257
Hibernating Bears Conserve Muscle Strength and Maintain
Fatigue Resistance
T. D. Lohuis1,*
H. J. Harlow2
T. D. I. Beck3
P. A. Iaizzo4 Introduction
1Alaska Department of Fish and Game, Soldotna, Alaska
99669; 2Department of Zoology and Physiology, University of The black bear (Ursus americanus) spends 4–7 mo each winter
Wyoming, Laramie, Wyoming 82071; 3Colorado Division of hibernating in its den (Beck 1991). Bears do not arouse during
Wildlife, 317 West Prospect Road, Fort Collins, Colorado the winter to eliminate metabolic waste or eat and drink during
80536; 4Department of Anesthesiology, University of hibernation (Nelson et al. 1973, 1983; Harlow et al. 2004), but
Minnesota, Minneapolis, Minnesota 55455 they are capable of initiating a profound locomotor response
to a threat or disturbance even while in a state of mild hy-
Accepted 1/25/2007; Electronically Published 2/9/2007 pothermia. Indeed, bears demonstrate unimpaired locomotor
function during the winter and are able to sustain walking and
running through heavy snow without noticeable strength loss
or fatigue (T. D. I. Beck, personal observation of several hun-
ABSTRACT dred denned bears). The apparent retention of locomotor func-
Black bears spend several months each winter confined to a tion is remarkable considering that the bear is confined to a
small space within their den without food or water. In non- small space with limited mobility, has no access to food or
hibernating mammals, these conditions typically result in severe water, and maintains a body temperature that is only a few
degrees below its activity range for an entire winter (Nelson et
muscle atrophy, causing a loss of strength and endurance. How-
al. 1973; Harlow et al. 2004). These observations prompted our
ever, an initial study indicated that bears appeared to conserve
studies on overwintering bears with a focus on comparison
strength while denning. We conducted an in vivo, nonsubjective
with traditional models of skeletal disuse atrophy and with
measurement of strength, resistance to fatigue, and contractile
studies on smaller vertebrate species that either hibernate or
properties on the tibialis anterior muscle of six hibernating
estivate.
bears during both early and late winter using a rigid leg brace
There is a large body of literature quantifying muscle disuse
and foot force plate. After 110 d of anorexia and confinement,
atrophy on four model systems: microgravity (Edgerton et al.
skeletal muscle strength loss in hibernating bears was about 1995; Lambertz et al. 2000; LeBlanc et al. 2000), bed rest (Dud-
one-half that in humans confined to bed rest. Bears lost 29% ley et al. 1989; LeBlanc et al. 1992; Berg et al. 1997; Widrick
of muscle strength over 110 d of denning without food, while et al. 1998; Alkner and Tesch 2004), limb immobilization by
humans on a balanced diet but confined to bed for 90 d have casting (Hortobagyi et al. 2000; Thom et al. 2001; Krawiec et
been reported to lose 54% of their strength. Additionally, mus- al. 2005) or by suspension (Berg et al. 1991; Ploutz-Snyder et
cle contractile properties, including contraction time, half- al. 1996), and denervation with spinalization or spinal isolation
relaxation time, half–maximum value time, peak rate of de- (Talmadge et al. 1995, 1999; Haddad et al. 2003). The response
velopment and decay, time to peak force development, and is similar for all models and is predominantly described in rats,
time to peak force decay did not change, indicating that no mice, and humans (reviewed in Baldwin and Haddad 2001;
small-scale alterations in whole-muscle function occurred over Adams et al. 2003).
the winter. This study further supports our previous findings Uniformly, all atrophy models show a profound decrease in
that black bears have a high resistance to atrophy despite being myofibrillar and sarcoplasmic protein (Larsson et al. 1996; Berg
subjected to long-term anorexia and limited mobility. et al. 1997; Gamrin et al. 1998; Lecker et al. 1999; reviewed in
Lecker and Goldberg 2002) and whole-muscle cross-sectional
* Corresponding author. Address for correspondence: Alaska Department of Fish area and mass (LeBlanc et al. 1992; Narici et al. 1997; Krawiec
and Game, Kenai Moose Research Center, 43961 Kalifornsky Beach Road, Suite et al. 2005; reviewed in Adams et al. 2003) as well as fiber size
B, Soldotna, Alaska 99669; e-mail: thomas_lohuis@fishgame.state.ak.us.
(Edgerton et al. 1995; Hortobagyi et al. 2000; Rittweger et al.
Physiological and Biochemical Zoology 80(3):257–269. 2007. 
 2007 by The 2005). Atrophy models are also characterized by a loss of skel-
University of Chicago. All rights reserved. 1522-2152/2007/8003-5202$15.00 etal muscle strength (Dudley et al.1989; Hortobagyi et al. 2000;
258 T. D. Lohuis, H. J. Harlow, T. D. Beck, and P. A. Iaizzo
Roy et al. 2002; reviewed in Adams et al. 2003) and reduced or are protected from such losses. Estivating amphibians have
aerobic capacity and resistance to fatigue (McDonald et al. 1992; also been found to exhibit protein sparing and maintenance of
Zhan and Sieck 1992). In addition, under some conditions, the muscle integrity during winter (Hudson and Franklin 2002a,
time course of in vivo muscle contractile properties may be 2002b; Layne and Rice 2003; Hudson et al. 2006; West et al.
altered as a result of changed myosin heavy chain (MHC) iso- 2006), which may be due in part to cellular mechanisms in
forms (reviewed in Baldwin and Haddad 2001; Caiozzo 2002) concert with very low body temperatures (Hudson and Franklin
or changed sarcoplasmic reticulum calcium kinetics (Schulte et 2002a).
al. 1993) in response to unloading. In contrast to hibernating bears, which exhibit mild hypo-
Skeletal muscle of adult mammals is polymorphic. Muscle thermia of only 2–5C below normal and do not arouse unless
fibers generally referred to as slow-twitch oxidative predomi- disturbed (Nelson et al. 1973; Harlow et al. 2004), small-
nantly contain the slow type I MHC isoforms, while fast-twitch mammal hibernators decrease their body temperature 30–35C
glycolytic fibers contain greater, yet variable, amounts of the below normothermia but arouse every 1–3 wk to urinate, def-
faster IIa, IIx, or IIb MHC isoforms (reviewed in Caiozzo 2002). ecate, and even drink water, depending on the species (Harlow
The relative expression of these MHC isoforms in the muscle and Menkens 1985; Barnes 1989; Boyer and Barnes 1999). Glu-
is subject to resculpturing as a result of atrophy (reviewed in coneogenic demands associated with repeated arousal from
Baldwin and Haddad 2001). In most animal models, skeletal multiple torpor bouts necessitate protein catabolism (Burling-
muscle atrophy affects predominantly slow-twitch oxidative ton and Klain 1967; Galster and Morrison 1975), which may
rather than fast-twitch glycolytic fibers, though human models account for measurable losses of muscle protein in some small-
do not appear to show the same characteristic response (Thom- mammal hibernators (Yacoe 1983; Steffen et al. 1991; Wickler
ason and Booth 1990; Fitts et al. 2000, 2001; Haddad et al. et al. 1991) but not in others (Cotton 2005). However, while
2003; Alkner and Tesch 2004). This atrophy is associated with repeated arousal bouts may require protein catabolism, it is
a loss of type I MHC slow isoform composition of skeletal believed that they may also be important to facilitate the main-
muscle (reviewed in Baldwin and Haddad 2001). Changes in tenance of skeletal muscle integrity as a result of vigorous shiv-
MHC composition have complex effects on muscle function ering episodes (Rourke et al. 2004a, 2004b). Also unlike small-
(reviewed in Caiozzo 2002), but some researchers report that mammal hibernators, bears appear to exhibit nitrogen balance
atrophy models, including unloading, spaceflight, immobili- throughout the winter (Koebel et al. 1991; Barboza et al. 1997;
zation, limb suspension, and denervation, may result in partial Tinker et al. 1998; Lohuis et al. 2007) and may exhibit subtle
or complete conversion of slow to fast fibers or slow to fast muscular activity that does not result in complete arousal (Har-
MHC transformations (Desplanches et al. 1987; Andersen et low et al. 2004) and therefore would not require substantial
al. 1999; reviewed in Baldwin and Haddad 2001). Because the protein catabolism. The combination of relatively unaltered
isoform composition of a muscle influences the force-velocity muscle structure (Tinker et al. 1998), nitrogen balance (Barboza
relationship and therefore the amount and time course of power et al. 1997; Lohuis et al. 2007), and the lack of the repeated
output from a muscle (reviewed in Caiozzo 2002; Adams et al. arousals experienced by small-mammal hibernators (Harlow et
2003), it follows that atrophy can result in altered whole-muscle al. 2004) may result in greater strength retention by bears than
contractile parameters (Caiozzo et al. 1998; Haddad et al. 2003). expressed by traditional disuse atrophy models and similar or
These include contraction time, half-relaxation time, and as- greater strength retention by bears than by small-mammal
sociated measurements that evaluate the time course of muscle deep-hibernating species exposed to prolonged periods of re-
movement. Indeed, alterations in some of these parameters duced activity and food intake. Indeed, our initial study (Har-
have been shown to accompany functional decrements as a low et al. 2001) demonstrated a lower percentage loss of skeletal
result of unloading or immobilization even in cases where large- muscle force by overwintering bears than predicted by human
scale atrophy or complete fiber type conversion may not be or other animal models of muscle atrophy.
observed (Larsson et al. 1996; Hortobagyi et al. 2000; reviewed The objectives of this study are to measure muscle strength,
in Caiozzo 2002). Thus, changes in contractile properties could contractile characteristics, and resistance to fatigue nonsubjec-
serve as further indicators of compromised whole-muscle tively in vivo on naturally denning black bears during early and
function. late winter. We address three questions: (1) How do muscle
The skeletal muscle of hibernating mammals may not express strength and associated contractile characteristics change be-
many of the above effects. Previous studies have suggested that tween early and late hibernation? (2) Do skeletal muscles of
hibernating small mammals, such as prairie dogs (Harlow and bears maintain fatigue resistance, indicative of slow oxidative
Menkens 1985; Cotton 2005), hamsters (Wickler et al. 1987), fiber retention, between early and late hibernation? (3) How
ground squirrels (Steffen et al. 1991; Wickler et al. 1991; Reid does the bear compare to traditional disuse atrophy models
et al. 1995; Rourke et al. 2004a, 2004b), and bats (Yacoe 1983), and to small vertebrate species engaged in deep hypothermia?
either suffer limited losses of muscle size, protein, strength, and Data from this study will help us to understand the extent of
muscle fiber integrity compared to atrophy in nonhibernators strength retention in hibernating bears and to draw a closer
Hibernating Bears Preserve Muscle Function 259
association between fiber morphology, contractile properties, sition, with the foot firmly secured against a platform and a
and fatigue resistance of an animal exposed to prolonged pe- Wheatstone bridge force transducer (Fig. 1). Internal motion that
riods of dramatically reduced activity and food intake while in could dampen the signal was eliminated by application of pres-
a state of mild torpor. sure to the top of the knee. The knee, leg, and foot were then
secured to the apparatus with 2.5-cm nylon straps. Supramaximal
Material and Methods electrical stimulation of the common peroneal nerve caused dor-
siflexion of the tibialis anterior, resulting in force reading from
Bear Capture and Handling
the foot platform. The system was controlled by a Compaq Ar-
Six adult (five female, one male) bears were captured in Middle mada 1575DM laptop computer with the Labview 5.0 program
Park, Colorado (4005
N, 10559
W; 2,580–3,550 m elevation) used for stimulus generation and data acquisition. Labview 5.0
during the summer in 1# 1# 2-m woven metal box traps calculated force output according to the following equation: force
with a spring door activated by a foot treadle. Bears were an- in newton-metersp (5.211# volts)/0.17. This equation was de-
esthetized with tiletamine-zolezapam (Telazol; 7.0 mg/kg) ad- rived by using a series of known masses hung from a lever arm
ministered with a 1-m spring-operated jab pole. They were then to depress the foot pedal, recording the voltage output, and then
placed on a Therm-a-Rest pad, and their body temperature was generating a standard curve of voltage output as a function of
continuously monitored rectally with a digital thermometer. torque calculated for each mass at the known distance of the
Radio telemetry collars (ATS, Isanti, MN) transmitting in the lever arm. Electrical stimulation and signal amplification were
150–152-MHz range were placed around the neck of each provided by custom-built hardware (University of Wyoming Di-
animal. vision of Research Support, machine and electrical shops). A
Bears were located in their winter dens using aerial and constant current stimulus (range 100–300 V) was delivered by
ground telemetry and tested within approximately 2 wk of the a pair of 8.5-mm-diameter silver disk electrodes placed 3 cm
initiation of their hibernation season (median sampling apart (Nicolet Biomedical, Madison, WI; Fig. 1). Power was pro-
date p November 28). Four of these bears were also tested vided by two 12-V deep-cycle gel-cell batteries with a 12-V DC-
again in late winter before emergence from the den (median AC power converter.
sampling date p March 15). The mean interval between den An area of approximately 6 cm# 6 cm was shaved over the
visits was 110 d. Of the original six bears sampled in early fibular head, allowing electrical stimulation of the common
winter, two left their dens and could not be located. Thus, we peroneal nerve, and the bear’s lower hindlimb was placed in
substituted two bears (one male, one female) that had not been the brace. Estimated supramaximal electrical stimuli were de-
sampled but were of similar age and body mass as the original livered, and strength output was monitored while the ankle
two lost bears. These bears responded to all tested parameters joint was rotated in the brace in order to locate the optimal
similarly to the four sequentially sampled animals and were length-tension relationship of the fibers in the tibialis anterior.
included as a part of the late-winter group. The optimal length-tension position is achieved when the foot
Bears were anesthetized in their den during early and late is positioned so that actin and myosin filaments in the muscle
winter using the same protocol described for summer animals. maximally overlap and provide the greatest possible force pro-
After removal from the den, bears were placed on a Therm-a- duction. The aluminum leg brace allowed rotation of the foot
Rest pad, covered with a blanket to prevent abnormal hypo- around the ankle joint to stretch the tibialis anterior and was
thermia, and fitted with a leg brace and force plate for muscle marked in 10 increments to allow consistent, repeatable po-
strength and fatigue assessment as described below. sitioning. We established the optimal length-tension relation-
After sampling, bears were placed back into their den, and ship by first increasing stimulus intensity until the measured
the entrance was covered with pine boughs and snow to en- force production no longer increased, ensuring that a supra-
courage them to remain in their original den sites. All handling maximal stimulus had been reached. At no time was the max-
and experimental procedures were approved by the Colorado imum possible stimulus voltage required to elicit a supramax-
Division of Wildlife and University of Wyoming animal care imal stimulus. Typically, as defined above, a supramaximal
and use committees. stimulus was attained at between 60% and 80% of the maxi-
mum possible stimulus voltage. Then, the ankle joint was ro-
tated in 10 increments to determine which position generated
In Vivo Strength and Fatigue Assessment
the strongest contraction, representing the optimal length-
The method for assessing strength and fatigue of an intact muscle tension position. After the tibialis anterior was positioned, stim-
of a bear used in this study is an adaptation of a system used ulus intensity was again increased to ensure activation of all
for the evaluation of neuromuscular disease progression in hu- fibers in the muscle. The ankle was then returned to a neutral
mans (Brass et al. 1996; Schulte-Mattler et al. 2003). The ap- position, and the stimulus intensity was lowered. This process
paratus consisted of an aluminum leg brace that held the lower was repeated in triplicate before data collection to ensure re-
hindlimb of the anesthetized bear in a defined, repeatable po- peatable, nonsubjective determination of optimal length and
260 T. D. Lohuis, H. J. Harlow, T. D. Beck, and P. A. Iaizzo
Figure 1. In vivo strength and fatigue assessment apparatus. The aluminum leg brace (A) is designed to provide consistent, repeatable positioning
of the lower hindlimb. Strength and fatigue are measured by the foot plate and Wheatstone bridge strain gauge (B). Single-pulse stimuli for
strength assessment and trains of stimuli at 0.2, 0.5, 1.0, 3.0, and 5.0 Hz for fatigue assessment are delivered by a custom-built stimulator (C).
Stimulus generation and data collection are controlled by a Compaq laptop computer (D). Power is supplied by a 12-V deep-cycle gel-cell
battery (E). Figure reprinted with permission from Harlow et al. (2001).
stimulus intensity. Both the stimulus intensity and ankle po- three such contractions were compared between early- and late-
sition necessary to generate a supramaximal stimulus were re- winter sampling seasons.
corded in all bears tested. In all animals that were repeatedly Core body temperature was continuously monitored during
tested, optimal ankle positions were identical between early and testing procedures with an Atkins model 396K thermocouple
late hibernation. thermometer (Gainesville, FL), with a lubricated 0.5-cm blunt
Muscle force, in addition to six other parameters describing probe inserted 7 cm into the rectum. Tibialis anterior tem-
muscle function and fiber composition, were calculated and perature was measured with the same device but using a sterile
recorded by the Labview program each time a contraction was 18-ga needle probe inserted 2 cm into the muscle at the con-
generated. Parameters were defined according to Brass et al. clusion of the strength testing procedure. Mean late-winter core
(1996) and included contraction time (the interval between the body and extremity temperatures were both 1.3C lower com-
onset of force and the time of peak force), half-relaxation time pared with early-winter temperatures, while extremity temper-
(the time from peak force development to the time at which atures averaged 2.4C colder than core temperatures in both
force decays to one-half of peak force), rate of peak force de- early and late winter. Force generated by mammalian muscle
velopment and rate of peak decay (the maximum rate of force has been shown to decrease with temperature (Buller et al. 1984;
development and decay), time to peak decay (the time interval Iaizzo and Poppele 1990). Therefore, at the conclusion of late-
between the peak rate of development and the peak rate of winter strength and fatigue measurements, we applied chemical
decay), and the half–maximum value time (the time when the heat packs to the tibialis muscle and recorded muscle force
generated force is maintained at a level of one-half or greater values in response to the same test stimuli used to assess
of the peak force). strength at progressively increasing muscle temperatures rang-
Supramaximal stimuli were given 1 min apart with the ankle ing from 29.2 to 34.6C. Strength was measured between four
secured to measure maximum muscle force. Mean values of and seven times at different temperatures on each bear tested.
Hibernating Bears Preserve Muscle Function 261
Using these data, we calculated a temperature coefficient (Q10) the Student’s t-test on six bears. All results are presented here
factor of 3.3, which was then used to standardize all force values as the mean (SE) of six bears.
and contractile properties to a stereotypical muscle temperature Fatigue indices were normalized so that the initial twitch
of 34C, the mean extremity temperature measured in the early- force in a train represented a maximal value expressed as 100%,
winter-hibernating bears. While Q10 coefficient correction fac- which then declined over time as a result of muscle fatigue.
tors for muscle contraction strength become nonlinear at ex- Data expressed as percentages were then arcsine transformed.
treme temperatures, we never measured a muscle temperature Data expressed as percentages tend to be distributed binomially
lower than 29 or warmer than 36C in these bears. Therefore, rather than normally. Arcsine transformation ensures normal
we assumed that within the range of temperatures we measured distribution (Zar 1999, p. 239). Differences were then tested
in these bears, Q10 correction factors were linear (De Ruiter with a paired t-test (four bears that were studied in both early
and De Haan 2000; Hilber et al. 2001; Wang and Kawai 2001) and late winter) and a Student’s t-test (entire cohort of six
Muscle fatigue was measured using a 15-min protocol pre- early-winter and six late-winter bears, including two substi-
viously employed on humans (Schulte-Mattler et al. 2003). The tutes). All results are again presented as the mean (SE) of
protocol alternated 1 min of stimulation beginning at 0.5 Hz six bears.
with 2 min of rest. Stimulus frequency was increased after every
2-min rest interval, so that there were five 1-min periods of
repeated supramaximal stimuli delivered at 0.5, 1.0, 2.0, 3.0,
Results
and 5.0 Hz. Choice of this stimulus protocol was due to ex-
perimental goals. In order to compare the response of denned
bears to decreased mobility and anorexia with the response of Skeletal Muscle Strength and Contractile Properties
human clinical patients, we needed to use a protocol similar
to one employed on humans. Stimulus rates greater than 5 Hz Muscle strength measured in vivo based on maximum twitch
are not well tolerated by awake human patients. In addition, force in the tibialis anterior of denning bears was 29% greater
stimulus frequencies greater than 5 Hz can generate tetanic (Fig. 2) in bears in early hibernation (24.08  4.4 newton-
potentiation (Schulte-Mattler et al. 2003). We did not observe meters) than in bears in late hibernation (17.11  2.33 newton-
potentiation at any stimulus frequency presented. meters; P p 0.04). Figure 3 compares single muscle twitches
Data were then used to calculate a fatigue index, the differ- from a representative bear (844) in both early (November) and
ence in force output between the mean of the first 10 and mean late (March) hibernation. Seven contractile parameters indic-
of the last 10 stimuli of a stimulus interval, expressed as a ative of muscle fiber type composition were also measured in
percentage (Haida et al. 1989; Zhan and Sieck 1992; Reid et the course of this study. Contraction time, half-relaxation time,
al. 1993; Schulte-Mattler et al. 2003). The fatigue index at each half–maximum value time, peak rates of development and de-
stimulus frequency was compared between early- and late- cay, and time to peak force development and decay were not
winter animals. significantly different (P 1 0.05) between early and late hiber-
nation (Table 1).
Statistical Analysis
Paired t-tests were performed using Sigmastat (Systat, Point
Richmond, CA) to assess significant differences in the mean
values of the measured parameter between seasons on the four
animals that were measured in both early and late denning (Zar
1999). Alpha levels were set at 0.05. Because two of the initial
six bears left the study area after early-winter sampling, two
additional bears of the same size and age class were located
and sampled in their dens during late-winter hibernation to
increase sample size. Additionally, Student’s t-tests were per-
formed using Sigmastat to assess significant differences in the
mean values of the measured parameter between the initial six Figure 2. Skeletal muscle strength, measured in vivo over a 110-d
(early-winter cohort) bears and the final six (late-winter cohort) interval of denning inactivity and anorexia. Vertical bars show the mean
to include the two substitutes (Zar 1999). Alpha levels were force output in newton-meters by the tibialis anterior of six bears in
response to a multiple-pulse stimulus to the peroneal nerve. Vertical
again set at 0.05. All of the significant differences we detected lines represent 1 SEM. Asterisk indicates significant (P ! 0.05) de-
between early- and late-winter muscle performance were iden- crease in strength between early-winter (left bar) and late-winter (right
tical (P ! 0.05) using either the paired t-test on four bears or bar) bears.
262 T. D. Lohuis, H. J. Harlow, T. D. Beck, and P. A. Iaizzo
for several months during winter but can still retain muscle
protein (Tinker et al. 1998) and display sustained activity if
disturbed, we measured skeletal muscle strength, fatigue resis-
tance, and in vivo contractile properties of intact muscle in
bears within their natural dens.
We reported in an earlier study (Harlow et al. 2001) that
black bears appeared to conserve skeletal muscle strength dur-
ing hibernation. In that article, however, we were unable to
provide information on muscle contractile properties, muscle
temperature, and endurance to increase understanding of mus-
cle performance. Here, we present results on those contractile
properties and muscle fatigue while adjusting measurements of
muscle strength output for temperature.
In Vivo Measurement of Skeletal Muscle Strength
Peak force production, adjusted for temperature, by bear tibialis
anterior muscle in this study decreased by 29% over about 110
d of confinement while without food or water. This value is
Figure 3. Representative muscle twitch recordings from the tibialis
anterior muscle. Upper trace was recorded during early denning (No- similar to that in our earlier study showing a loss of 23% in
vember); lower trace was recorded during late denning (March). force production by a different cohort of overwintering bears
(Harlow et al. 2001). For comparison, in humans confined to
bed rest for 90 d, peak force production, measured as the max-
Skeletal Muscle Fatigue imum voluntary contraction generated by plantar flexor mus-
Bears in late hibernation were more susceptible to fatigue than cles, decreased by 54% of pretrial levels, while peak force pro-
were bears in early hibernation. Stimulation at 0.5, 1.0, and 2.0 duction from knee extensors decreased by 51%–60% (Alkner
Hz did not cause measurable fatigue in tibialis anterior of either and Tesch 2004). These values are of similar magnitude to the
early- or late-winter-hibernating bears. For all bears tested, 3- findings of Koryak (1998), a loss of 45% by triceps surae.
Hz stimulation caused a decrease in force output by the tibialis Studies on immobilized humans show a similar magnitude of
anterior over 1 min of stimulation in late-winter-hibernating loss over a shorter time course. For example, Hortobagyi et al.
bears (initial force normalized to 100%, force after 1 min p (2000) report a 47% decline in strength after only 3 wk of knee
71.29%, P p 0.011) but not in early-winter-hibernating bears immobilization by casting. Spinal isolation results in a 33%
(initial force normalized to 100%, force after 1 min of 3-Hz drop in strength after 60 d (Roy et al. 2002). Spaceflight (Tesch
stimulation p 99.8%, P p 0.17). Stimulation at 5 Hz also et al. 2004) induces a 9%–11% decrease in maximum voluntary
caused a greater decrease in force output, and therefore greater force production by knee extensor muscles after an extremely
fatigue, by the tibialis anterior in late-winter-hibernating bears short (relative to the length of the current study) 17-d space-
(initial force normalized to 100%, force after 1 min p flight, despite exercise sessions throughout the duration of the
54.34%, P p 0.019) than in early-winter-hibernating bears (ini- flight. While the progression of atrophy and accompanying
tial force normalized to 100%, force after 1 min p 85.68%, strength loss is not linear over the time course of immobili-
P p 0.018; Fig. 4). Figure 5 is an example of evoked twitches zation or bed rest and appears to proceed at a greater rate at
at 2-, 3-, and 5-Hz stimulation from bear 804 in early (No- the onset of immobilization than later (Thomason and Booth
vember) and late (March) hibernation. Stimulation at 5 Hz 1990; LeBlanc et al. 1992; Haddad et al. 2003; Alkner and Tesch
produced a greater fatigue response than did stimulation at 3 2004), Tesch et al. (2004) report that this magnitude of strength
Hz, while stimulation at 2 Hz was not sufficient to induce loss (9%–11% in 17-d spaceflight) is similar to that experienced
fatigue in either early or late hibernation. by other human models, and it is therefore to be expected that
longer-duration spaceflight would have an effect similar to
Discussion those found in long-term bed rest studies such as those con-
ducted by Alkner and Tesch (2004) and Koryak (1998). Indeed,
Fasting (Knapik et al. 1987; Koerts-DeLang et al. 1998), un- reports of strength loss from several muscle groups during
weighting (Adams et al. 2003; Haddad et al. 2003), or im- longer-duration spaceflights extend up to 48% over a 90–180-
mobility (Hortobagyi et al. 2000; Ferretti et al. 2001) results in d period (Greenleaf et al. 1989; Zange et al. 1997; Lambertz et
compromised muscle function resulting from the loss of muscle al. 2000).
contractile protein. Because bears are confined and anorexic Further, the magnitude of strength loss by human experi-
Hibernating Bears Preserve Muscle Function 263
Table 1: Twitch parameters of tibialis anterior during early- and late-
denning seasons
Early Denning Late Denning
Contraction time (ms) 41.9 (1.39) 41.7 (1.9)
Half-relaxation time (ms) 74.4 (9.5) 95.0 (14.9)
Peak rate of development (Nm/ms) 84.77 (37.18) 84.14 (30.13)
Peak rate of decay (Nm/ms) 
47.83 (13.48) 
48.1 (11.24)
Time to peak development (ms) 46.7 (16.9) 44.7 (16.2)
Time to peak decay (ms) 28.1 (6.5) 25.8 (4.1)
Half–maximum value time (ms) 18.8 (8.4) 12.8 (7.9)
Note. Values are given as means with SEM in parentheses. Nm p newton-meter.
mental subjects may be greater than reported because of the estimation is similar to the measurements of protein loss in
subjectivity of the monitoring protocol. For example, strength gastrocnemius (loss of 4%–10% of muscle protein), biceps fe-
is generally assessed in humans by measuring force generated moris (10%), and extensor hallucis longus (10%) by overwin-
through voluntary contractions (Gogia et al. 1988; LeBlanc et tering bears reported by Tinker et al. (1998) and Koebel et al.
al. 1992; Berg et al. 1997) rather than force evoked by an es- (1991). These muscles represent both hindlimb flexors and ex-
tablished supramaximal stimulus at an optimal length-tension tensors and, in combination with the tibialis anterior (this
relationship as employed in this study. In addition, other factors study), provide evidence for limited protein loss in black bear
such as nutrition and energy intake can influence strength. For hindlimb muscles during hibernation.
example, fasting compounds the effect of immobility in humans In a companion study on biopsies from the vastus lateralis,
and has been shown to profoundly reduce strength expression a knee extensor muscle, no significant loss of protein or loss
of skeletal muscles. Specifically, Knapik et al. (1987) docu- of strength, measured in vitro (Lohuis 2002; Lohuis et al. 2007),
mented a 10% decrease in isokinetic strength of elbow flexors was observed over the same hibernation interval. It is well
after only 3.5 d of fasting. Black bears are completely fasted documented that protein loss, as a result of either limited mo-
during the winter, and even though they do not undergo bility or fasting, varies between muscle groups (Li and Goldberg
weightlessness, casting, or denervation, they do experience lim- 1976; Gogia et al. 1988; LeBlanc et al. 1992). In addition, pen-
ited mobility. Their strength loss during this time is markedly guins (Cherel et al. 1994), geese (LeMaho et al. 1981), bats
lower than that of animal models subjected to only a single (Yacoe 1983), and ground squirrels (Steffen et al. 1991; Wickler
perturbation, much less a combination of food deprivation and et al. 1991), all species that are adapted to endure long-term
immobility that induces muscle atrophy. fasting, appear to catabolize protein from certain skeletal mus-
Despite the bears’ ability to preserve skeletal muscle and cles while preserving protein, mass, and function in others. It
maintain muscle strength and function over 110 d of denning- is apparent that the hibernating black bear has developed a
imposed anorexia and inactivity, the protein sparing effect is similar strategy. In addition, penguins (Groscolas and Robin
not complete. Metabolic demands for gluconeogenic precursors 2001) and bats (Yacoe 1983) put on extra muscle mass before
and tricarboxylic acid cycle intermediates necessitate a mod- migration or fasting, which is later catabolized as a nutritional
erate amount of protein catabolism (Bintz et al. 1979; Yacoe reserve without apparent compromise of function. This skeletal
1983). However, Barboza et al. (1997) proposed that small muscle protein reserve may be augmented by the catabolism
losses of protein from several muscle groups would be sufficient of smooth muscle, organ tissues, or extracellular matrix such
to meet the denning bears’ metabolic demands for protein as collagen (Cherel et al. 1994; Hissa et al. 1998). By utilizing
without severely compromising muscle function. Both Koebel small amounts of protein from some specific skeletal muscles
et al. (1991) and Barboza et al. (1997) estimated that a total while completely conserving protein in others and at the same
loss of body protein of approximately 10% would occur during time utilizing alternate potentially labile protein reserves, the
winter dormancy. This supposition was confirmed by Harlow denned bear is able to meet metabolic demands without severely
et al. (2002), who reported losses of 7.6% of total body protein compromising strength during extended fasting and limited
in overwintering, nonreproductive bears. mobility.
Interestingly, if we use the estimation of Hortobagyi et al. This study strongly supports our claim that bears exhibit
(2000) that strength loss is approximately four times greater limited atrophy with marginal strength loss compared to stan-
than protein loss, the 29% reduction in strength reported here dard disuse models, but how do they compare to small hiber-
translates to a 7.25% loss of protein content in the tibialis nating and estivating animals? Overwintering golden-mantled
anterior over the 110 d of denning anorexia and inactivity. This ground squirrels (Spermophilus lateralis) exhibit a 15%–20%
264 T. D. Lohuis, H. J. Harlow, T. D. Beck, and P. A. Iaizzo
Figure 4. Muscle fatigue index measured in vivo. Vertical bars show percentage of strength remaining after 1 min of 3-Hz (light bars) and 5-
Hz (dark bars) stimulation during early (left bars) and late (right bars) denning. Different letters indicate values that are significantly different
(P ! 0.05) between early and late denning. Vertical lines represent 1 SEM.
drop in wet weight of the soleus and extensor digitorum longus body temperatures during cold winters (Hudson and Franklin
(EDL; Musacchia et al. 1989; Steffen et al. 1991, Wickler et al. 2002a).
1991), with a 20% reduction in muscle protein and 30% de-
crease in muscle fiber cross-sectional area (Steffen et al. 1991).
The white-tailed prairie dog (Cynomys leucurus) and black- Muscle Fatigue
tailed prairie dog (Cynomys ludovicianus) show a slightly lower
Equally important to mobility is the retention of protein within
loss in EDL muscle wet weight (10%–15%) and reduction in
the sarcoplasmic reticulum for calcium release and uptake
protein content (14%–17%) with no loss in cross-sectional area,
(Schulte et al. 1993; reviewed in Allen and Westerblad 2001).
culminating in an overall 22%–25% strength loss over the win-
In addition, loss of vascular smooth muscle and changes in
ter (Cotton 2005). In comparison, the hibernating black bear
myoglobin content and enzyme activity are also associated with
exhibited no loss in fiber cross-sectional area of the biceps
changes in muscle fiber function. We indirectly assessed these
femoris or gastrocnemius, no reduction in protein content of
changes by measuring not only contractile force but also muscle
the gastrocnemius or vastus lateralis, and a drop of only about fatigue and contractile kinetic properties.
10% in the protein content of the biceps femoris (Tinker et al. Muscle fatigue is a result of repeated use or stimulation of
1998; Lohuis et al. 2007). A similar conservation of muscle muscle (Westerblad et al. 2000) without sufficient recovery in-
protein and size in the tibialis anterior may account for its terval and is characterized by a decrease in force output over
marginal (23%–29%) strength loss in bears that is similar in time (reviewed in Allen and Westerblad 2001; Adams et al.
magnitude to that of small-mammal hibernators (Cotton 2003). Fatigue results from decreased oxygen delivery (Mc-
2005). Donald et al. 1992), limited ATP availability, or reduced calcium
It is noteworthy that estivating amphibians, such as the release from sarcoplasmic stores (Allen et al. 1995; Favero
green-striped burrowing frog (Cyclorana alaboguttata), show 1999). Muscles that show a reduced oxidative capacity or loss
no loss in wet weight of several hindlimb muscles or in con- of slow-oxidative fibers as a result of atrophy will fatigue more
tractile properties of the gastrocnemius (Hudson and Franklin rapidly (Booth 1977; Fell et al. 1985; Witzmann et al. 1992).
2002a, 2002b). In addition, the isovelocity power output of Black bears appeared to sustain only a moderate loss of fa-
aquatic Rana tempararia was unaltered during hibernation tigue resistance over the winter. At the end of 110 d of fasting
(West et al. 2006). However, ectotherms generally have a met- and confinement, bears still exhibited a profile of 29% and 44%
abolic rate five to seven times lower than that of mammals decrease in force with 1 min of 3-Hz and 5-Hz stimulus, re-
(Else and Hulbert 1985; Brand et al. 1991) and may also be spectively. This increased susceptibility to fatigue measured in
protected from muscle atrophy, in part because of very low late winter is not one that should severely limit locomotor
Hibernating Bears Preserve Muscle Function 265
Figure 5. Representative early- and late-denning fatigue profiles from ankle dorsiflexors of bear 804. Note that figure shows response only to
2-, 3-, and 5-Hz stimuli because 0.5-Hz and 1.0-Hz stimulation did not produce a measurable fatigue response. Timescale of horizontal axis
is adjusted accordingly. Spikes in force output every 10 s were muscle contractions generated by a single stimulus as part of the experimental
protocol. These contractions were designed to ensure consistency throughout and between the measurement periods.
performance. For example, measures of fatigue in the tibialis 2003). The MHC isoform composition of a muscle influences
anterior of healthy, fed, active human subjects tested with a contraction velocity. A greater number of fibers in a muscle
stimulus protocol identical to that used in this study exhibit expressing type II fast MHC isoforms as a result of unloading
approximately a 25% and 50% reduction in force production would potentially result in increased whole-muscle contraction
in response to 1 min of 3-Hz and 5-Hz stimulation, respectively velocities and a tendency for that muscle to exhibit more fast-
(Schulte-Mattler et al. 2003). After 110 d of fasting and limited glycolytic characteristics and less resistance to fatigue (Diffee
mobility imposed by denning, bears still exhibited a fatigue et al. 1991; reviewed in Caiozzo 2002). In addition, relaxation
profile similar to that of healthy, active, fed humans. kinetics are altered by unloading as sarcoplasmic reticulum cal-
cium uptake is increased, leading to faster relaxation times
(Schulte et al. 1993; Caiozzo et al. 1998). It is possible that
In Vivo Assessment of Muscle Contractile Properties changes in either of these parameters would result in a muscle
Mammalian skeletal muscle is a heterogeneous composition of that was functionally more fast-twitch with increased suscep-
slow-twitch type I and fast-twitch type II fibers with contractile tibility to fatigue. However, none of the seven whole-muscle
properties that differ in part because of their MHC expression. contractile properties measured in our study on bears—con-
Slow-twitch muscles contain mostly the MHC I slow isoform traction time, half–maximal value time, half-maximum dura-
and some MHC IIa, which is the slowest of the fast MHCs. tion, time to peak force development and decay, and rate of
Fast-twitch muscles contain more MHC IIx and MHC IIb fast force development and decay—were altered in response to an-
isoforms (reviewed in Baldwin and Haddad 2001; Caiozzo orexia and confinement imposed by 110 d of denning in bears
2002). Unloading or immobilization causes atrophy as well as (Table 1).
an increase in the proportion of fast relative to slow MHC We did not directly test MHC isoform expression in this
isoforms, measured at both the single-fiber and whole-muscle study of the tibialis anterior during early and late hibernation.
levels (Caiozzo et al. 1998; reviewed in Baldwin and Haddad However, in a study of overwintering bears in the same region,
2001; Caiozzo 2002). A notable type I to type II transition is there was either no reduction or an increase in the MHC I slow
evident in spaceflight (Caiozzo et al. 1996), and hindlimb sus- isoform expression in the biceps femoris and gastrocnemius
pension (Thomason et al. 1987; reviewed in Baldwin and Had- muscles (Rourke et al. 2006). This may be more typical than
dad 2001), with up to an 80% decrease in MHC I in slow- atypical of hibernators. Recent studies suggest that the mRNA
twitch muscles after 15 d of spinal isolation (Haddad et al. (Rourke et al. 2004a) and expression of the MHC I slow isoform
266 T. D. Lohuis, H. J. Harlow, T. D. Beck, and P. A. Iaizzo
was retained in hibernating ground squirrels (Rourke et al. than small-mammal hibernators. A distinct benefit would be
2004b) and even modestly increased in prairie dogs (Rourke achieved by the bears’ uncompromised capacity of protein and
et al. 2006). It is certainly noteworthy that muscle function at strength retention of locomotor muscles in responding to threat
the level of contractile properties was preserved in the tibialis or disturbance during winter dormancy or in foraging upon
anterior of bears in our study. These data are consistent with emergence from the den in the spring.
previous morphological studies (Tinker et al. 1998) that show
either no change or marginal alteration in skeletal muscle fiber
Acknowledgments
type ratios or MHC isoform expression (Rourke et al. 2006)
of specific muscles in hibernating bears. Therefore, the mod-
This research was supported by National Science Foundation
erate increase in susceptibility to fatigue observed in the tibialis
grant IBN-9808785 to H.J.H. We thank the Colorado Division
anterior of our bears during late hibernation could have been
of Wildlife and the Wyoming Game and Fish Department for
due to a combination of factors, such as loss of myofibrillar
their cooperation and logistical support during this study. This
and sarcoplasmic protein, impaired calcium release, decreased
project could not have been successful without technicians
ATP availability, or reduced vascularization and associated ox-
Mike Hooker, Craig Jamison, Joe Koloski, and Lyle Willmarth.
ygen delivery, in concert with an impaired oxidative capacity
We would also like to thank the many volunteers, especially
of this muscle. Any or all of these processes could result in
Kevin McDonough, Mark Murphy, John Perdue, and Todd
altered contractile properties, contraction strength, or resistance
Perdue, who, along with many others, provided excellent as-
to fatigue (reviewed in Allen and Westerblad 2001).
sistance with bear tracking and den locating, often in snowy,
subzero conditions. Many thanks to Gary Williams for his com-
Conclusions puter programming assistance and expertise.
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