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92:354-361, 2002.J Appl PhysiolJuha Oksa, Michel B. Ducharme and Hannu Rintamkifunction and fatigueCombined effect of repetitive work and cold on muscle

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Combined effect of repetitive work and coldon muscle function and fatigue

JUHA OKSA,1 MICHEL B. DUCHARME,2 AND HANNU RINTAMAKI1

1Oulu Regional Institute of Occupational Health, 90220 Oulu, Finland; and 2Defenceand Civil Institute of Environmental Medicine, Toronto, Ontario, Canada M3M 3B9

Received 10 May 2000; accepted in final form 27 August 2001

Oksa, Juha, Michel B. Ducharme, and Hannu Rinta-maki. Combined effect of repetitive work and cold on musclefunction and fatigue. J Appl Physiol 92: 354361, 2002.This study compared the effect of repetitive work in thermo-neutral and cold conditions on forearm muscle electromyo-gram (EMG) and fatigue. We hypothesize that cold andrepetitive work together cause higher EMG activity andfatigue than repetitive work only, thus creating a higher riskfor overuse injuries. Eight men performed six 20-min workbouts at 25C (W-25) and at 5C while exposed to systemic(C-5) and local cooling (LC-5). The work was wrist flexion-extension exercise at 10% maximal voluntary contraction.The EMG activity of the forearm flexors and extensors washigher during C-5 (31 and 30%, respectively) and LC-5 (25and 28%, respectively) than during W-25 (P 0.05). On thebasis of fatigue index (calculated from changes in maximalflexor force and flexor EMG activity), the fatigue in theforearm flexors at the end of W-25 was 15%. The correspond-ing values at the end of C-5 and LC-5 were 37% (P 0.05 inrelation to W-25) and 20%, respectively. Thus repetitive workin the cold causes higher EMG activity and fatigue thanrepetitive work in thermoneutral conditions.

neuromuscular performance; cooling; electromyogram

THE EFFECTS OF SUBNORMAL MUSCLE temperature and fa-tigue on human muscle function have been found to bevery much alike (6): both decrease maximal muscleforce and increase time to peak tension (TPT) andrelaxation time (RT) (5, 7). With subnormal muscletemperatures, more muscle fibers must be recruited tosuccessfully perform a given work output (14). Simi-larly, fatiguing muscle has to recruit more fibers tomaintain the required force level (18). The effect ofdecreased muscle temperature and fatigue may be seenas increased amplitude of surface electromyogram(EMG) (9, 27).

Literature reports that, during high-intensity dy-namic work, fatigue is developed earlier when muscletemperature is lowered (4). However, literature doesnot report how low-intensity repetitive work and coldtogether affect muscle function, EMG activity, andfatigue. Because overuse injuries and musculoskeletaldisorders are a worldwide problem, especially in indus-tries where repetitive work and cold are combined [e.g.,

food processing industry (23)], it is important to moreaccurately identify the risk that the combined effect ofcold and repetitive work may produce. If the combina-tion of low-intensity repetitive work and cold acceler-ates the onset of fatigue, as we hypothesize on the basisof previous results with high-intensity work (4), theymay increase the risk for overuse injuries and, in thelong run, induce musculoskeletal disorders (8).

The possible additive effects of cold and repetitive

work should be seen as increased amplitude of surfaceEMG. This could be caused by increased excitability ofthe motoneuron pool [reflected by increased amplitudeof short-latency (SL) and medium-latency (ML) compo-nents of stretch reflex, i.e., reflex regulation of forceproduction] and/or increased neural drive from thecentral nervous system [increased amplitude of long-latency (LL) component of stretch reflex, i.e., centralforce regulation]. The mechanisms underlying the ef-fects of cold on muscle function may be many; e.g., theeffect of cold air on human skin can increase the num-ber of motor units recruited (32), motor unit recruit-ment pattern may be altered (14), or increased antag-

onist activity due to cold has to be balanced withincreased agonist activity (2, 25).We hypothesize that, compared with the thermoneu-

tral condition, during local and systemic cold condi-tions, EMG activity in the forearm muscles is higherfor a given level of work. During local cooling, this isdue to increased reflex activity; however, during sys-temic cooling, increased neural drive from the centralnervous system will also be required.

Our specific questions are as follows: 1) In relation tothe thermoneutral condition, what is the level of EMGactivity and fatigue in the forearm muscles duringrepetitive work during exposure to systemic and localcooling? 2) Are the possible changes in forearm EMG

activity and fatigue associated with changes in periph-eral or central force regulation?

MATERIALS AND METHODS

Subjects. Eight healthy, nonsmoking men volunteered astest subjects for the study. Their (mean SD) age was 31 6 yr, mean body height was 176 6 cm, mean body mass was

Address for reprint requests and other correspondence: J. Oksa,Oulu Regional Institute of Occupational Health, Aapistie 1, FIN-90220, Oulu, Finland (E-mail: [email protected]).

The costs of publication of this article were defrayed in part by thepayment of page charges. The article must therefore be herebymarked advertisement in accordance with 18 U.S.C. Section 1734solely to indicate this fact.

J Appl Physiol92: 354361, 2002.

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72 9 kg, and mean skinfold thickness of biceps, triceps,subscapularis, and crista iliaca was 31.2 6.7 mm (meanbody fat 14 3%). All subjects were informed of all details ofthe experimental procedures and the associated risks anddiscomforts. After a medical examination, each subject gavewritten informed consent. The experimental protocol wasapproved by the Human Ethics Committee of the Defenceand Civil Institute of Environmental Medicine. The subjectswere asked to abstain from exhaustive exercise and the

consumption of caffeine and alcohol for 12 h before theexperimental sessions.

Thermal exposures and temperature measurements. Eachsubject was exposed once to 25C (thermoneutral control) andtwice to 5C. During each exposure, the subjects performedsix 20-min work bouts. In the control condition at 25C, thesubjects were dressed with a T-shirt, shorts, and joggingshoes. In this trial, the body and arms of the subjects weremaintained at thermoneutrality (W-25). The first exposure at5C aimed at having the whole body cooled to an average skintemperature 58C lower than in the W-25 condition, inaddition to subnormal muscle temperature for the forearm(systemic cooling, C-5). In this condition, the clothing was thesame as that worn by the subjects in W-25, with the additionof a two-piece sweat suit with the right-hand sleeve cut off.

The other exposure at 5C aimed at subnormal muscle andskin temperature on the right forearm, with the rest of thebody maintained at thermoneutrality (local cooling, LC-5). Inthis condition, the clothing was a two-piece sweat suit (as inC-5), Canadian Army insulated trousers, a winter jacket withthe right sleeve cut off, a pair of wool socks, a pair ofmukluks, a glove over the left hand, and a toque. Because theseverity of the cold exposure may affect the level of responsesas well as the physiological mechanism, which may accountfor the possible changes, these two different types of coldexposures were chosen. The subjects were exposed to thethree conditions in a random order. The intervening timebetween exposures was 4 days, and the experiments tookplace during late spring and early summer.

During the three conditions, rectal temperature (Tre, 15

cm depth) and muscle and skin temperatures from 14 differ-ent sites (forehead, chest, scapula, abdomen, lower back,upper arm, flexor and extensor side of the lower arm, hand,front thigh, back thigh, calf, shin, and foot; model 400, YellowSprings Instrument, Yellow Springs, OH) were continuouslyrecorded on a data acquisition unit (model HP 3497, HewlettPackard, Palo Alto, CA). Mean skin temperature (Tsk) wascalculated by weighing the local skin temperatures by repre-sentative areas (22). Muscle temperature was measured atthe wrist joint flexor (flexor carpi radialis muscle) 58 cmbelow the proximal head of the ulnar bone at the bulk of themuscle. The skin above the muscle was disinfected withiodine solution and then anesthetized with 2% lidocaine (2.0ml). A sterilized catheter (18 gauge) was introduced perpen-dicularly into the muscle at a depth of 1.5 cm. A sterilized

multicouple probe (0.9 mm OD) (11) was inserted through thecatheter into the muscle, and the catheter was carefullywithdrawn. The probe had thermocouple junctions at 5-mmintervals, and therefore muscle temperature was measuredfrom depths of 1.5, 1.0, and 0.5 cm. At 1.5 cm, the probe wasapproximately at midthickness of the muscle, and thereforethe temperature gradient of approximately half (superficial)of the muscle was measured. To prevent the multicoupleprobe from sliding out of the puncture site, the probe wastaped at the puncture site with Tegaderm tape.

Force measurement and exercise type. At the beginning ofthe first and at the end of each work bout, maximal voluntarycontraction (MVC) of the wrist flexors was measured while

the subject was seated with his hip and elbow angle adjustedat 90. The armrest of the seat supported the relaxed forearm(alongside the torso). The subject was holding a handle in hishand (the handle and the palm of the hand were in a verticalposition). Via a pulley, the metal wire from the handle wasattached to a strain gauge (BLH Electronics, Canton, OH)capable of measuring the force produced by the maximalflexion of the wrist. The strain gauge was fixed to the floorand connected to a printer (model TA 2000, Gould, Duluth,

MN). The forearm was fixed to the armrest of the chair, sothat only the motion of the wrist joint was allowed. Themaximal force level, TPT (the rise of force from resting levelto maximum value), and RT (restoring the force from maxi-mum to resting level) were analyzed from the MVC data. Adecrease in the MVC was considered a sign of muscle fatigue.

During each condition, the subjects performed 10% MVCwrist flexion-extension repetitive work in the same positionin which the MVC was measured. This workload was chosenbecause it is recommended that during dynamic work, whichlasts1 h, the load corresponding to 10% MVC should not beexceeded (16). The subjects performed six 20-min work boutsfor a total work period of 120 min in each condition. Betweeneach work bout, there was a period of1 min during whichthe MVC and reflex (see Stretch reflex) measurements were

performed. The proper load corresponding to 10% MVC wasapplied to the wrist joint with the same pulley system usedfor the MVC measurements. Starting with their wrist fullyextended, the subjects flexed their wrist every 3rd s to the fullfree range of joint motion and returned their hand to thestarting position. These dynamic concentric-eccentric con-tractions were paced with a light that flashed every 3rd s. Atthe beginning of the first and at the end of each work bout,EMG activity from the four forearm muscles was measuredfor 30 s.

Electromyography and fatigue index. To evaluate the levelof muscle activity during work, surface EMG activity (modelME3000P8, Mega Electronics, Kuopio, Finland) was mea-sured from the wrist flexors (flexor carpi radialis and flexordigitorum superficialis muscles) and wrist extensors (brachio-

radialis and extensor carpi radialis muscles). The EMG sig-nals from the skin above the working muscles were acquiredwith a sample rate of 2,000 Hz with the use of pregelledbipolar surface electrodes (M-OO-S, Medicotest, lstykke,Denmark). The electrodes were placed over the belly of themuscle, and the distance between recording contacts was 2cm. Ground electrodes were attached above inactive tissue.To ensure the constant location of the electrodes on the skinover the three conditions, their initial locations were care-fully marked on the skin with permanent waterproof draw-ing ink. The markings were clearly visible throughout theexperiments. The measured EMG signal was amplified 2,000times (preamplifier situated 6 cm from the measuring elec-trodes), and the signal band between 20 and 500 Hz wasfull-wave rectified and averaged with a 10-ms time constant.

To assess the frequency component of the EMG, the powerspectrum was estimated by moving fast Fourier transform(FFT window, 512 points). From the power spectra, meanpower frequency, median frequency, and zero crossing ratewere calculated to describe changes in the frequency compo-nent. The EMG data were analyzed separately for concentric(wrist flexion) and eccentric (wrist extension) muscle contrac-tion, these two phases being clearly distinct from each other(Fig. 1A). In RESULTS, EMG data from the two flexor and twoextensor muscles are presented as means.

The second variable used to define the level of fatigue wasthe fatigue index (FI), which consisted of changes in wristflexion MVC and the amplitude of surface EMG from wrist

355COLD AND MOTOR PERFORMANCE

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flexors during work. For each condition, FI was calculated asfollows

FI MVC0 min/EMG0 min/MVC120 min/EMG120 min

where MVC0 min is MVC at the beginning of the exposure (N),MVC120 min is MVC at the end of the exposure (N), EMG0 minis the average EMG of wrist flexors at the beginning ofexposure (V), and EMG120 min is the average EMG of wristflexors at the end of exposure (V) (24). When FI 1.0,fatigue is not apparent. The higher FI is above 1.0, the

greater is the fatigue. The fatigue in this study is expressedas a percentage. At W-25, the FI is the difference between 1.0(no fatigue) and the FI calculated from the results obtainedat W-25. The FIs calculated from the results obtained at C-5and LC-5 are compared with the FI obtained at W-25.

Stretch reflex. To evaluate whether the possible changes inmuscle activity (changes in amplitude of surface EMG) wereperipherally or centrally mediated, stretch reflex responsesfrom the forearm flexors were measured at the beginning ofthe first and at the end of each work bout.

A stretch reflex was evoked by inducing a sudden displace-ment of the wrist joint (a pull to the handle the subjects wereholding in their hand by a solenoid-driven motor; Gjardian

Electric, Chicago, IL) with the forearm supported in theposition described above. The pull was 1.6 cm, and the dis-tance was covered in 0.3 s. The reflex was elicited 10 timeswithin 30 s during each measurement session, with the last7 being analyzed. Every time the solenoid motor started itsdisplacement, a signal was simultaneously sent to the EMGdevice to mark the starting point of the pull for determina-tion of the latency of the SL, ML, and LL components of thereflex response. In addition to reflex latencies, the maximum

amplitudes (from baseline to peak, baseline being the activitybefore the stretch, see example in Fig. 1B) of SL, ML, and LLresponses were analyzed.

SL and ML components are thought to reflect changes inthe reflex regulation (peripheral) of force production. AnLL component is thought to travel through a supraspinalpathway and to reflect changes in the neural drive from thecentral nervous system (12, 21). An increase in the ampli-tude of the SL or ML component is considered to reflectincreased excitability of the motoneuron pool (3). Simi-larly, an increase in the amplitude of the LL component isthought to reflect an increased neural drive from the cen-tral nervous system (12, 21).

Forearm blood flow. At the beginning of the first, in themiddle of the fourth, and at the end of the last work bout,

forearm blood flow was measured using venous occlusionplethysmography (Totalab 400, Hokanson, Bellevue, WA)according to the method of Whitney (31).

Statistics. Analysis of variance with repeated measureswas used. When a significant F ratio was obtained, one-wayanalysis of variance with a Duncans post hoc test wasapplied, and significance was accepted at P 0.05. Theresults obtained from the two cold conditions were testedagainst the thermoneutral reference value (value at 0 minin the beginning of the 1st work bout at W-25) and againstthermoneutral data at the same time of exposure. Valuesare means SE.

RESULTS

Thermal responses. Tre

was only slightly affected bythe thermal exposures, the highest value being 37.2 0.1C at the beginning of W-25 and the lowest 36.8 0.2C (P 0.05) at the end of C-5. Despite a significantdecrement, rectal temperature during the three expo-sures were considered to be at thermoneutrality (20).Tsk was 33.2 0.2C during W-25 and did not changesignificantly over the duration of the exposure. Expo-sure to C-5 decreased Tsk from a starting value of32.2 0.2C to 30.3 0.3C (P 0.05) after the first20-min work period. Tsk continued to steadily decreaseand reached its lowest value of 27.8 0.3C at the endof C-5. This final value at C-5 is 5.4C lower thanduring W-25, and thus the aim of having the whole

body cooled to an average skin temperature 58 lower(than at W-25) was achieved. However, it is probablymore appropriate to compare the final C-5 value withthe initial C-5 value, and then the difference in Tsk is4.4C, which is slightly less than the aim (58C).During LC-5, the initial Tsk of 33.3 0.3C signifi-cantly decreased to 32.5 0.3C (P 0.05) after thefifth work bout at 100 min and to 32.2 0.3C by theend of LC-5. Tsk during LC-5 differed significantly fromthat during C-5, but not from that at W-25.

The local skin temperature (average during eachexposure) at the forearm flexor side was significantly

Fig. 1. A: electromyogram (EMG) activity of the forearm flexor(flexor digitorum superficialis muscle) during 1 flexion (concentric)-

extension (eccentric) contraction in 1 subject. B: short-latency (SL),medium-latency (ML), and long-latency (LL) reflex EMG responses[average EMG (aEMG)] of 1 subject.

356 COLD AND MOTOR PERFORMANCE



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different (P 0.05) between each exposure: 34.3 0.2,27.8 0.2, and 29.3 0.1C at W-25, C-5, and LC-5,respectively. At the extensor side, the correspondingvalues were 33.2 0.2, 25.6 0.5, and 25.0 0.4C.The values at C-5 and LC-5 were significantly lowerthan at W-25 (P 0.05) but did not differ from eachother.

Muscle temperature increased due to the work bouts

but remained 2.04.5C lower during C-5 and LC-5(Fig. 2). The mean temperature gradient between 1.5-and 0.5-cm depths was greatest (2.9 0.1C) at C-5.The respective gradients for W-25 and LC-5 were 0.8 0.2 and 2.3 0.1C, respectively. The average muscletemperatures at 1.5 cm were 35.3 0.8, 32.8 0.6, and32.6 0.8C during W-25, C-5, and LC-5, respectively.The corresponding values for average muscle temper-atures at 1.0 cm were 35.0 0.8, 31.8 0.5, and 31.9 0.8C. The average values at 0.5 cm were 34.5 0.8,30.0 0.6, and 30.3 0.8C. The average muscletemperatures at C-5 and LC-5 did not differ from eachother but were significantly (P 0.05) lower than atW-25.

MVC, TPT, and RT. The maximal isometric wristflexion force decreased significantly after the first 20-min work bout during C-5 and LC-5 in relation to thethermoneutral reference value (Table 1). The MVCvalues during C-5 and LC-5 did not differ significantlyfrom each other.

TPT varied between 150 and 240 ms and RT between120 and 230 ms, but no significant differences werefound between the conditions.

FI and electromyography. The FI was 1.15, 1.82, and1.44 at the end of W-25, C-5, and LC-5, respectively.Expressed as a percentage, repetitive work at W-25caused 15% fatigue of the forearm flexors (FI 1.15 is

related to the time point at the beginning of W-25;where there is no fatigue, FI 1.0). In relation to FI atW-25 (1.15), the FIs at C-5 and LC-5 were 37% (P 0.05) and 20% higher, respectively. The difference be-tween C-5 and LC-5 is not significant.

During the concentric contraction, the EMG activityof the forearm flexors and extensors was significantlyhigher (i.e., increased coactivation) during C-5 andLC-5 in relation to thermoneutral activity (Fig. 3).During the eccentric contraction, the EMG activity ofthe forearm flexors was significantly higher in C-5, andthe activity of the extensors was higher at the begin-ning of the first work bout in relation to thermoneutralactivity (Fig. 4). There were no significant differences

between C-5 and LC-5. No significant changes in thefrequency components of the EMG were observed.Stretch reflex. The latencies of SL, ML, and LL re-

sponses were slightly higher (not significant) duringC-5 and LC-5 than during W-25. The average SL val-ues were 31.6 0.5, 32.8 0.8, and 33.1 0.8 msduring W-25, C-5, and LC-5, respectively. The corre-sponding ML values were 65.7 0.6, 69.2 0.9, and68.1 1.3 ms. For average LL latencies, the valueswere 103.5 0.6, 106.0 1.1, and 106.4 1.3 ms.

The amplitude of the SL and ML responses washigher or had a tendency to be higher during C-5 and

LC-5 than during W-25. However, the amplitude of theLL response was higher or had a tendency to be higheronly during C-5 (Fig. 5). There were no significantdifferences between C-5 and LC-5. The average SLamplitudes were 149 8, 253 18, and 212 13 Vduring W-25, C-5, and LC-5, respectively. The corre-sponding values for the average ML amplitudes were418 15, 516 16, and 434 15 V and for averageLL 136 5, 188 8, and 138 7 V, respectively.

Fig. 2. Forearm muscle temperature (Tm) at depths of 1.5 cm (A), 1.0cm (B), and 0.5 cm (C). *Significant difference in relation to thermo-neutral reference value [0 min at 25C (W-25)]. #Significant differ-ence in relation to thermoneutral value at the same time of exposure.

Values are means SE; n 8. C-5, systemic cooling at 5C; LC-5,local cooling at 5C.




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Forearm blood flow. In relation to W-25, forearmblood flow during C-5 was significantly lower through-out the experiment. During LC-5, blood flow was sig-nificantly lower after 65 min of exposure (Table 2).There were no significant differences between C-5 andLC-5.

DISCUSSION

This study focused on comparing the force-generat-ing capability, EMG activity, and fatigue of the fore-

arm muscles during low-intensity repetitive work inthermoneutral and cold conditions. The main resultsindicate that, compared with repetitive work in thethermoneutral condition, repetitive work in the coldcauses enhanced muscle EMG activity, increased levelof coactivation of the agonist-antagonist muscle pairs,and enhanced fatigue of the forearm muscles.

During all exposures, the work done by the forearmmuscles increased the muscle temperature during thefirst 2040 min of work, to reach a plateau that wasmaintained until the end of each condition. For allmeasuring depths, the plateau level was significantlyhigher for W-25 than for C-5 and LC-5. It has been

suggested that if muscle temperature is below normal,the muscle perfusion for a given workload will bereduced (26). The present study supports this observa-tion, since the blood flow was significantly lower duringthe two cold conditions than during the thermoneutralcondition.

Changes in MVC force have conventionally beenused as an indicator of muscle fatigue (17). It wasobserved in the present study that by the end of W-25the MVC had decreased by 15%, while the same vari-able decreased by 26% and 17% for C-5 and LC-5,respectively. The other indicator of muscle fatigue, FI

Table 1. MVC of wrist flexion

Time ofExposure, min W-25 C-5 LC-5

0 31426 30926 3102820 28132 29329* 29326*40 28625 29027* 28125*60 28421 27628* 27324*80 26724 26423* 27223*

100 26624 24424* 26522*120 26825 22822* 25723*

Values are means SE in N; n 8. MVC, maximal voluntarycontraction; W-25, thermoneutrality (i.e., 25C); C-5, systemic cool-ing at 5C; LC-5, local cooling at 5C. *Significantly different(P 0.05) from W-25; significantly different (P 0.05) from W-25at the same time of exposure.

Fig. 3. EMG activity of forearm flexors (A) and extensors (B) duringconcentric contractions. *Significant difference in relation to thermo-neutral reference value (0 min at W-25). #Significant difference inrelation to thermoneutral value at the same time of exposure. Valuesare means SE; n 8.

Fig. 4. EMG activity of forearm flexors (A) and extensors (B) duringeccentric contractions. *Significant difference in relation to thermo-neutral reference value (0 min at W-25). #Significant difference inrelation to thermoneutral value at the same time of exposure. Valuesare means SE; n 8.




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(24) used in this study, showed changes similar toMVC: 37% and 20% higher (in relation to W-25) in C-5and LC-5, respectively. The larger change in FI than inMVC could be attributed to the fact that the calculationof FI also takes into account the changes in the EMGactivity of the working muscles. This is because in-creased EMG activity for a given work performancecan also be considered an indicator of muscle fatigue(13). Because FI combines the changes in MVC andEMG activity of the muscles during work, it may beconsidered a more true indicator of muscle fatigue.

However, regardless of which variable is used to indi-cate fatigue, the results show that the combination ofrepetitive work and cold is more harmful when theability of the muscles to function is considered. To-gether, they clearly induce a higher level of fatigue andaffect the force-generating capability and EMG activityto a greater degree than repetitive work only. If it isconsidered that the greater effort required to maintain

a given level of work is a risk factor for musculoskeletaldisorders, then these results clearly show that workand cold exposure together create a substantiallyhigher risk than either creates alone.

It is also important to note that, according to the FI,at the end of C-5 the amount of fatigue was more thantwice that during W-25. At the end of LC-5, the amountof fatigue was also much higher than at W-25, but theeffect of local cooling in increasing the amount offatigue was less than the effect of systemic cooling.This emphasizes the importance of keeping thewhole body in a thermoneutral state during work incold conditions.

An increase in the duration of TPT and RT after

muscle cooling has been related to slowed ATP hydro-lysis (15), slowed Ca2 release and uptake from thesarcoplasmic reticulum (19), and decreased Ca2 sen-sitivity of actomyosin (30). Because TPT and RTshowed no change in this study, it may be possible thatmuscle biochemistry was not substantially altered dur-ing repeated muscle contractions in the cold. However,it is also possible that the decrease in muscle temper-ature induced by exposures to C-5 and LC-5 simply wasnot large enough to produce measurable changes inTPT or RT or that the measuring system was notsensitive enough.

In comparison to the thermoneutral condition, both

cold conditions caused significantly higher levels ofEMG activity in the forearm flexor muscles, thus re-flecting significantly higher muscular effort. Whenonly the forearm is exposed to cold (while the rest of thebody was thermoneutral), the EMG activity in theforearm had a tendency to be lower than when thewhole body was exposed. It has been reported thatreducing muscle temperature can increase the EMGactivity of the working muscles (25, 28). However,because the muscle temperature was approximatelythe same between C-5 and LC-5, factors other thanmuscle temperature are responsible for the higherEMG activity during C-5 than during LC-5.

It has been shown that cooling of the skin could

increase the recruitment of motor units (32) and in-

Table 2. Forearm blood flow

5 min 65 min 115 min

W-25 9.90.8 10.11.6 10.01.1C-5 6.51.0* 5.20.7* 4.60.6*LC-5 7.50.8 8.20.9* 7.40.5*

Values are means SE in %/min; n 8. *Significantly different(P 0.05) from W-25; significantly different (P 0.05) from W-25at the same time of exposure.

Fig. 5. Peak to peak (PTP) amplitude of SL (A), ML (B), and LL (C)stretch reflex responses. *Significant difference in relation to ther-moneutral reference value (0 min at W-25). #Significant difference inrelation to thermoneutral value at the same time of exposure. Valuesare means SE; n 8.




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crease the excitability of the motoneuron pool (1). Inthe present study, the local skin temperatures in theforearm, in addition to Tsk, were lowest during C-5. Itcould be possible that the cutaneous afferent input tothe spinal cord was more intense during C-5 thanduring LC-5, causing an increased recruitment of mo-tor units (higher level of EMG), therefore being onemechanism causing the increased muscular effort.

This assumption is supported by the stretch reflexresults, where somewhat higher reflex activity wasdetected during C-5 than during LC-5, especially to-ward the end of the exposure. The same mechanismcould be responsible for the higher EMG activity ob-served at LC-5 than at W-25.

The EMG results very clearly indicate that, duringconcentric contractions, the level of coactivation (i.e.,increased EMG activity of flexor and extensor muscles)increases significantly during both cold conditions andremains elevated throughout the exposures. In thestudy of Rissanen et al. (29), a similar cooling-inducedincrease in the level coactivation of lower leg muscles

was observed at the beginning of exercise. However,along with increasing the temperature of the workingmuscles (due to work), the coactivation gradually dis-appeared. This was not the case in this study, possiblybecause the muscle temperature during the cold con-ditions did not rise to a high enough level. It may beconcluded that the increased efforts of the flexor mus-cles and the increased coactivation of the agonist-an-tagonist muscle pairs (induced by the increase in EMGactivity of the extensor muscles during concentric con-traction, the activity that has to be overcome by flexoractivity) are mechanisms causing the enhanced fatigueof the forearm observed during both cold conditions.

The increases in latency of SL, ML, and LL re-sponses have been related to slowed nerve and muscleconduction velocity (10). Slowed conduction velocitymay result in an increased temporal summation lead-ing to an increased EMG amplitude response of thefollowing muscle contraction. The results of this studyshow that there is no statistically significant differencein the latencies of different components during W-25,C-5, and LC-5 exposures. Therefore, it may be assumedthat slowed nerve and/or muscle conduction velocitywas not responsible for the increased EMG activityduring the cold exposures.

In the cooled condition, the amplitude of EMG may

be affected by many other factors: cooling may modifythe shape of the action potential, and cold skin andmuscle may act as a low-pass filter. In this study,during both cold conditions the muscle and skin tem-peratures remained stable after the first 20-min workbout and the work was the same throughout the expo-sures. Still, a rising tendency during C-5 and LC-5 canbe seen in the EMG activity of the forearm flexormuscles, especially during the concentric contraction.Because ambient temperature, muscle temperature,local flexor skin temperature, and work remain con-stant, it may be assumed that physiological, rather

than methodological, factors are causing the increasingtendency of EMG in the cold conditions.

The peak amplitudes of SL and ML were higher orhad a tendency to be higher during C-5 and LC-5 thanduring W-25. Because the constant stimulus (stretch)was able to induce an increased response in relation toW-25 (although not always statistically significant), itmay be considered that these changes reflect increased

excitability of the motoneuron pool and/or increasedsensitivity of the muscle spindles due to the cold expo-sure (3). It may be considered that, to meet the cold-induced increased effect of the repetitive work, theEMG activity of the working forearm muscles is in-creased via reflex pathways.

The amplitude of the LL component during LC-5remained at a thermoneutral level, indicating that theincreased effort required by the working muscles dur-ing that exposure was met just by increasing the reflexactivity of the forearm flexors. However, during C-5,the amplitude of LL was significantly higher by the endof the exposure. This indicates that, during C-5, in-creasing the reflex activity was not sufficient, but ad-ditional increased drive from the central nervous sys-tem was required to maintain the level of workrequired by the experiment in the cold.

The authors thank Drs. Gary Gray and Bill Bateman for assis-tance with insertion of the muscle temperature probes and RobertLimmer, Pierre Meunier, John Ulrichsen, and Allan Keefe (Defenceand Civil Institute of Environmental Medicine) for excellent techni-cal assistance.

This study was financially supported by the Finnish Work Envi-ronment Fund.

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