees. In the second test, subjects performed isometric knee extensor contractions following muscle shortening produced by isokinetic knee extensions at two speeds: 20 and 240 degrees /s. Knee extensor moments, surface electromyographical (EMG) signals of quadriceps femoris, and interpolated twitch moments were measured during all contractions and were compared with the corresponding values obtained during purely isometric contractions. Force depression following muscle shortening for the voluntary contractions tested in this study did not depend on the distance or the speed of muscle shortening. These results are in contrast to the corresponding results in the literature obtained using artificial electrical stimulation in which force depression was always found to be directly related to the distance of shortening and inversely related to the speed of shortening. The difference in force depression as a function of the distance and speed of muscle shortening between voluntary and artificial electrical stimulation may be associated with changes in activation following the voluntary shortening contractions, whereas activation is controlled and constant in all artificial stimulation protocols.     

Effect of aging on human adductor pollicis muscle function.

The effect of aging on the voluntary and electrically evoked contractile properties of the human adductor pollicis muscle was investigated in 70 healthy male subjects aged 20-91 yr, 10 subjects for each decade. Maximum isometric voluntary force declined significantly (range of P values less than 0.001-0.05) after the age of 59 yr, dropping by the eighth decade to 57.6% of the level recorded in the second decade. A significant shift (P range less than 0.001-0.05) to the left of the frequency-force curve after ulnar nerve supramaximal stimulation at 1, 10, 20, 30, and 50 Hz was observed in the most elderly group (greater than 80 yr) compared with the youngest group (20-29 yr). Maximum relaxation rate dropped by 48.7% from the second to the eighth decade. The decrease became significant (P range less than 0.05-0.001) with the sixth decade. Isometric endurance, evaluated during 30 s of stimulation at 30 Hz, showed a linear (P less than 0.001) increase with age. Aged muscle is thus weaker, slower, and tetanized at lower fusion frequencies but, paradoxically, more resistant to static fatigue.     

Stimulation frequency and force potentiation in the human adductor pollicis muscle

The effect of stimulation frequency on twitch force potentiation was examined in the adductor pollicis muscle of ten normal subjects. The ulnar nerve was supramaximally stimulated at the wrist and isometric twitch force was measured from a 3-Hz train lasting 1 s. Test stimulation frequencies of 5, 10, 20, 25, 30, 40, 50 and 100 Hz were applied for 5 s each in random order (5 min apart) and the twitches (3 Hz) were applied immediately before and after (1 s) the test frequency and at intervals up to 5 min afterwards (10 s, and 1, 2 and 5 min). Poststimulation twitches were expressed as a percentage of the prestimulation twitch. Low frequency fatigue was not induced by the protocol since the 20:50 Hz ratio did not alter within each session. The degree of twitch potentiation was frequency dependent, with potentiation increasing up to 50 Hz [mean 173 (SD 16)%] but the effect was markedly less at 100 Hz [mean 133 (SD 25)%, P less than 0.01] for all subjects. The reduced potentiation at 100 Hz may have occurred due to high frequency fatigue produced by the 100-Hz test stimulation train. The optimal frequency of those examined in the experimental group was 50 Hz but this only produced maximal potentiation in six of the ten subjects and 100 Hz always produced less potentiation. These findings have implications for electrical stimulation of muscle in the clinical setting.     

Activation-induced force enhancement in human adductor pollicis

It has been known for a long time that the steady-state isometric force after muscle stretch is bigger than the corresponding force obtained in a purely isometric contraction for electrically stimulated and maximal voluntary contractions (MVC). Recent studies using sub-maximal voluntary contractions showed that force enhancement only occurred in a sub-group of subjects suggesting that force enhancement for sub-maximal voluntary contractions has properties different from those of electrically-induced and maximal voluntary contractions. Specifically, force enhancement for sub-maximal voluntary contractions may contain an activation-dependent component that is independent of muscle stretching. To address this hypothesis, we tested for force enhancement using (i) sub-maximal electrically-induced contractions and stretch and (ii) using various activation levels preceding an isometric reference contraction at 30% of MVC (no stretch). All tests were performed on human adductor pollicis muscles. Force enhancement following stretching was found for all subjects (n = 10) and all activation levels (10%, 30%, and 60% of MVC) for electrically-induced contractions. In contrast, force enhancement at 30% of MVC, preceded by 6 s of 10%, 60%, and 100% of MVC was only found in a sub-set of the subjects and only for the 60% and 100% conditions. This result suggests that there is an activation-dependent force enhancement for some subjects for sub-maximal voluntary contractions. This activation-dependent force enhancement was always smaller than the stretch-induced force enhancement obtained at the corresponding activation levels. Active muscle stretching increased the force enhancement in all subjects, independent whether they showed activation dependence or not. It appears that post-activation potentiation, and the associated phosphorylation of the myosin light chains, might account for the stretch-independent force enhancement observed here.     

The shape of the force-elbow angle relationship for maximal voluntary contractions and sub-maximal electrically induced contractions in human elbow flexors

The force-length relationship is a basic property of skeletal muscle. Knowledge of this relationship is necessary for most analyses of human movement, and in simulation models predicting movement control strategies. Studies on animal muscles have shown that force-length relationships for sub-maximal contractions are not related through a simple scaling procedure to the relationship for maximal contractions. Furthermore, potentiation might produce a shift of sub-maximal relative to maximal force-length relationships. In this study, we tested the hypothesis that human force-elbow angle relationships for sub-maximal unpotentiated contractions are shifted to larger elbow angles (i.e. larger muscle lengths) compared to the relationship for maximal voluntary contractions (MVC), and that this shift is reduced, or even abolished, for sub-maximal potentiated contractions. Force-elbow angle relationships (48-160 degrees) were obtained from healthy subjects (n=13). At each of nine tested elbow angles, the test set consisted of a single twitch (ST(pre)) and a doublet twitch (DT(pre)) stimulation of m. biceps brachii, followed by an MVC, followed by another single twitch (ST(post)) and a doublet twitch (DT(post)) stimulation. The single and doublet twitches induced sub-maximal contractions. The force-elbow angle relationships for the pre-MVC (unpotentiated) twitch contractions were shifted to larger angles compared to those obtained for MVC. The force-elbow angle relationships for the post-MVC (potentiated) twitch contractions were shifted to smaller angles compared to those obtained for the unpotentiated twitch contractions. These results support the idea that the shift to larger muscle lengths for the sub-maximal, unpotentiated force-length relationships relative to the relationship for maximal contractions may be caused by a length-dependent Ca(2+) sensitivity that may be offset, at least in part, by potentiation.     

Postactivation potentiation in a human muscle: effect on the load-velocity relation of tetanic and voluntary shortening contractions.

Recently it was demonstrated that postactivation potentiation (PAP), which refers to the enhancement of the muscle twitch torque as a result of a prior conditioning contraction, increased the maximal rate of torque development of tetanic and voluntary isometric contractions (3). In this study, we investigated the effects of PAP and its decay over time on the load-velocity relation. To that purpose, angular velocity of thumb adduction in response to a single electrical stimulus (twitch), a high-frequency train of 15 pulses at 250 Hz (HFT(250)), and during ballistic voluntary shortening contractions, performed against loads ranging from 10 to 50% of the maximum torque, were recorded before and after a conditioning 6-s maximal voluntary contraction (MVC). The results showed an increase of the peak angular velocity for the different loads tested after the conditioning MVC (P < 0.001), but the effect was greatest for the twitch ( approximately 182%) compared with the HFT(250) or voluntary contractions ( approximately 14% for both contraction types). The maximal potentiation occurred immediately following the conditioning MVC for the twitch, whereas it was reached 1 min later for the tetanic and ballistic voluntary contractions. At that time, the load-velocity relation was significantly shifted upward, and the maximal power of the muscle was increased ( approximately 13%; P < 0.001). Furthermore, the results also indicated that the effect of PAP on shortening contractions was not related to the modality of muscle activation. In conclusion, the findings suggest a functional significance of PAP in human movements by improving muscle performance of voluntary dynamic contractions.     

The contribution of the supraspinatus muscle at sub-maximal contractions

During maximum effort, the supraspinatus muscle contributes approximately 50% of the torque need to elevate the arm, but this has not been examined at sub-maximal levels. The purpose of this study was to determine the contribution of the supraspinatus muscle to shoulder elevation at sub-maximal levels. Seven healthy subjects (four males, three females) performed isometric ramp contractions at the shoulder. Middle deltoid electromyography (EMG) and force applied at the wrist were collected before and after a suprascapular nerve block. For the same level of deltoid EMG, less external force will be measured after the nerve block as the supraspinatus muscle no longer contributes. The difference between the EMG/force curve was the contribution of the supraspinatus muscle. The supraspinatus contributed 40%, 95% CI [32%–48%], to shoulder elevation. The effect of angle (p = .67) and % maximal voluntary contraction (p = .13) on supraspinatus contribution were not significant. The maximum is slightly less than reported in a previous suprascapular nerve block study using maximal contractions. The results from this study can be used to assess supraspinatus contribution in rotator cuff tears, after rehabilitation interventions, and as a restraint in computation modelling.     

Tetanic force development of adductor pollicis muscle in anesthetized man.

The tetanic force development of the human adductor pollicis muscle was studied under light anesthesia with nitrous oxide, oxygen, and Demerol, by the use of tetanic stimulation of the ulnar nerve at frequencies ranging from 10 to 100 Hz. The time necessary for the tetanic contraction to reach a plateau was longest at frequencies between 15 and 20 Hz. Fusion of tetanus occurred between 40 and 45 Hz. The mean maximal force of 6.92 kg was developed at a mean frequency of approximately 75 Hz. The maximal force was well maintained up to a stimulation frequency of 100 Hz. The results indicate that in lightly anesthetized man, the maximal force is developed at higher stimulation frequencies than those observed in conscious man and that it is well sustained at higher frequencies.     

The measurement of force/velocity relationships of fresh and fatigued human adductor pollicis muscle

The purpose of the study was to obtain force/velocity relationships for electrically stimulated (80 Hz) human adductor pollicis muscle (n = 6) and to quantify the effects of fatigue. There are two major problems of studying human muscle in situ; the first is the contribution of the series elastic component, and the second is a loss of force consequent upon the extent of loaded shortening. These problems were tackled in two ways. Records obtained from isokinetic releases from maximal isometric tetani showed a late linear phase of force decline, and this was extrapolated back to the time of release to obtain measures of instantaneous force. This method gave usable data up to velocities of shortening equivalent to approximately one-third of maximal velocity. An alternative procedure (short activation, SA) allowed the muscle to begin shortening when isometric force reached a value that could be sustained during shortening (essentially an isotonic protocol). At low velocities both protocols gave very similar data (r2 = 0.96), but for high velocities only the SA procedure could be used. Results obtained using the SA protocol in fresh muscle were compared to those for muscle that had been fatigued by 25 s of ischaemic isometric contractions, induced by electrical stimulation at the ulnar nerve. Fatigue resulted in a decrease of isometric force [to 69 (3)%], an increase in half-relaxation time [to 431 (10)%], and decreases in maximal shortening velocity [to 77 (8)%] and power [to 42 (5)%]. These are the first data for human skeletal muscle to show convincingly that during acute fatigue, power is reduced as a consequence of both the loss of force and slowing of the contractile speed.     

Behavior of human muscle fascicles during shortening and lengthening contractions in vivo.

The aim of the present study was to investigate the behavior of human muscle fascicles during dynamic contractions. Eight subjects performed maximal isometric dorsiflexion contractions at six ankle joint angles and maximal isokinetic concentric and eccentric contractions at five angular velocities. Tibialis anterior muscle architecture was measured in vivo by use of B-mode ultrasonography. During maximal isometric contraction, fascicle length was shorter and pennation angle larger compared with values at rest (P < 0.01). During isokinetic concentric contractions from 0 to 4.36 rad/s, fascicle length measured at a constant ankle joint angle increased curvilinearly from 49.5 to 69.7 mm (41%; P < 0.01), whereas pennation angle decreased curvilinearly from 14.8 to 9.8 degrees (34%; P < 0.01). During eccentric muscle actions, fascicles contracted quasi-isometrically, independent of angular velocity. The behavior of muscle fascicles during shortening contractions was believed to reflect the degree of stretch applied to the series elastic component, which decreases with increasing contraction velocity. The quasi-isometric behavior of fascicles during eccentric muscle actions suggests that the series elastic component acts as a mechanical buffer during active lengthening.     

The effect of age and gender on the relative fatigability of the human adductor pollicis muscle

The purpose of this study was to examine the relative influence of such factors as age, gender, and absolute force on the fatiguability of the human adductor pollicis muscle. 12 young males (YM, 25.3 +/- 2.1 y), 12 young females (YF. 23.5 +/- 2.1 y), 12 older males (OM, 71.7 +/- 5.6 y) and 12 older females (OF, 69.5 +/- 4.6 y) participated. Three minutes of intermittent (5 s contraction, 2 s rest) maximal voluntary contractions (MVC) were used to fatigue the adductor pollicis muscle; the ulnar nerve was also stimulated in each 2 s rest period to evoke a maximal twitch. Males were stronger than females in both voluntary and evoked force (PT) in the young age group (MVC: YM, 10.0 +/- 2.7 kg vs. YF, 6.6 +/- 1.1 kg, P < 0.05) (PT: YM, 0.99 +/- 0.21 kg vs. YF, 0.71 +/- 0.12 kg, P < 0.05). In the older adults, however, males were stronger only in the evoked twitch (OM, 0.73 +/- 0.24 kg vs. OF, 0.48 +/- 0.07 kg, P < 0.05). There was no significant effect of gender or absolute muscle force on relative fatigability; the only variable found to significantly affect fatigability was age. Older adults were significantly less fatigable than young adults as indicated by the voluntary fatigue index (FI) (percentage of force reduction from baseline; FI-young, 40.2 +/- 12.6% vs. FI-old, 25.2 +/- 12.3%). This age effect, however, was more prominent in males than females (FI-YM, 44.7 +/- 10.5% vs. FI-OM, 24.2 +/- 10.7%, P < 0.01; FI-YF, 37.8 +/- 14.1% vs. FI-OF, 26.3 +/- 14.5%, P = 0.13). In conclusion, age was found to be the strongest single predictor of fatigability during short duration, intermittent exercise in human adductor pollicis muscle.     

Gender alters impact of hypobaric hypoxia on adductor pollicis muscle performance.

Recently, we reported that, at similar voluntary force development during static submaximal intermittent contractions of the adductor pollicis muscle, fatigue developed more slowly in women than in men under conditions of normobaric normoxia (NN) (Acta Physiol Scand 167: 233-239, 1999). We postulated that the slower fatigue of women was due, in part, to a greater capacity for muscle oxidative phosphorylation. The present study examined whether a gender difference in adductor pollicis muscle performance also exists during acute exposure to hypobaric hypoxia (HH; 4,300-m altitude). Healthy young men (n = 12) and women (n = 21) performed repeated static contractions at 50% of maximal voluntary contraction (MVC) force of rested muscle for 5 s followed by 5 s of rest until exhaustion. MVC force was measured before and at the end of each minute of exercise and at exhaustion. Exhaustion was defined as an MVC force decline to 50% of that of rested muscle. For each gender, MVC force of rested muscle in HH was not significantly different from that in NN. MVC force tended to decline at a faster rate in HH than in NN for men but not for women. In both environments, MVC force declined faster (P < 0.01) for men than for women. For men, endurance time to exhaustion was shorter (P < 0.01) in HH than in NN [6.08 +/- 0.7 vs. 8.00 +/- 0.7 (SE) min]. However, for women, endurance time to exhaustion was similar (not significant) in HH (12.86 +/- 1.2 min) and NN (13.95 +/- 1.0 min). In both environments, endurance time to exhaustion was longer for women than for men (P < 0.01). Gender differences in the impact of HH on adductor pollicis muscle endurance persisted in a smaller number of men and women matched (n = 4 pairs) for MVC force of rested muscle and thus on submaximal absolute force and, by inference, ATP demand in both environments. In contrast to gender differences in the impact of HH on small-muscle (adductor pollicis) exercise performance, peak O(2) uptake during large-muscle exercise was lower in HH than in NN by a similar (P > 0.05) percentage for men and women (-27.6 +/- 2 and -25.1 +/- 2%, respectively). Our findings are consistent with the postulate of a higher adductor pollicis muscle oxidative capacity in women than in men and imply that isolated performance of muscle with a higher oxidative capacity may be less impaired when the muscle is exposed to HH.     

Experimental muscle pain increases mechanomyographic signal activity during sub-maximal isometric contractions.

This study was designed to investigate the local effect of experimental muscle pain on the MMG and the surface EMG during a range of sub-maximal isometric contractions. Muscle pain was induced by injections of hypertonic saline into the biceps brachii muscle in 12 subjects. Injections of isotonic saline served as a control. Pain intensity and location, MMG and surface EMG from the biceps brachii were assessed during static isometric (0%, 10%, 30%, 50% and, 70% of the maximal voluntary contraction) and ramp isometric (0-50% of the maximal voluntary contraction) elbow flexions. MMG and surface EMG signals were analyzed in the time and frequency domain. Experimentally induced muscle pain induced an increase in root mean square values of the MMG signal while no changes were observed in the surface EMG. Most likely this increase reflects changes in the mechanical contractile properties of the muscle and indicates compensatory mechanisms, i.e. decreased firing rate and increased twitch force to maintain a constant force output in presence of experimental muscle pain. Under well-controlled conditions, MMG recordings may be more sensitive than surface EMG recordings and clinically useful for detecting non-invasively increased muscle mechanical contributions during muscle pain conditions.