lnspiratory Muscle Fatigue Following Running to ...

Physiology and Biochemistry

169

lnspiratory Muscle Fatigue Following Running to Volitional Fatigue: The Influence of Baseline Strength A. K. McConnell, M. P. Caine, C. R. Sharpe

A. K. McConnell, M.P. Caine and G. R. Sharpe, lnspiratory Muscle Fatigue Following Running to Volitional Fatigue: The Influence of Baseline Strength. Int. J. Sports Med.. Vol. 18. No. 3. pp. 169- 173.1997.

Accepted after revision: August 3,1996 Respiratoty muscle fatigue has been demonstrated following short-term exercise to volitional fatigue. as well as following prolonged submaximal exercise. There is some suggestion that the respiratory muscles of 'athletic' individuals have superior strength and greater fatigue resistance but it is not known whether inspiratoty muscle strength influences fatigueability of the inspiratory muscles. The present study examined this question in 24 moderately trained young men. lnspiratory muscle strength was measured a t residual volume using a hand held Mouth Pressure Meter before and after an incremental. multistage shuttle run to volitional fatigue. Following the run, there was a significant fall in inspiratory mouth pressures (- 10.5 &SO 8.2 %: p < 0.001 Pre- vs Post P,,,,,). The subjects with the weakest inspiratory muscles exhibited significantly greater fatigue than those with the strongest (- 17.0 f SD 7.8% c.f. 6.8 f SD 4.4% for the 25th and 75th percentiles respectively p < 0.01). These data support existing evidence that the respiratory muscles fatigue following high intensity exercise. In addition. they provide new evidence that this phenomenon occurs in moderately trained young men and that the severity of the fatigue is related t o the baseline strength of the inspiratory muscles. Key words: lnspiratory muscles, shuttle run. inspiratoty muscle fatigue

@

haustion (4.5). Both studies compared highly trained subjects with sedentary subjects. Neither Coast et al. (5) nor Choukroun et al. (4) noted any statistically significant difference i n the baseline (pre-exercise) inspiratory muscle strength of their trained and untrained groups. However, there was a tendency for the trained group i n Coast et al.'s ( 5 ) study to have a higher pre-exercise inspiratory muscle strength but, surprisingly, the 30% difference was not statistically significant. This observation hints at a we!l accepted phenomenon in other skeletal muscles, viz.. superior strength i s associated with fatigue resistance (19). The purpose of the present study was to explore two questions in a single group of moderately trained young men: 1. Does incremental shuttle running to volitional fatigue induce inspiratory muscle fatigue in this group? 2. Does baseline (pre-exercise) inspiratory muscle strength influence post-exercise inspiratory muscle strength? Materials and Methods

Subjects Twenty-four moderately-trained male volunteers took part i n this study. Some of their physical characteristics are shown i n Table 1.None had any history of cardiorespiratory disease. Informed, written consent and local ethics committee approval were obtained. Table 1 Some basic anthropometric and demographic data on the

subjects studied. VO,maxP is the maximal oxygen uptake predicted using the multi-stage shuttle run to volitional fatigue (18).

Introduction

Age (Y r)

Height (m)

Weight (kg)

V02maxP (ml . min-I . kg-')

23 2.8 18 29

1.83 0.1 1 1.66 2.05

93.3 15.8 72.1 126.0

53.8 5.2 40.8 64.2

Fatigue of the respiratory muscles has been demonstrated following both short term exercise to volitional fatigue (511) and following prolonged submaximal exercise such as marathon running (12).

Mean SD Min Max

Two previous studies have demonstrated that in both children and adults, inspiratory muscle fatigue occurs in untrained, but not trained individuals following short term exercise to ex-

Measurement of inspiratory muscle strength

Int.]. Sports Med. 18 (1997) 169-173 @ Georg Thieme Verlag Stuttgart . New York

Respiratory muscle force generation, or strength, was assessed indirectly from measurements of maximum static inspiratory pressure at the airway opening during occluded inspiratory ef-

Downloaded by: Thieme Verlagsgruppe. Copyrighted material.

School of Sport and Exercise Sciences. The University of Birmingham. Birmingham, United Kingdom

A K. McConnell. M. P. Caine. G. R. Shame

Int.l.S~ortsMed.18(1997)

haw

Pupeat

A absolute

A PI,,^

Pre Post (cm H20)

Pre Post (cm HIO)

8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

187 160 123 168 148 137 135 191 200 240 161 165 182 178 161 212 182 171 206 153 129 161 203 163

176 135 113 149 130 95 113 179 180 212 134 177 163 181 151 209 173 140 186 122 110 125 199 154

177 155 110 148 137 107 130 183 185 211 146 154 155 171 150 203 185 154 193 120 124 152 191 146

170 130 100 144 122 83 110 169 175 186 124 168 150 179 146 201 168 136 178 108 109 121 193 135

-11 -25 -10 -19 -18 -42 -22 -12 -20 -28 -27 12 -19 3 -10 -3 -9 -31 -20 -31 -19 -36 -4 -9

-7 -25 -10 -4 -15 -24 -20 -14 -10 -25 -22 14 -5 8 -4 -2 -17 -18 -15 -12 -15 -31 2 -11

Mean SD

171.5 28.3

154.4 33.1

157.8 28.8

146.0 32.5

-17.1 12.5

-11.7 10.8

Subject

1 2 3 4

5 6 7

A PI, (cm Hz01

forts. Measurements were made using a hand-held Mouth Pressure Meter (Precision Medical Ltd., Pickering, North Yorlcs. UK). The device incorporates a valve with a small leak, a ceramic solid state pressure transducer, a microprocessor and a digital pressure display. The microprocessor is programmed to and maximum pressure determine the peak pressure (P,,) for both inspiratory and exaveraged over one second )P(,I piratory efforts. For the purposes of the present study only inspirdtory pressure was measured and this was made at residual volume. The mouth pressure meter was used with a flanged mouthpiece (P.K. Morgan, UK). Values obtained using this device have been shown to be both reliable and accurate for normal subjects and patients with respiratory disease (10). We have extensive experience with this technique and it is highly reproducible in our hands. Procedure

lnspiratory muscle strength measurements were made before (baseline) and after a multi-stage incremental shuttle run to volitional fatigue which was conducted in a gymnasium. The shuttle test is designed to elicit exhaustion within 10- 15 min. Subjects run back and forth between markers placed 20m apart and are paced by an audible cue. The interval between cues decreases gradually and subjects must increase their speed to keep pace with the cue. The shuttle stage at which the subjects drop out of the test is used for the prediction of maximal oxygen uptake (VO,maxP)(18).

A%

,,P ,,I

PI,

(%)

- 5.9 - 15.6 -8.1 -11.3 -12.2 - 30.7 - 16.3 - 6.3 - 10.0 -11.7 -16.8 7.3 - 10.4 1.7 - 6.2 - 1.4 - 4.9 -18.1 - 9.7 - 20.3 -14.7 - 22.4 - 2.0 - 5.5

-3.6 - 16.1 -9.1 -2.7 -11.0 - 22.4 - 15.4 -7.7 - 5.4 - 11.9 -15.1 9.1 - 3.2 4.7 - 2.7 -1.0 - 9.2 -11.7 7.8 - 10.0 -12.1 - 20.4 1.1 - 7.5

-10.5 8.2

-8.0 7.5

Table 2 Pre- and post-exercise values for peak inspiratory pressure (P3,l and maximum inspiratory pressure averaged over 1 sec P (J,l. Aabsolute is the difference between pre- and post-exercisevaluesin cm H20;A % is the percentage change between pre- and postexercise (p < 0.001 pre- vs post,,,PI and .)P ,I

-

Measurements of mouth pressures were taken by a single investigator and subjects made a number of efforts (not normally more than 3) until they achieved two values that were within 5 % of one another; the highest value for, , IP and ,PI was recorded. Post-exercise measurements were made within 3 min of termination of the shuttle run to volitional fatigue. On a separate occasion 4 weeks later, 6 subjects repeated the test to evaluate the reproducibility of the measurements and the influence of the exercise test. Statistical analyses

Paired comparisons were made using a Student's t-test (twotailed). Simple linear regression analysis was used to determine the relationship between two variables, significance of the correlationship was assessed by ANOVA. All significance levels were set at 5 %. Results

There was no correlation between height, mass, body mass or. ,PI Thus. index, or VO,maxP and either pre-exercise,,,P, those subjects with the highest inspiratory pressures were not necessarily the largest, or most aerobically fit. Both,,,PI and ,P,I declined significantly following shuttle running to volitional fatigue (Table 2:Figs.1 and 2). There was a statistically significant difference in the extent of the fatigue


170

Int. 1. Sports Med. 18 (1997) ,

lnspiratory Muscle Fatigue Following Running tovolitional Fatigue Fig. 1 Relationship between Plpeak measured pre- and post-exercise to volitional fatigue (n 24). The line represents the line of identity.

Fig. 2 Relationship between P,I measured pre- and post-exercise to volitional fatigue (n = 24). The line represents the line of identity.

220 200 180 -

-

60 O

O

O

O

O

O

O

O

O

C

' O m S 2 3 4 2 a M : Pre-exercise Ppeak (c-0)

,,I for the 6 subjects who Table 4 Test-retest values for Pupelk and P were tested on two separate occasions (Pre and Post refer to measurements made pre- and post-exercise to volitional fatigue). A = difference between the two tests. -

Plperk

Test 1 Test 2 A

-

plm

Pre (cm H20)

Post (cm H,O)

Pre (cm H,O)

156.6f 33.8 156.6k24.6 0.0k18.2

140.4f 31.5 ?38.2*24.5 - 2 . 2 f 10.6

143.6 k31.8 146.2f26.6 2.6k16.0

Post

(cmH,O) 123.2 k34.9 129.0f 25.1 5.8f 12.6

Fig.3 Relationship between preexercise PIwk and the percentae (A )P ,,I following exercise to volitional fatigue (n 24). change in,,P ,, The line represents the line of best fit (r = 0.399; p < 0.054).

Fig.4 Relationship between pre-exercise P ,,I and the percentage change in ,P ,I (A ),P ,, following exercise to volitional fatigue (n = 24). The line represents the line of best fit (r = 0.404; p < 0.0502).

exhibited by the 25th and 75th percentiles (the 6 weakest and strongest members of the group, respectively); P,,I was significantly higher in the 75th percentile compared to the 25th ,, was significantly smaller for the 75th percentile, whilst AP percentile (Table 3). In addition, there was a negative correlation between the size of the fall in mouth pressure and AP,) and the pre-exercise ,,P,, and P ,,I which fell just outside the 5 % significance level (p < 0.054 and p < 0.0502, Figs. 3 and 4. respectively).

In the six subjects for whom the test was repeated after an interval of 4 weeks, there was no significant difference between either the pre-exercise or post-exercise measurements of either ,,P,I or.,P ,I In addition, the magnitude of the fall in both ,,P,I and ,,PI on the second test was not significantly different from that observed in the first test (Table 4).


,I and P,I and the percentage Table 3 Pre-exercise values for ,,P change in Pipeak and ,PI for the 25th and 75th percentile members of the group ('p < 0.05; * 'p < 0.01 ; ' "p < 0.001; each percentile group contains 6 subjects).

Int. I. Sports Med. 18 (1997) Discussion These moderately-trained young men displayed a highly significant reduction in their ability to generate inspiratory pressure following an incremental shuttle run to volitional fatigue. A decline in the force generating capacity ofa muscle, or group of muscles is generally accepted to illustrate the presence of fatigue (9). These data therefore lend further support to the notion that the inspiratory muscles exhibit fatigue following strenuous exercise (3,11,12) and argue against the doctrine that the respiratory system is somewhat unique in not displaying symmorphosis. since fatigue would not occur if the muscle group were working well within its capability (19). Perhaps more striking than the presence of post-shuttle run inspiratory muscle fatigue was the finding that the subjects with the strongest inspiratory muscles displayed the least fatigue (Table 3).Since baseline strength was not a function of body size, its potentially confounding influence can be excluded. Thus, these data suggest that strong inspiratory muscles afford some protection against inspiratory muscle fatigue. The most liltely explanation for this is that greater absolute strength leads to a smaller relative demand for force generation during exercise. The absence ofthe usual relationship between morphologyand skeletal muscle strength begs an important question; why do some subjects have stronger inspiratory muscles than others? We have previously demonstrated a very strong influence of habitual physical activity upon respiratory muscle strength in healthy elderly men and women (6). But the question of whether whole body training improves inspiratory muscle strength and fatigue resistance in young adults remains open. Coast and colleagues (5) demonstrated that maximum inspiratory pressure was reduced following an incremental cycle test to exhaustion in sedentary subjects but not in elite crosscountry skiers. They concluded that the skiers had fatigue resistant inspiratory muscles and that the respiratory muscles adapt to training in a similar way to the locomotor muscles. Similarly, Martin and Stager (14) compared the ventilatory endurance of athletes and non-athletes, finding that the athletes had greater fatigue resistance. However, neither of these studies provided sufficient evidence of an association between athletic training and fatigue resistant respiratory muscles, since neither could exclude the influence of a common genetic predisposition for 'athleticism' and strong respiratory muscles. In a subsequent study Martin and Chen (15)explored this question in athletic and non-athletic siblings, their data demonstrated that the fatigue resistance was a product of physical training and not of genetic predisposition. We observed no correlation between aerobjc fitness, as predicted using the shuttle run (VO,maxP),and either pre-exercise Plpeak or Plavc.This suggests that either aerobic training does not influence inspiratory muscle strength, or that the relationship is not a straightforward one. We favour the latter explanation for a number of reasons. Firstly, our observations in elderly men and women (6) suggest that the respiratory muscles become trained and detrained in response to their use or disuse. respectively. Although aerobic fitness reflects training status it may not reflect respiratory muscle usage in a predictable way. Secondly, if inspiratory muscle fatigue contributes to the decision to terminate exercise, then the shuttle stage (and thus

A. K. McConnell, M. P. Caine, C. R. Shorpe

VO,maxP)becomes both a dependent as well as an independent variable. Thirdly, our index of inspiratory muscle function is 'one dimensional' in that training alters both the strength and the biochemical properties of a muscle. We currently have no means of assessing the latter easily and this is likely to be much more sensitive to training than isometric strength. Finally, our study population was quite homogeneous, with a range of VO,maxP of 41 to 64 ml. min-' . kg-', a larger range of training status may yet yield a positive relationship between respiratory muscle strength and training status. An important question raised by the data relates to the functional relevance of inspiratory muscle fatigue. Limitation to performance is not implicit in fatigue. The concept that the respiratory system might limit exercise in healthy human beings is not widely held and most researchers believe that the respiratory system is 'over-built' relative to the rest of the oxygen transport system, i.e., it is not symmorphotic (7).However. evidence is accruing in favour of some form of respiratory limitation to human performance and comes from two sources. Firstly, from experiments in which the respiratory muscles have been fatigued deliberately prior to exercise (16.13); secondly, from experiments in which the respiratory muscles have been trained (1,2,8,17). Martin et al. (16) induced respiratory muscle fatigue prior to a short tenn maximal running test to exhaustion. Respiratory muscle fatigue was induced by a sustained maximal isocapnic hyperpnoea. Time to exhaustion was reduced from 7.6min to 6.5 min and subjects ceased work at signifiantly lower ventilation, heart rate and oxygen uptake after ventilatory work. More recently, Mador and Acevedo (13) used a similar rationale but induced inspiratory muscle fatigue by breathing against a load equivalent to 80%of the subjects' baseline inspiratory muscle strength. Following inspiratory muscle fatigue. endurance time at 90 % W0,max was reduced. Thus, prior fatigue of the respiratory muscles can limit short term high intensity exercise. Its influence upon more prolonged exercise remains untested. Four studies have examined the influence of respiratory muscle training (RMT) upon endurance time, all have used voluntary hyperpnoea as the training stimulus. At first sight, the data from these studies appear contradictory. being split evenly in favour and against a beneficial influence of RMT. The studies that have demonstrated no influence of RMT have both used tests of'endurance' that have utilised intensities of exercise which induce volitional fatigue in less than 10 minutes (8.17). In contrast. the two studies using endurance testsat intensities eliciting fatigue in excess of 20 minutes duration have shown improvements in cycle endurance time in both sedentary and athletic subjects following RMT (1. 2). Thus, the influence of RMT may be confined to exercise lasting more than 10 minutes; implicit within these observations is the recognition that any influence of inspiratory muscle fatigue upon performance may also be confined to exercise lasting more than 10 minutes. In summary, the data from the present study lend further support to existing observations that inspiratory muscle fatigue occurs following strenuous exercise to volitional fatigue. In addition, the study has demonstrated that this phenomenon occurs in moderately trained young men and that there is a relationship between inspiratory muscle strength and the fatigueability of these muscles. In the light of previous studies


1k

Int. 1. Sports Med. 18 (1997)

Inspiratow Muscle Fatique Followinq Runninq to Volitional Fatique

References Boutellier U., Buchel R., Kunder A., Spengler C.: The respiratory system as an exercise limiting factor in normal trained subjects. Eurj Appl Physiol 65: 347 -353. 1992. 2 Boutellier U.. Piwko P.: The respiratory system as an exercise limiting factor in normal sedentary subjects. EurJ Appl Physiol 64: 145 - 152, 1992. Bye P. T. P., Esau S. A., Walley K R., Mecklem P. T., Pardy R 1.: Ventilatory muscles during exercise in air and oxygen in normal men. J Appl Physiol 56: 464-471. 1984. Choukroun M. 1.. Kays C., Gioux M.. Techoueyres P.. Guenard H.: Respiratory muscle function in trained and untrained adolescents during short-term high intensity exercise. Eur J Appl Physiol 67: 14-19. 1993. Coast 1. R. Clifford P. S., Henrich T. W.. Stray-Gundersen J., Johnson J. R: Maximal inspiratow pressure following maximal exercise in trained and untrained subjects. Med Sci Spom Exerc 22: 811 -815, 1990. Copestake A. I., McConnell A. K: The influence of physical activity on maximum static respiratory pressures in healthy elderly human beings._l Physiol475P: 14, 1994. Dempsey J., Hanson P., Pegelow D., Claremont A., Rankin J.: Limitations to exercise capacity and endurance: pulmonary system. Can J Appl Sport Sci 7: 4- 13, 1982. a Fairbarn M. 5.. Coutts I