Changes in Maximum Muscle Strength and Rapid Muscle Force Characteristics after Long-Term Special Support and Reconnaissance Missions: A Preliminary Report

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 (Jaegerkorpset, 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 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 aifected 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 eifect 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.

METHODS

The study was designed as a prospective observational study and included five healthy male elite soldiers (age, 32.2 ± 7.7 years; height!84.6 ± 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 segmentai 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).

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 147deg;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 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 90°. 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 90°.

Maximum Voluntary Contraction

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 counter-movement 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.

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 (Amoment/Atime) in time intervals of O to 30, O to 50, O to 100, and O 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.

Countermovement Jumping (CMJ)

After the dynamometer tests, the subjects performed CMJ10 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, OuIu, 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 . g . t^sup 2^, g = 9.81m/s^sup 2^, in which t = flight time off the ground, including ascending and descending flight phase).

Assessment of Body Composition

Bioimpedance and the segmentai methodology are increasingly used for the assessment of total body water and its components. ' ' 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 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.

Blood and Urine Sampling

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).

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).

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 O 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 O 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).

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 prone-covert SSR mission with no possibility of muscle exercise.

In addition to the above indications of muscle wasting, we found that MVC of the knee extensors was significantly reduced by 9.5%. The reduction in MVC seemed to be consistent with the observed loss in muscle mass in the lower extremities. Notably, even more pronounced decrements (18-26%) in RFD were observed after the 8-day SSR mission, which was not restored for the late-phase RFD (0-200 ms) after short-term (3- hour) recovery, while strongly tending to remain suppressed also at the earlier time intervals (50-100 ms). The ability to quickly generate muscle force (i.e., RFD), which is highly needed during combat and exfiltration in rough terrain, obviously is of utmost importance. Reduction in the ability of very quick movements and balance is a survivability issue during rapid exfiltration.

Contractile RFD was reduced in most measured time intervals, i.e., O to 50, 100, and 200 ms. Andersen and Aagaard12 showed that RFD is increasingly related to MVC as the time from the onset of contraction increases and that RFD in the time interval from O to 200 ms is strongly correlated to MVC. Any reduction of MVC, in this study caused by prone positioning and operational stress muscle atrophy, will also reduce the ability for movements as jumping or sprinting, which are movements that rely on RFD.13 These findings of impaired rapid muscle strength capacity are consistent with the significant reduction in maximal CMJ height of 8.2% observed after the SSR mission.

Even quicker movements, such as kicking and balancing through rough terrain, rely on explosive muscle strength generated during even earlier time spans of the muscle contraction (i.e., 0-50 ms). These changes are thought to be caused by alterations in intrinsic muscle properties and neural drive, while less affected by changes in MVC.9,12

The present findings are consistent with previous studies that have used bed rest as a model of limb immobilization.14 These studies, and others (see below), have indicated that prolonged periods of skeletal muscle inactivity attributable to immobilization, bed rest, denervation, limb unloading, or microgravity inevitably result in significant muscle atrophy.15 The muscle atrophy is characterized by decreased muscle fiber cross-sectional area and protein content, reduced force, increased insulin resistance, as well as a slow-to-fast fibertype transition.16 Much of this muscle mass decrease is a result of atrophy of the lower limb muscles, especially of the knee extensors. Decreased muscle mass is accompanied by reduced muscle strength. Thus, Dudley et al.17 showed that 30 days of bed rest resulted in a 20% decrease in MVC of the knee extensors and that within 30 days after the end of bed rest strength recovers to within 92% of pre-bed rest levels. Apart from changes related to the potential decrease in muscle mass with immobilization, there seems to be evidence for decreased motor neuron excitability and an impaired ability to activate motor units during maximal contractions as an additional mechanism for the observed decrease in muscle strength.18 The actual causes for the decline in muscle mass, 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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