| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
1Canadian Centre for Activity and Aging, School of Kinesiology, Faculty of Health Sciences, and 2Department of Anatomy and Cell Biology, Faculty of Medicine and Dentistry, The University of Western Ontario, London, Ontario, Canada N6A 3K7
Submitted 11 September 2003 ; accepted in final form 15 October 2003
 |
ABSTRACT |
|---|
 TOP ABSTRACT METHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
|---|
31%) and relaxation was slower compared with the young men, as assessed by the maximal relaxation rate constant of the 50-Hz tetanus (young 12.1 ± 0.2 vs. old 9.2 ± 0.5 s-1; P < 0.05) and a leftward shift in the force-frequency relationship. The NF protocol revealed a decreased fatigability in the quadriceps with old age (percentage of 1st contraction force remaining at 180th: old 63.4 ± 1.5 vs. young 58.2 ± 1.7%; P < 0.05) that was masked during the CF protocol (old 60.7 ± 1.6 vs. young 58.6 ± 2.3%; P > 0.05). Irrespective of the protocol, the maximal relaxation rate was reduced to 73 and 57% of the prefatigue value in the young and old men, respectively. The age-related leftward shift in the force-frequency relationship of the quadriceps contributed to an underestimation of the fatigue resistance with old age during the CF protocol. However, when the stimulation frequency used in the NF protocol was adjusted to account for the age-related shift in the force-frequency relationship, the quadriceps muscles of the old men were less fatigable than those of the young men. Thus we suggest that whole muscle fatigability is better examined by electrical stimulation protocols that are adjusted for inter- and intragroup differences in the force-frequency relationship. electrical stimulation; slowing of relaxation; isometric contraction
It has been suggested that the age-related slowing of relaxation is an important factor in the fatigability comparison between young and old adults, because it likely alters the degree of circulatory occlusion during the fatigue protocol (7, 8, 14). Slower relaxation delays the return of force to the baseline level following intermittent contractions and causes a "leftward shift" in the relative relationship between the force outputs from low vs. high frequencies of stimulation when old are compared with young adults. Therefore, during a low-frequency stimulation protocol at a constant frequency (CF) of stimulation (e.g., 15 Hz), the muscles of old adults are stimulated at a greater percentage of their available maximal tetanic force than those of young adults (1, 10). Both the delay in force recovery and the shift in the force-frequency curve may cause old adults to experience a greater relative degree of circulatory occlusion during each contraction, and fatigability may increase compared with young adults. However, the delayed recovery of force following the intermittent contractions does not explain the increased fatigability with old age during low, but not higher, frequency protocols, because the prolonged relaxation is present for stimulated contractions at a large range of frequencies (e.g., 1–50 Hz) (14, 24). Previous electrical stimulation studies have not adjusted the frequency of stimulation to account for the age-related shift in the force-frequency curve during the fatigability comparison of young and old humans.
We examined the effect of an age-related leftward shift of the force-frequency relationship on muscle fatigue during low-frequency electrical stimulation by comparing the quadriceps fatigability of young and old men during two different intermittent electrical stimulation protocols: 1) a CF protocol at 14.3 Hz; and 2) a normalized frequency (NF) protocol, which stimulated the muscle at the frequency corresponding to 60% of each subject's force-frequency curve. Therefore, the stimulation frequency used during the NF protocol was adjusted for each subject to account for the age-related shift in the force-frequency relationship. Because the rate of relaxation of the quadriceps is slowed with old age (15) and there is a leftward shift in the force-frequency curve of old compared with young adults (25, 27), we hypothesized that a constant, low-frequency protocol would induce greater fatigue in the quadriceps of the old vs. the young men. Moreover, it is reasonable to hypothesize that a normalized protocol, which was adjusted to account for this shift, would mitigate the increase in quadriceps fatigability.
|
METHODS |
|---|
|
TOPABSTRACTMETHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
|---|
Experimental setup. Subjects were seated in a custom-built isometric dynamometer with the knee joint at 85° of flexion, and their dominant lower limb was attached to a strain gauge via a strap just proximal to the malleoli (25). The knee joint angle did not differ between subjects or between visits. A seatbelt over the hips and two diagonal straps secured across the chest minimized extraneous movements during the quadriceps contractions. Voluntary and electrically stimulated isometric quadriceps force was recorded by the strain gauge (model 363-D3-300-20P3, InterTechnology, Don Mills, Ontario), where the output was amplified (x2), filtered (60-Hz notch filter) (Neurolog System, Digitimer, Welwyn Garden City, Hertfordshire, UK), and sampled at 500 Hz using a 12-bit analog-to-digital converter (model 1401 Plus, Cambridge Electronics Design, Cambridge, UK). The online force signal was displayed on an oscilloscope to provide visual feedback to the subject.
Two stimulation electrodes, consisting of aluminum foil wrapped in a thin conductive gel-soaked cloth, were secured transversely to the proximal (one-third of the distance from the anterior superior iliac spine to the superior border of the patella) and distal portions (2.5 cm from the patella) of the dominant thigh (25). To ensure stimulation of the largest mass of the quadriceps without interference from antagonist muscles, the stimulation electrode pair was either 7.5 x 12 cm or 7.5 x 16 cm, depending on the size of the thigh. Electrical stimulation was applied via a computer-triggered stimulator (model DS7H, Digitimer) at a pulse width of 50 µs, 400 V, and current level ranging from 230 to 650 mA.
Experimental protocol. Subjects participated in two visits, which differed principally in the frequency of stimulation used during the fatigue protocol: the CF visit and the NF visit. Unless otherwise stated, the following experimental procedures were conducted on both visits.
Subjects performed three quadriceps maximum voluntary contractions (MVCs), with each held for 3–4 s and separated by 3 min of rest. Voluntary muscle activation was assessed during the second and third MVC using the ratio of the force amplitude of an interpolated doublet (Ts, 2 pulses at 100 Hz) and a doublet following the MVC (Tr) {%activation = [1 - (Ts/Tr)] x 100%}. Because of subject discomfort, supramaximal doublets were not used in the assessment of voluntary muscle activation. Instead, the stimulation intensity was set so that the Tr force amplitude was 20% of the MVC force of each subject (combined CF visit and NF visit results: young 21.6 ± 0.3%, old 21.5 ± 0.5%).
After 3-min rest, the stimulation intensity for the force-frequency curve was determined. This procedure differed between the two visits to account for the different stimulation frequencies to be used in the subsequent fatigue protocols. During the CF visit, the stimulation intensity of a six-pulse CF train (CFT) of 14.3 Hz (70-ms interpulse interval, 350-ms duration) was increased until the peak force of the CFT was 25% of the subject's greatest MVC of the visit. Ten seconds separated each CFT in this incremental sequence, and five to six trains were delivered to match 25% of the MVC. Because all subjects would experience a fatigue protocol of the CFT, it was useful to derive the force-frequency curve based on a stimulation intensity determined at 14.3 Hz. Conversely, during the NF visit, a 1-s train at 100 Hz was applied once per 10 s (5–6 times), as the stimulation intensity was increased, until 40% of the greatest MVC of the NF visit was achieved. We did not know what frequency each subject would require in the NF fatigue protocol until after the force-frequency curve had been generated (see below). Thus it was useful to derive the curve based on the stimulation intensity determined with the tetanic maximum (100 Hz). Although the procedures differed on the two visits, they yielded the same result: a 1-s 100-Hz train, eventually derived from the force-frequency curve, produced 40% of each subject's greatest MVC of each visit.
After 3 min of rest, a force-frequency curve was obtained from the relationship of the force amplitudes at various frequencies, expressed as a percentage of the 100-Hz force. The force-frequency protocol consisted of 14 different frequencies (1, 5, 7.5, 10, 12, 14.3, 17, 20, 25, 30, 40, 50, 75, and 100 Hz) delivered in incremental order at one frequency every 10 s. Excluding the 1-Hz pulse, each stimulation frequency train was applied for 1 s. Because immediate analysis of the force-frequency relationship was required to normalize the fatigue protocol sequence of the NF visit, a 15-min rest period was provided on both visits before the next experimental procedure was performed. During the NF visit, the frequency corresponding to 60% of the 100-Hz force (f60) was extrapolated (to the nearest 0.5 Hz) from each subject's force-frequency curve, and this frequency was used in the subsequent fatigue protocol.
After a 15-min rest period, additional prefatigue measures were recorded to further characterize the quadriceps muscle of the young and old men (see Data reduction and statistics). This involved a prefatigue stimulation sequence (PSS) that consisted of a six-pulse train (either the CFT or the f60, depending on the visit), followed by a 1-s train at 50 Hz, with 1 s of rest separating the low- and high-frequency stimulation. After a 1-min rest, a single CFT or f60 train was applied to the quadriceps to determine whether the stimulation intensity elicited 25% of the greatest MVC of the session. If the force amplitude of this test train did not correspond to 25% MVC, the stimulation intensity was adjusted accordingly before the next PSS, which was delivered 1 min later. The interaction between a loss of potentiation within the muscle during the 15-min rest period and the potentiation reintroduced by the 50-Hz train necessitated the slight adjustment in stimulation intensity. The PSS was performed three times, and the fatigue protocol commenced 1 min following the third sequence at the same stimulation intensity. This ensured that the first contraction of the stimulated fatigue protocol generated a force equal to 25% of the subject's MVC.
The CF fatigue protocol consisted of 180 CFTs of six pulses at 14.3 Hz, each separated by 650 ms (4). The NF fatigue protocol consisted of 180 stimulation trains of six pulses at the f60 from the force-frequency relationship, and each train was separated by 650 ms. Following the 180th contraction of each fatigue protocol, the PSS was delivered (end) and then repeated at 0.5, 1, 2, 3, 4, and 5 min of recovery (R0.5 to R5).
Data reduction and statistics. Peak force and half relaxation time (HRT) were recorded from the 1st, 45th, 90th, 135th, and 180th contraction of the CF and NF protocols, and the values throughout the fatigue protocol were normalized to the values of the 1st contraction. The maximal relaxation rate (MRR) constant of the 50-Hz tetanus (s-1) and the HRTs of the low-frequency trains (HRT-CFT, HRT-f60) were analyzed from the third PSS. The MRR was calculated by dividing the peak rate of change of force during the relaxation (N/s) by the peak force of the 50-Hz tetanus (N) (2). This normalization of the rate of relaxation was necessary to eliminate the effect of differences in peak force on the comparison of age-related slowing (14). To assess recovery, the low- (CFT or f60) and high-frequency (50-Hz) forces and the relaxation measures (HRT-CFT or HRT-f60 and MRR) were normalized to the prefatigue values (i.e., third PSS).
Subject characteristics (height and mass), the prefatigue measures of MVC, HRT-CFT, HRT-f60, MRR, and the force-frequency curve (f60 and percentage of 100-Hz force at each frequency) were compared with an unpaired t-test. The voluntary muscle activation (%activation), which was not normally distributed, was compared with a Mann-Whitney U-test (nonparametric). The normalized forces (i.e., %prefatigue values) during the fatigue protocols and the recovery profiles were analyzed using two-way repeated-measures analysis of variance, with age as one factor for comparison and time as the other. Tukey's honestly significant difference was used for post hoc analysis when statistically significant interactions were observed between the two factors. Values for all data are presented in the text and Figs. 1, 2, 3, 4, 5 as means ± SE. The level of significance was set at P < 0.05.
|
|
|
|
|
|
RESULTS |
|---|
|
TOPABSTRACTMETHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
|---|
|
During both the CF visit and NF visit, the 100-Hz tetanus of the force-frequency curve produced a force that was 40% of each subject's greatest MVC of the visit (combined CF visit and NF visit results: young, 41.3 ± 0.8%; old, 39.1 ± 0.5%). The force-frequency curves did not differ between the CF visit and NF visit for either age group. During both visits, there was an age-related shift in the percentage of the 100-Hz force produced by the lower frequencies of stimulation (<30 Hz). Figure 1 shows that a greater percentage of 100-Hz force was produced by the 5- to 30-Hz stimulation in the old compared with the young men during the NF visit, resulting in a leftward shift of the force-frequency relationship. The f60 was significantly higher in the young (14.9 ± 0.4 Hz) compared with the old men (12.7 ± 0.5 Hz; P < 0.05) (Fig. 1). Thus the NF of stimulation used for the NF fatigue protocol was significantly higher in the young vs. the old men. During both visits, electrical stimulation at 14.3 Hz, which was the frequency used in the CF fatigue protocol, produced a significantly greater percentage of the 100-Hz force in the old compared with the young men (CF visit: young, 60.2 ± 1.7 vs. old, 66.2 ± 1.1%, P < 0.05; NF visit: young, 58.3 ± 1.5 vs. old, 66.3 ± 1.9%, P < 0.05).
Fatigue. The force of the first contraction of both the CF and NF fatigue protocols was 25% of each subject's MVC (CF protocol: young, 24.7 ± 0.2 vs. old, 25.1 ± 0.2%; NF protocol: young, 24.8 ± 0.2 vs. old, 24.5 ± 0.3%), with no difference between age groups or between visits. During the CF and NF protocols, force decreased in a curvilinear fashion in the young and old men (Fig. 2, A and B, respectively). There was no age-related difference in the loss of force during the CF protocol, and, at the 180th contraction, force was reduced equally in the young and old men to 60% of the force of the first contraction (young, 58.6 ± 2.3 vs. old, 60.7 ± 1.6%) (Fig. 2A). Conversely, during the NF protocol, force was reduced more in the young compared with the old men at the 90th, 135th, and 180th contraction (percentage of 1st contraction remaining at 180th: young, 58.2 ± 1.7 vs. old, 63.4 ± 1.5%, P < 0.05) (Fig. 2B).
The old men experienced a significantly greater relative increase in the HRTs of the low-frequency trains during both the CF protocol (percentage of 1st contraction at 180th: young, 134.9 ± 5.3 vs. old, 163.1 ± 6.7%; P < 0.05; Fig. 3A) and the NF protocol (HRT-f60: young, 136.2 ± 4.0 vs. old, 162.7 ± 6.3%; P < 0.05; Fig. 3B). This age-related increase in the normalized HRTs during fatigue was equivalent between protocols.
Recovery. The acute recovery profile of the low-frequency (CF protocol: 14.3 Hz; NF protocol, f60: young, 14.9 ± 0.4 vs. old, 12.7 ± 0.5 Hz) and high-frequency (50 Hz) force is presented for the young and old men in Fig. 4, A and B. Following both protocols, the 50-Hz force recovered faster in the old men compared with the young men (significant time x age interaction and main effect for age, P < 0.05). The 50-Hz force approached complete recovery in the quadriceps of the old men (97%), whereas it remained 12% impaired in the young at R5 (Fig. 4, A and B). Similar to the lack of age-related difference in force loss during the CF protocol, the recovery of the CFT force did not differ between groups in the recovery period (P > 0.05) (Fig. 4A). Conversely, a significant interaction and main effect for age group was found for the f60 force recovery following the NF protocol (P < 0.05), such that the old men had recovered more than the young men (Fig. 4B). However, no individual time points reached statistical significance using Tukey's honestly significant difference post hoc analysis.
The increase in normalized HRTs with old age during the fatigue protocol (Fig. 3) persisted in the recovery period (Fig. 5) and was accompanied by a significantly greater slowing of the MRR in the old vs. the young men following the CF protocol (%prefatigue MRR at end: young, 73.2 ± 2.3 vs. old, 57.9 ± 4.2%; P < 0.05; Fig. 5A) and the NF protocol (young, 73.3 ± 2.3 vs. old, 56.3 ± 2.3%; P < 0.05; Fig. 5B). Although the normalized HRTs and MRR showed an exacerbated slowing of relaxation with old age following both protocols, the age-related differences persisted for a longer duration in the recovery period following the NF protocol (Fig. 5, B vs. A).
|
DISCUSSION |
|---|
|
TOPABSTRACTMETHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
|---|
Prefatigue measures. The quadriceps muscles of the old men were significantly weaker than those of the young men, and there was a slowing of relaxation with old age, as shown by the reduction in the MRR constant of the 50-Hz tetanus, the increase in the HRTs of the low-frequency trains (HRT-CFT, HRT-f60), and the leftward shift in the force-frequency curve. Similar to the findings from the quadriceps of young and old women (15), the MRR was reduced 24% in the old compared with the young men, indicative of a slowing of the intrinsic speed of relaxation. At the level of the whole muscle, a relative increase in type I fiber composition with aging can prolong the relaxation with old age, because these fibers contain contractile proteins with inherently slower contractile properties than fast-twitch fibers (14). Also, within each fiber, an age-related impairment in excitation-contraction coupling (19, 23), specifically an altered dissociation of the actin-myosin cross bridge (11), or an impaired calcium sequestering activity of the sarcoplasmic reticulum (SR) (17, 21) could slow relaxation. However, Hunter et al. (15) suggested that the maximal rate of SR calcium uptake was not the rate-limiting mechanism in the slowing of relaxation in the quadriceps of old women because, following 12 wk of resistance training, a significant increase in SR calcium uptake rate was not accompanied by an increased rate of relaxation. Therefore, although slowing of relaxation is a common observation in most muscles with old age, the principal mechanism(s) remains unclear.
However, it is apparent that slowing of relaxation is responsible for an age-related leftward shift in the force-frequency relationship. This has been shown for the quadriceps (25, 27), triceps surae (8), and adductor pollicis muscles (22). The shift in the force-frequency curve can be quantified by directly comparing the percentage of maximal tetanic force produced at each of the lower frequencies of stimulation (e.g., at 14.3 Hz: young, 58.3 ± 1.5 vs. old, 66.3 ± 1.9%), or by extrapolating from the curve to determine the stimulation frequency corresponding to a given percentage of the maximal tetanic force (f60: young, 14.9 ± 0.4 vs. old, 12.7 ± 0.5 Hz). In the present study and similar to the previous studies cited above, both the direct comparison and the extrapolation technique identified an age-related leftward shift in the force-frequency relationship of the quadriceps. Although the force-frequency relationship has been characterized by extrapolating the curve at 50% of the maximal tetanic force (16), we used the f60 value because the extrapolated frequencies were closer to those used in the previous studies that compared the effect of old age on fatigue induced by low frequencies of stimulation (6–8, 18), and pilot testing showed that the f60 value in the young subjects was similar to the stimulation frequency (14.3 Hz) used in the CF protocol (4).
Fatigue. Despite the age-related leftward shift in the force-frequency curve, quadriceps fatigability was not different in the old compared with the young men during the CF protocol (Fig. 2A). In contrast, an age-related increase in fatigability was found in previous studies in the adductor pollicis (18), triceps surae (7, 8), and tibialis anterior muscles (6) during relatively low frequencies of intermittent or continuous stimulation. This discrepancy may be due to a greater degree of slowing of relaxation in distal vs. proximal limb muscles with old age. For example, when the contraction duration of an evoked twitch is used to compare the different muscle groups, the triceps surae and tibialis anterior slow 30% with old age (5, 7, 8, 28), whereas the quadriceps slows 10% (25).
In contrast to the CF protocol, the NF protocol showed a decreased fatigability with old age. It has been suggested that prefatigue age-related slowing of relaxation, coupled with an exacerbated slowing induced by fatigue, contributes to an increased fatigability with old age because circulatory occlusion persists for a longer duration after each contraction as the force takes longer to return to baseline (7, 8, 14). Thus it is reasonable to speculate that the improved fatigue resistance with old age found only during the NF protocol occurred because that protocol induced less slowing of relaxation in the old men than the CF protocol. This was not the case: the fatigue-induced slowing of relaxation was not different between the CF and NF protocols, for either age group (Fig. 3). Alternatively, quadriceps fatigability indeed may be decreased with old age, but this condition was masked during the CF protocol because the quadriceps of the old men were stimulated at a greater percentage of their maximal tetanic force than that of the young men during each contraction.
The decreased fatigability with old age found during various voluntary and electrical stimulation fatigue tasks may result from an age-related atrophy or loss of type II fibers and the inherent fatigue resistance of the surviving type I fibers (3, 10, 22). Because Gonzalez and Delbono (12) found no age-related difference in fatigability of single isolated fibers from either the soleus or extensor digitorum longus muscle of mice, they suggested that, when a whole muscle is fatigued, it is the shift in fiber-type composition of the muscle, and not intrinsic changes within the remaining fibers, that accounts for the altered fatigability with old age. As discussed previously, a slowing of whole muscle relaxation may be a functional consequence of this shift in fiber-type composition. It is worth noting that not all human limb muscles show the same degree of age-related slowing of relaxation (14, 26). Therefore, we suggest that normalized fatigue protocols be used to compare whole muscle fatigability with old age. In addition to studies on aging, fatigability comparisons of muscles that have experienced alterations in the relaxation properties, such as pathological conditions or following training interventions, may be improved by using a normalization procedure. As demonstrated in this study, differences in fatigability could be masked by an altered force-frequency relationship.
Recovery. Irrespective of the protocol, the 50-Hz force recovered faster in the old vs. the young men (Fig. 4). The recovery profile of the CFT force did not differ between groups after the CF protocol (Fig. 4A), whereas the f60 force was greater in the old compared with the young men throughout the recovery period following the NF protocol (Fig. 4B). The low-frequency force likely recovered faster in the old compared with the young men during the NF but not CF protocol because, unlike the CF protocol in which both groups lost the same force, the old men fatigued less during the NF protocol. In addition, it is possible that the prolonged slowing of relaxation that persisted in the old men following the NF protocol compared with the CF protocol (Fig. 5, B vs. A) was a factor in the age-related difference in recovery of the f60 force. In contrast to during the fatigue protocol per se, in which an exacerbated slowing could increase fatigability (see above), a prolonged slowing of relaxation during recovery could enhance the force output because there would be a greater degree of fusion of the force response at the given low frequency of stimulation.
Similar to the proposed mechanisms for the age-related slowing of relaxation, altered cross-bridge kinetics or calcium handling is thought to be the general mechanism responsible for fatigue-induced slowing of relaxation (29). However, Hill et al. (13) did not find a significant correlation between relaxation rates and the reduced rate of calcium uptake by the SR following intense fatiguing quadriceps exercise. Therefore, those authors suggested that the rate-limiting process during fatigue is due to a reduced rate of cross-bridge detachment (13). Irrespective of the mechanism(s), the relatively greater fatigue-induced slowing of relaxation in the old vs. the young men (Figs. 3 and 5), coupled with the prefatigue differences in relaxation (Table 1), likely confounded the fatigability comparison between young and old humans during electrical stimulation protocols.
In conclusion, the age-related leftward shift in the force-frequency relationship of the quadriceps contributed to an underestimation of the fatigue resistance with old age during the CF protocol. However, when the stimulation frequency used in the NF protocol was adjusted to account for the age-related shift in the force-frequency relationship, the quadriceps muscles of the old men were less fatigable than those of the young men. Thus we suggest that normalized fatigue protocols provide a more objective description of the changes in muscle fatigability with old age than do CF fatigue protocols.
|
ACKNOWLEDGMENTS |
|---|
|
TOPABSTRACTMETHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
|---|
This work was supported in part by the Canadian Institutes of Health Research and the Natural Sciences and Engineering Research Council of Canada.
|
FOOTNOTES |
|---|
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
|
REFERENCES |
|---|
|
TOPABSTRACTMETHODSRESULTSDISCUSSIONACKNOWLEDGMENTSREFERENCES |
|---|
This article has been cited by other articles:
|
|
 |
|
  L. H. Chung, D. M. Callahan, and J. A. Kent-Braun Age-related resistance to skeletal muscle fatigue is preserved during ischemia J Appl Physiol, November 1, 2007; 103(5): 1628 - 1635. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 I. R. Lanza, R. G. Larsen, and J. A. Kent-Braun Effects of old age on human skeletal muscle energetics during fatiguing contractions with and without blood flow J. Physiol., September 15, 2007; 583(3): 1093 - 1105. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 C. J. McNeil and C. L. Rice Fatigability Is Increased With Age During Velocity-Dependent Contractions of the Dorsiflexors J. Gerontol. A Biol. Sci. Med. Sci., June 1, 2007; 62(6): 624 - 629. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| Visit Other APS Journals Online |
) and old (
) men during the normalized frequency (NF) visit. For each subject, peak force at each stimulation frequency was normalized to the peak force of his 100-Hz tetanus. Values are means ± SE. The stimulation frequency extrapolated from 60% of the 100-Hz force was significantly lower in the old men (12.7 ± 0.5 Hz) compared with the young men (14.9 ± 0.4 Hz; P < 0.05). A greater percentage of the 100-Hz force was produced by the 5- to 30-Hz stimulation in the old vs. the young (*P < 0.05), resulting in a leftward shift in the force-frequency relationship with old age.
