MILITARY MEDICINE, 173, 9:889, 2008
Changes in Maximum Muscle Strength and Rapid Muscle Force
Characteristics after Long-Term Special Support and
Reconnaissance Missions: A Preliminary Report
Captain Peter A. Christensen, Special Operations Forces*; Ole Jacobsen, MD†;
Jonas B. Thorlund, MSc‡; Thomas Madsen, MSc‡; Carsten Møller, WO1§; Claus Jensen, WO1§;
Charlotte Suetta, MD PhD¶; Per Aagaard, PhD¶
INTRODUCTION
Military Special Forces have been met with increasing demands of number and duration of missions during the recent
years. These missions can be long-term covert Special Support and Reconnaissance (SSR) missions with difficult infiltration/exfiltration circumstances, camouflage, and nutrition
and hydration logistics. All aspects of stealth, observation,
and communication techniques are among the many considerations that must be taken in mission planning. During these
observation missions, the elite soldiers can be immobilized
for as long as 10 to 12 days. Danish Army Special Operating
Forces (Jægerkorpset, SOF Denmark) have participated in
numerous SSR missions during Operation Enduring Freedom
and these type of missions are considered as one of their
specialities.
On one occasion, a patrol was operating in the Tora Bora
Mountains, Afghanistan, for a 10-day SSR mission in hostile
environmental conditions with restricted food and water rations. The Lay-up-Point (LUP) and Observation Post were
located in a high threat area with no space for movement,
which prohibited the soldiers from engaging in physical exercise to prevent deconditioning effects. Subjectively, it was
*Jaegerkorpset, DK, Toggerbovej 4, DK-8420 Knebel, Denmark.
†Langelinie Allé 29, DK-2100, Copenhagen E, Denmark.
‡Institute of Sports Science and Clinical Biomechanics, University of
Southern Denmark, Campusevej 55, DK-5230, Odense M, Denmark.
§Jaegerkorpset, Thisted Landevej 53, DK-9430 Vadum. Denmark.
¶Department of Integrative Biology, University of California, Berkeley,
3060 Valley Life Science Building, Berkeley, CA 94720-3140.
Address for reprints: Peter A. Christensen, Toggerbovej 4, DK-8420
Knebel, Denmark; e-mail: toggerbo@privat.dk.
This manuscript was received for review in October 2007. The revised
manuscript was accepted for publication in April 2008.
Reprint & Copyright © by Association of Military Surgeons of U.S., 2008.
MILITARY MEDICINE, Vol. 173, September 2008
apparent that their muscle strength and ability to perform
explosive muscle actions were severely compromised at the
end of the mission. The extent of these reductions could
potentially have presented operational problems in case of a
difficult and/or rapid exfiltration. It was apparent, when the
patrol was debriefed, that muscular weakening and weight
loss during SSR missions was not an uncommon phenomenon and numerous accounts of lower extremity injuries due to
reduced strength, speed, and balance during exfiltration were
given.
Decreases in skeletal muscle size and strength after muscular
inactivity have been reported after bed rest during illness,1,2
during limb immobilization,3,4 or when exposed to weightlessness during space travel.5,6 The antigravity muscles, especially of
the lower extremities, are most severely affected with reduction
in both muscle mass and strength and this reduction is already
prominent after only a few days of space flight.7
To the best of our knowledge, no previous study has been
conducted to examine the effect of immobilization during
dangerous and stressful military operations in highly trained
Special Forces soldiers. Specific knowledge on the effect of
this type of immobilization on this very specific population
could potentially give valuable information for future intervention studies to counteract the effect of immobilization. Therefore,
our intention was to study the effect of a long-term covert SSR
mission with immobilization on muscle mass, strength, and
contraction dynamics after an 8-day simulated SSR patrol. This
study focused on changes in leg muscle function since reduced
leg muscle strength is likely to highly affect the ability of rapid
exfiltration/evacuation. The study was undertaken in Denmark
under controlled conditions to validate the method and study
design, which will later be used in interventional studies during
field conditions in future task force operations.
889
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ABSTRACT Purpose: The purpose of the present study was to examine the impact of 8 days of immobilization during
a Special Support and Reconnaissance mission (SSR) on muscle mass, contraction dynamics, maximum jump
height/power, and body composition. Methods: Unilateral maximal voluntary contraction, rate of force development,
and maximal jump height were tested to assess muscle strength/power along with whole-body impedance analysis before
and after SSR. Results: Body weight, fat-free mass, and total body water decreased (4 –5%) after SSR, along with
impairments in maximal jump height (⫺8%) and knee extensor maximal voluntary contraction (⫺10%). Furthermore,
rate of force development was severely affected (⫺15–30%). Conclusions: Eight days of immobilization during a covert
SSR mission by Special Forces soldiers led to substantial decrements in maximal muscle force and especially in rapid
muscle force capacity. This may negatively influence the ability for rapid exfiltration and redeployment, respectively.
Maximum Muscle Strength and Rapid Muscle Force after SSR Missions
Field Exercise
The 8-day exercise was conducted as a real-time simulated
SSR mission from a small wooded area against a farmhouse
where suspected arms trading was to take place. It was to be
covert with stealthy infiltration, a LUP with the Operative
Post no further than a few feet away. The farmhouse, 200
meters away, could easily be observed. The patrol could not
build semipermanent structures and relied on natural camouflage and equipment brought with them. All normal procedures as 3-hour rotation, noise discipline, and radio reporting
were kept. The soldiers were required to remain prone throughout the exercise, except during toilet activities where squatting
was allowed. To ensure normal hydration and nutrition, the
soldiers were allowed 3 L of water and the Norwegian meal,
ready to eat FR 3500 (DryTech AS; 3,564 kcal, protein, 84 g;
carbohydrates, 547 g; and fat, 116 g) a day.
The study was conducted during early spring with an average
day temperature between 8 and 14°C and 3 and 9°C at night.
A relative high degree of threat was maintained during the
8-day period with search patrols with dogs, helicopter
searches, and noticeable suspicious activity at the farmhouse.
At the termination of the exercise, the pickup was conducted
at the LUP; the soldiers were not to carry their equipment and
were driven, while prone, directly to the test center where the
post 1 test series was immediately carried out. Subsequently,
post 2 testing was conducted after 3 hours of rest and dinner.
Assement of Muscle Strength and Rapid
Force Capacity
Force Sampling
Maximal voluntary contraction strength (MVC) was measured
for the knee extensors of the left leg, as described earlier,8,9 using
890
FIGURE 1.
One subject in the portable dynamometer.
a custom-made transportable dynamometer (Fig. 1) where the
subjects were seated in an upright position with back support
and the knee and hip flexed at 90o. A steel cuff was strapped
around the lower leg, ⬃2 cm above the medial malleoli, and was
connected via a rigid steel bar to a strain gauge load cell, again
connected to a preamplifier and amplifier. The strain gauge
signal was sampled at 1000 Hz into a stationary computer. The
arms were crossed over the chest to avoid any influence on the
force measurement from the uninvolved leg and upper body.
After a few submaximal habituation trials, four maximum attempts were performed at a static knee joint angle of 90o.
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METHODS
The study was designed as a prospective observational study
and included five healthy male elite soldiers (age, 32.2 ⫾ 7.7
years; height184.6 ⫾ 1.1 cm; body mass. 85.9 ⫾ 3.7 kg;
means ⫾ SEM) from the Danish National Guard, SSR Unit.
This group was included because the soldiers were all trained
in SSR techniques, were superb athletes, and were mentally
prepared for a SSR exercise. All subjects gave their informed
consent to the procedures of the study. The local ethics committee approved the study.
Baseline measurements (PRE) included assessment of body
weight, maximal contractile muscle strength, and maximal
jump height along with blood and urine tests. Furthermore,
fat mass, fat-free mass, total body water content, and approximation of segmental muscle mass were estimated by bioimpedance methods. All PRE tests were conducted during the
day prior to planned mission infiltration, which was initiated
at midnight the same day. At the completion of the mission,
the subjects were tested immediately after arrival at home
base (post 1) and again after 3 hours (relative to arrival at
home base) that involved rehydration, nourishment, and unrestricted ambulatory activity (sitting, standing, walking) (post 2).
Maximum Muscle Strength and Rapid Muscle Force after SSR Missions
Maximum Voluntary Contraction
Rate of Force Development (RFD)
Contractile RFD was determined from the MVC trial. RFD
was derived as the average slope of the moment-time curve
(⌬moment/⌬time) in time intervals of 0 to 30, 0 to 50, 0 to
100, and 0 to 200 ms relative to the onset of contraction.8,9
Onset of contraction was defined as the instant when the knee
extensor exceeded baseline ⬎7.5 Nm. In addition peak, RFD
was recorded as the maximal (peak) slope of the ascending
part of the force-time curve over two consecutive data points.
Blood and Urine Sampling
Countermovement Jumping (CMJ)
10
After the dynamometer tests, the subjects performed CMJ to
assess the ability of the muscles to generate power to the
body’s center of gravity. Maximal jump height is mainly
determined by the power generated by the knee extensor
muscles during the concentric push-off phase. After a dynamic warm-up period consisting of 2 to 3 submaximal
vertical jumps, the subjects performed three maximal CMJs
with 30 seconds pause on a mobile digitest jumping mat
(Newtest, Oulu, Finland).
From an upright standing position with both hands on the
hip, the subjects performed the CMJ by rapidly moving
downward (knee and hip flexion combined with dorsiflexion
of the ankle), immediately followed by a fast upward movement. The subjects were verbally encouraged to jump as high
as possible. The maximal vertical jump height was determined from the flight time (H ⫽ 1⁄8 䡠 g 䡠 t2, g ⫽ 9.81m/s2, in
which t ⫽ flight time off the ground, including ascending and
descending flight phase).
Assessment of Body Composition
Bioimpedance and the segmental methodology are increasingly used for the assessment of total body water and its
components.11 A mobile Tanita Body Composition Analyzer
(Tanita BC 418 MA; Tanita Corp., Tokyo, Japan) was used to
measure body composition using a constant current source
with a high frequency current (50 kHz, 500 mA). A total of
eight electrodes was positioned so that electric current was
applied to the electrodes on the tips of the toes of both feet
MILITARY MEDICINE, Vol. 173, September 2008
To evaluate hydration, measurements of hematocrit, SE-sodium, SE-potassium, SE-creatinine, SE-albumin, SE-creatine
kinase, and urine osmolarity (digital refractometer, UG-alpha;
ATAGO, Tokyo, Japan) were performed at baseline (PRE), 1
hour (post 1), and 3 hours (post 2) after the exercise.
Statistical Analysis
Data are given as group mean values ⫾ SEM. All PRE to
post 1 and post 2 exercise changes were evaluated with a
Wilcoxon signed-rank test for paired samples (two tailed,
0.05 level of significance). All data were analyzed using
StatView 5.0 (SAS Institute, Cary, North Carolina).
RESULTS
Body Composition
All subjects underwent bioimpedance and the obtained data
points in the direction of a general body weight loss of ⬃4%.
Thus, body weight decreased by 3.2 ⫾ 0.97 kg (SEM) from
85.9 ⫾ 3.66 kg (SEM) to 82.7 ⫾ 3.69 kg (SEM) (p ⬍ 0.05).
The post 1 weight loss was due to decreases both in fat-free
mass (FFM) (⫺5.0%, p ⬍ 0.05) and total body water (TBW)
(⫺5.2%, p ⬍ 0.05; see Table I).
Total muscle mass was reduced with 5.1% or 2.9 kg (p ⬍
0.05) at post 1. Specifically, a 6% or 1.5-kg decrease in
muscle mass was observed for the lower extremities (p ⬍
0.05; Table I).
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The subjects were carefully instructed to contract the knee
extensors (quadriceps femoris muscle) “as fast and hard as
possible.” Each maximal voluntary isometric contraction of
the quadriceps muscle was sustained for ⬃3 seconds with a
rest period of 60 seconds in between. Strong verbal encouragement and online visual feedback of the strain gauge signal
was given. Trials with initial countermovement were discarded and an extra trial was performed. This was only
observed on one occasion in which the subject was asked to
refrain from the countermovement. The force signal was
multiplied by the lever arm length to obtain knee joint torque.
MVC was defined as the highest peak torque value of the four
attempts.
and the fingertips of both hands, respectively, and voltage
was measured at the heels of both feet and at the thenar side
of both hands.
In deriving the body fat percentage, fat mass, and fat-free
mass for both the whole body and specific body parts (right
arm, left arm, right leg, left leg, trunk), the Body Composition
Analyzer uses data acquired by dual-energy X-ray absorptiometry as well as a regression formula derived through repeated regression analysis using variables as height, weight,
age, and impedance between right hand and foot and between
individual body parts, respectively. A high degree of correlation has been found to exist between body fat percentage,
fat mass, and fat-free mass for individual parts and for the
entire body calculated with this predictive formula and the
figures obtained by dual-energy X-ray absorptiometry, respectively, and the results are thus highly reproducible.11
The subjects were to stand still on the platform, unclothed,
after having voided their bladder. The measurements were
performed three times in a row and average approximation of
body fat percentage, fat-free mass, fat mass, and predicted
muscle mass for specific body parts were noted. The post 1
and post 2 bioimpedance tests were performed before completion of the strength and jump tests, approximately at the
same time of day as the baseline (PRE) measurements.
Maximum Muscle Strength and Rapid Muscle Force after SSR Missions
TABLE I.
PRE
Post 1
Bioimpedance Measurements before (PRE) and Immediately after (post 1) Immobilization
TBW (kg)
FFM (kg)
M (kg)
MMLS (kg)
MMLL (kg)
MMRL (kg)
54.55 ⫾ 2.05
51.80 ⫾ 1.70a
74.51 ⫾ 2.80
70.65 ⫾ 2.33a
56.75 ⫾ 3.07
53.82 ⫾ 2.39a
23.77 ⫾ 0.64
22.37 ⫾ 0.54a
11.85 ⫾ 0.30
11.17 ⫾ 0.25a
11.93 ⫾ 0.39
11.20 ⫾ 0.29a
No data are presented for post 2 since these data only reflected the short-term recovery effect induced by intake of a meal and 3 hours of active rest.
FFM, Fat-free mass; M, total muscle mass; MMLS, muscle mass both legs; MMLL, muscle mass left leg; MMRL, muscle mass right leg.
a
Significantly different (p ⬍ 0.05) from PRE values. Values are expressed as means ⫾ SEM, n ⫽ 5.
Jump Height (CMJ)
$
Jump Height (cm)
30
25
5
200
50
0
0
PRE
POST 1
PRE
POST 2
FIGURE 2. Maximal jump height before (PRE, open bar), immediately
after (post 1, coarse-lined bar), and 3 hours after (post 2, fine-lined bar)
immobilization. ⴱⴱ, Significantly different from PRE values (p ⬍ 0.01). $,
Significantly different from post 1 values (p ⬍ 0.05). Values are expressed as
means ⫾ SEM, n ⫽ 5.
MVC and RFD
MVC decreased by 9.2% (p ⬍ 0.05) from PRE to post 1 as
a result of the 8-day immobilization period. The drop in MVC
tended to remain present after 3 hours of recovery (post 2;
⫺10.0%, p ⫽ 0.07; Fig. 3).
RFD was severely compromised following 8 days of immobilization (Fig. 4). Thus, peak RFD decreased by ⫺21.9%
(p ⬍ 0.05) at post 1, while fixed-time RFD was reduced in the
time intervals of 0 to 50 (⫺26.4%, p ⬍ 0.05), 100 (⫺19.4%,
p ⬍ 0.05), and 200 ms (⫺17.6%, p ⬍ 0.05). Furthermore, at
post 2, contractile RFD remained reduced in the later phase of
rising muscle force (time interval, 0 –200 ms; ⫺19.8%, p ⬍
0.05). In addition, a strong statistical tendency toward decreased explosive muscle force characteristics were observed
in almost all other time intervals examined: at post 1 at the
time interval of 0 to 30 ms (⫺28.0%, p ⫽ 0.07); assessment,
at post 2 at peak RFD (⫺27.9%, p ⫽ 0.07); and post 2 at time
intervals 0 to 50 ms (⫺29.8%, p ⫽ 0.07) and 0 to 100 ms
(⫺23.1%, p ⫽ 0.07) (Fig. 4).
Rate of Force Development (Nm/s)
Maximal Jumping Height
Maximal jump height decreased by 8.2% (p ⬍ 0.01) from
PRE to post 1. There seemed to be a short-term recovery in
jump height since jump height improved 5.3% from post 1 to
post 2 (p ⬍ 0.05; Fig. 2).
POST 1
POST 2
FIGURE 3. MVC before (PRE, open bar), immediately after (post 1,
coarse-lined bar), and 3 hours after (post 2, fine-lined bar) immobilization. ⴱ,
Significantly different from PRE values (p ⬍ 0.05). Values are expressed as
means ⫾ SEM, n ⫽ 5.
3000
892
(p=0.07)
*
250
Quadriceps RFD
PRE
POST 1
2500
POST 2
2000
*
1500
*
*
**
1000
500
0
Peak
0-30
0-50
0-100
0-200
ms
FIGURE 4. RFD before (PRE, open bars), immediately after (post 1,
coarse-lined bars) and 3 hours after (post 2, fine-lined bars) immobilization.
ⴱ, Significantly different from PRE values (p ⬍ 0.05). Values are expressed
as means ⫾ SEM, n ⫽ 5.
DISCUSSION
The subjects in the present study suffered a body weight loss
of 4% on average during the SSR mission that was accompanied by a corresponding loss of fat-free mass. There was
a significant reduction of 1.7 kg in total body water, with a
significant reduction of predicted total muscle mass with a
corresponding muscle mass loss of 1.4 kg in the lower extremities. These changes occurred after only 8 days of pronecovert SSR mission with no possibility of muscle exercise.
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**
35
Quadriceps MVC
300
Maximal Voluntary Contraction (Nm)
40
Maximum Muscle Strength and Rapid Muscle Force after SSR Missions
MILITARY MEDICINE, Vol. 173, September 2008
MVC, and RFD should be determined in future studies using
techniques like muscle biopsy sampling, twitch interpolation,
electromyography, etc.
The present study design also takes into consideration that
participants in SSR missions could be relatively dehydrated
and moderately malnourished because of ration logistics.
They have to carry their own rations of food and water into
the area of operation. In our study, we ensured sufficient food
and water by placing a ration cache at the LUP. We conducted blood and urine sampling with refractometry to evaluate hydration status and no significant pre- to postdifferences
were observed during the simulated SSR mission, thus indicating that hydration status was not compromised in the
present subjects, as might be the case during SSR missions in
hot environments.
CONCLUSIONS
We conclude that the weight loss, muscle atrophy, and reduction of muscle contraction dynamics seen in bed rest
studies and after space flight also appears to be found after
long-term covert SSR missions. Our preliminary data suggest
that such changes are apparent after only 8 days of SSR
mission. Consequently, appropriate countermeasures should
be taken into consideration when preparing for missions and
redeploying Special Forces even in the case of short-term
(8-day) SSR deployments. The present study was designed to
examine potential problems in retraining mechanical muscle
function and muscle mass with SSR missions and to validate
relevant assessment methods for future countermeasure studies in the field. In future studies, we intend to validate these
preliminary data in large-scale settings and also to study the
effect of various potential countermeasures such as employment of resistance training programs designed for cramped
spaces, electrical muscle stimulation,19 and nutritional amino
acid supplements,20 both which have shown promise in minimizing or preventing muscle atrophy in detraining studies.
Future studies will be conducted both in Denmark and on
international missions with regard to long-term covert SSR
and long-range mobility missions, and it is our hope that we
will be able to engage with other coalition units with similar
SSR interests. The present study methods are highly transportable, the research team will be able to travel abroad, and
our countermeasure studies could be of potential interest to
other Special Forces SSR units.
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