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Original Research

The Effects of Eccentric Exercise on Muscle Function and Proprioception of Individuals Being Overweight and Underweight

Paschalis, Vassilis1,2; Nikolaidis, Michalis G.1,3; Theodorou, Anastasios A.1,2; Deli, Chariklia K.1,2; Raso, Vagner4,5; Jamurtas, Athanasios Z.1,2; Giakas, Giannis1,2; Koutedakis, Yiannis1,2,6

Author Information
Journal of Strength and Conditioning Research: September 2013 - Volume 27 - Issue 9 - p 2542-2551
doi: 10.1519/JSC.0b013e31827fc9a6
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Abstract

Introduction

Being overweight or obese is associated with poor quality of life and several life-threatening diseases (3,41). Additionally, increased body weight has been correlated with decreased postural and balance stability (11,19). It has been established that overweight individuals are more sedentary (16), expend less energy in daily activities (38), and are more prone to eccentric exercise muscle damage than are lean individuals (30). On the other hand, being underweight is normally the result of insufficient energy consumption, a situation promoted by modern society. Epidemiological evidence suggests that the prevalence of underweight individuals might be comparable with that of obese individuals (5). Generally, undernutrition produces weight loss and decreases muscle mass, which could influence whole-body physiology (6).

Exercise is widely used by people as a means to control their weight and keep up their fitness level (17). Either being underweight, overweight, or lean, participation in unaccustomed and resistance training programs may lead to exercise-induced muscle damage (14,23). Some of the manifestations of eccentric exercise–induced muscle damage include increases in the concentration of muscle proteins in blood, loss of muscle force, and range of movement (ROM), and development of delayed-onset muscle soreness (DOMS) (12,21). More important to our discussion, muscle-damaging exercise disturbs position sense and joint reaction angle to release of arms (4) and legs (28,29) for up to 4 days after exercise cessation. It is a common experience that after participation in activities involving eccentric actions, such as downhill walking, people have difficulty in performing activities of daily living. Indeed, a study from our group has shown that gait biomechanics is disturbed the days after eccentric exercise (26).

The position sense task records position-matching errors of a human joint (40). The knee joint reaction angle test measures the angle that a limb needs to stop reacting to its release (free fall). During the knee joint reaction angle test, the muscles of the knee joint respond to a stimulus originating from the muscles themselves (28,29). The force mismatch task assesses the ability of humans to match a given target torque output (32). During the force mismatch assessment participants primarily use their sense of effort to estimate muscle force (32). Altogether, position sense, join reaction angle to release, and force mismatch are tasks assessing the ability of a person to perform delicate movements. Disturbances in these proprioception tasks may lead to injuries during either skilled exercise tasks or everyday life activities because of reduced motor control. To our knowledge, no study has compared the assessments of position sense, force matching, and knee joint reaction angle to release either at rest or after eccentric exercise among lean, overweight, and underweight populations. This is despite the fact that either being overweight (11,19) or underweight (6) has been reported to affect whole body and muscle physiology. Therefore, the purpose of this study was to examine the effect of being overweight or underweight on position sense, force matching, and knee joint reaction angle to release at rest and after muscle damaging eccentric exercise.

Methods

Experimental Approach to the Problem

To address the primary hypothesis presented herein, we selected healthy women with no experience in any form of structured resistance training. The participants were divided according to their body mass index (BMI) in lean, overweight, and underweight groups. The participants using their knee extensors of their dominant leg undertook an isokinetic eccentric exercise session using an isokinetic dynamometer (Cybex Norm, Ronkonkoma, NY, USA). Each participant was familiarized for 3 consecutive days before the beginning of the experimental procedures. Familiarization involved 8–10 isokinetic eccentric actions for all the participants at very low intensity not capable of inducing muscle dysfunction, and assessments for the evaluation of position sense, force matching error, and knee joint reaction angle. The exercise session consisted of 5 sets of 15 isokinetic eccentric maximal voluntary contractions (MVCs) in the seated position as previously described (27). The angular velocity during eccentric exercise was 60°·s−1 (knee range, 0–90°), and a 2-minute rest interval was incorporated between sets. Previous investigations had confirmed that the exercise session used in this investigation could cause muscle damage (28,30). During the exercise session, the subjects were verbally encouraged to maximally activate their knee extensors, even though because of fatigue, the performance was declined as the exercise progressed. Because muscle damage indices are altered the days post unaccustomed exercise (28,31), muscle damage indices and proprioception indices (i.e., position sense, reaction angle to release, and force matching) were assessed pre-exercise, immediately post-exercise and 24, 48, and 72 hours post-exercise.

Subjects

Thirty-two healthy women were divided into lean (n = 12; age, 20.2 ± 0.4 years; weight, 61.3 ± 1.9 kg; height, 166 ± 2 cm; BMI, 22.0 ± 0.3; body fat, 20.1 ± 0.9% of the body weight), overweight (n = 12; age, 20.8 ± 0.3 years; weight, 83.0 ± 1.5 kg; height, 168 ± 1 cm; BMI, 29.4 ± 0.5; body fat 31.4 ± 0.5% of the body weight), and underweight (n = 8; age, 21.4 ± 0.4 years; weight, 48.9 ± 1.2 kg; height, 167 ± 2 cm; BMI, 17.3 ± 0.2; body fat, 14.9 ± 0.5% of the body weight) groups according to their BMI (2). All the subjects were stable at their anthropometric characteristics for at least the last 2 years, and they had no injury or a surgery in their legs. Additionally, they did not undertake any scheduled eccentric exercise or other activities with large eccentric component for at least 6 months before the study, and they did not spend >1 h·wk−1 on sport activities. The project was conducted from November to February, and all the measurements for the data collection were performed between 09:00 and 12:00 AM after an 8-hour sleep, in a thermoregulated room (20 ± 1° C), and tap water was provided to the subjects ad libitum. The subjects were instructed to abstain from strenuous exercise for 3 days before and during data collection and were not taking any anti-inflammatory drugs. To evaluate the physical condition of all the subjects, V[Combining Dot Above]O2max was determined using a treadmill running test to exhaustion. All the volunteers were eumenorrhoeic (reporting their menstrual cycle as lasting 24–30 days). A written informed consent to participate in the study was provided by all the participants after the volunteers were informed of all possible risks, discomforts, and benefits involved. The Ethics Committee of the local University approved the study.

Procedures

Anthropometric Characteristics and Evaluation of V[Combining Dot Above]O2max

Body mass was measured to the nearest 0.1 kg (Beam Balance 710, Seca, Birmingham, United Kingdom) with the subjects lightly dressed and barefooted. Standing height was measured to the nearest 1 cm (Stadiometer 208, Seca). Percentage body fat was estimated from 7 skinfold measures (average of 2 measurements of each site) using a Harpenden caliper (John Bull, St. Albans, United Kingdom). The Siri skinfold equation was used to calculate body fat.

The V[Combining Dot Above]O2max was determined using a treadmill running test to exhaustion. The protocol began at 8 km·h−1 and was increased by 1 km every 2 minutes until V[Combining Dot Above]O2max was reached. The V[Combining Dot Above]O2max test was terminated when 3 of the following 4 criteria were met: (a) subject's exhaustion, (b) a <2 ml·kg−1·min−1 increase in V[Combining Dot Above]O2 with an increase in work rate, (c) a respiratory exchange ratio ≥1.10, (d) a heart rate within 10 b·min−1 of the theoretical maximum heart rate (220 − age). Respiratory gas variables were measured using a metabolic cart (Vmax29, Sensormedics, San Diego, CA, USA), which was calibrated before each test using standard gases of known concentration. Exercise heart rate was monitored by telemetry (Polar Tester S610TM, Electro Oy, Finland).

Muscle Damage Indices

The isokinetic dynamometer was used for the measurement of isometric peak torque of knee extensors at 90° knee flexion (30) and concentric peak torque at 60°·s−1 (knee range, 0–120°). The best of the 3 MVCs was recorded. To ensure that the subjects provided their maximal effort, we repeated the measurements if the difference between the lower and the higher torque value exceeded 10%. There was 1-minute rest between isometric efforts. Optimum knee angle was defined as the angle at which peak torque of the knee extensors was achieved during concentric contraction.

The assessment of pain-free ROM was performed manually on the isokinetic dynamometer. The investigator moved the calf at a very low angular velocity from full knee extension (0° knee joint angle) to the position where the subject felt discomfort. The angle was recorded to indicate the end of the pain-free ROM. All the participants were pain-free at full knee extension.

During the assessment of DOMS, the participants were asked to mark their perceived soreness of their muscle belly and the distal region of the quadriceps femoris on the scale during squat movement (90° knee flexion) and during walking. For the assessment of muscle pain, the visual rating scale was used, in which descriptors such as “no pain” and “unbearable pain” correspond to numbers 1 and 10, respectively.

Blood samples were drawn from a forearm vein pre-exercise and 24, 48, and 72 hours post-exercise for the evaluation of creatine kinase. Blood collection was performed 15 minutes before the experimental assessments. The blood was collected in plain tubes, was allowed to clot at room temperature for 30 minutes, centrifuged at 1,370g for 10 minutes at 4° C, and the serum was collected. The serum was stored at −80° C and thawed only once before analysis. Creatine kinase was determined spectrophotometrically in duplicate using a commercially available kit (Spinreact, Sant Esteve, Spain).

Position Sense at the Knee

The subjects were seated (120° hip angle) on the isokinetic dynamometer, and all the assessments were made on the knee extensors of the dominant leg. The angles were recorded by the dynamometer. The investigator positioned the leg in a random order, at the target angles of 30, 45, and 60° knee flexion and maintained it for 10 seconds. The investigator moved the leg manually to the target angle, guided by the information provided by the dynamometer, to ensure that the leg was positioned in exactly the same angle within and between subjects. Afterward, the subjects were asked to recall the target angles after the leg had been placed at that angle. The target angles were the angles chosen for the assessment of proprioception. The leg moved from 0° to 90° to the target position and then back to 0° before each of the 4 efforts. The subjects concentrically flexed their knee to the target angle and, when they were satisfied with the angle they had selected, they held it for about 2 seconds. The direction and the magnitude of the deviation from the target angle were recorded. Four trials were performed at each angle, and their mean value was used for the statistical analysis. The intraclass correlation coefficient of the position sense at the knee was r = 0.78 ([p < 0.001]; 95% confidence interval [CI]: 0.66–0.88) for 30°, r = 0.80 ([p < 0.001]; CI: 0.67–0.89) for 45°, and r = 0.85 ([p < 0.001]; CI: 0.75–0.92) for 60°. The intraclass correlation coefficient of the position sense at the knee was r = 0.86 ([p < 0.001]; CI: 0.76–0.92) for 30°, r = 0.81 ([p < 0.001]; CI: 0.69–0.89) for 45°, and r = 0.79 ([p < 0.001]; CI: 0.67–0.89) for 60°.

Force Mismatch

Isometric knee extensor torque mismatch was assessed with the subjects seated on the dynamometer (120° hip angle). The knee angle for the isometric contraction was set at 90°, and the duration of the contraction was 7 seconds. The torques used for the evaluation of isometric force mismatch was 10, 30, and 50% of the MVC, and they were performed in random order. The position of the subjects was fixed, and the angle was controlled and maintained by the dynamometer. The seat configuration and the joint positions were recorded for the follow-up measurements. The subjects were asked 4 times to produce the % torque of their MVC while they were looking at the computer monitor to receive feedback. Afterward, they were asked to reproduce the same percentage of force blindfolded. The torque where the subjects felt they reached their target was recorded. The interval between the assessments was 30 seconds. Four trials were performed at each torque, and their mean value was used for the statistical analysis. The intraclass correlation coefficient of the force mismatch during isometric contraction was r = 0.80 ([p < 0.001]; CI: 0.67–0.87) at 10% of MVC, r = 0.89 ([p < 0.001]; CI: 0.82–0.94) at 30% of MVC, and r = 0.91 ([p < 0.001]; CI: 0.85–0.95) at 50% of MVC.

Knee Joint Reaction Angle to Release

The isokinetic dynamometer was also used for the evaluation of knee joint reaction angle to release of the knee joint. The subjects were sited (120° hip angle) on the isokinetic dynamometer, and all assessments were applied to the knee extensors of the dominant leg. The angles were recorded by the dynamometer. Afterward, the leg was held by the investigator at 1 of the 3 different angles (30, 45, and 60°) in random order. When knee extensors were relaxed at the predetermined angle, the investigator without warning let the leg fall. The instruction given to the subjects was to stop the fall of the leg as soon as it was perceived as being released. The angle through which the leg moved before the subjects managed to stop the motion was recorded and considered the knee joint reaction angle to release. Four trials were performed at each target angle, and the mean of the 2 closest to the target angle was recorded and used for the statistical analysis. The intraclass correlation coefficient of the reaction angle to release of the knee joint position was r = 0.82 ([p < 0.001]; CI: 0.70–0.90) at 30°, r = 0.88 ([p < 0.001]; CI: 0.79–0.93) for 45°, and r = 0.81 ([p < 0.001]; CI: 0.68–0.89) for 60°.

Statistical Analyses

The distribution of all dependent variables was examined by the Shapiro-Wilk test and was found to be normally distributed. Two-way analysis of variance (ANOVA; group × time) with repeated measurements on time was used to analyze isometric peak torque, concentric peak torque, DOMS during squat and movement, ROM, and optimum muscle length. Three-way ANOVA (group × angle × time) with repeated measurements on time was used to analyze position sense and knee joint reaction angle to release. Three-way ANOVA (group × % of MVC × time) with repeated measurements on time was also used to analyze force mismatch. If a significant interaction was obtained, pairwise comparisons were performed through the Sidak test method. Effect size was calculated as the difference between pre-exercise values and the values at the point where the maximum exercise effect appeared divided by the pooled SD of pre-exercise and the same time point of the maximum exercise effect. The 1-way random single measures model was used for the calculation of the intraclass correlation coefficient while the values were obtained from all the subjects during the 3 consecutive days of the familiarization period. Data are presented as mean ± SEM. The level of significance was set at α = 0.05. The SPSS version 15.0 was used for all analyses (SPSS Inc., Chicago, IL, USA).

Results

Subjects

The BMI of the lean group was significantly different (p < 0.001) from that of the overweight and the underweight group. The body fat of the lean group was also significantly different (p < 0.001) from that of the overweight and the underweight groups (20.1, 31.4, and 14.9%, respectively). Pre-exercise assessments of isometric and concentric peak torques revealed that the overweight individuals exhibited a higher torque output than did the lean and the underweight individuals. Maximal knee extensors torque output of the lean group was 128.9 ± 6.0 and 118.7 ± 5.1 N·m for the isometric and concentric torques, respectively. Overweight individuals exhibited the greatest values among the groups (145.1 ± 4.6 and 132.8 ± 5.1 N·m, for isometric and concentric torques, respectively). Underweight individuals displayed the lowest values among the groups (119.3 ± 7.9 and 105.5 ± 6.2 N·m, for isometric and concentric torques, respectively). On the other hand, the 3 experimental groups did not exhibit any significant differences regarding their V[Combining Dot Above]O2max (i.e., 33.8 ± 1.1 ml·kg−1·min−1 for the lean group; 34.2 ± 1.5 ml·kg−1·min−1 for the overweight group; 36.4 ± 1.8 ml·kg−1·min−1 for the underweight group).

Eccentric Exercise Damage Indices

All indices of exercise-induced muscle damage were significantly different between baseline and post-exercise values for all the 3 groups. However, the overweight subjects experienced greater and more permanent alterations in these indices than did the lean and the underweight subjects (Table 1). Regarding optimum angle during maximal concentric contraction, significant group-by-time interaction (p = 0.024) appeared. The optimum angle appeared at longer muscle length after eccentric exercise for all the 3 groups, with the greatest change occurring at 48 hours after (Figure 1). However, the change in optimum angle was greater in the overweight and the underweight group than the lean group (Figure 1).

Table 1
Table 1:
Percent changes of muscle damage indices for the lean (n = 12), the overweight (n = 12), and the underweight (n = 8) women (mean ± SEM).*
Figure 1
Figure 1:
Optimum muscle angle during maximal concentric contractions at rest (○) and 2 days post-exercise (•) in the lean (A), the overweight (B), and the underweight (C) group. The arrow indicates the angle at which the maximal torque was achieved.

Position Sense in Absolute Values of Angle

There was main effect for group, angle and time (p < 0.001). Deterioration of position sense in absolute values of angle was observed after eccentric exercise in all 3 groups. A significant interaction between group by time and angle by time was also found (p < 0.001). The effect sizes of position sense in absolute values for the lean subjects were −2.09, −3.15, and −2.79, for the overweight subjects were −2.48, −4.07, and 6.08 and for the underweight subjects were −6.76, −4.70, and 3.23 regarding 30, 45, and 60°, respectively. These interactions indicate that the position sense in absolute values of angle at baseline were more accurate in the lean group than in the overweight and the underweight groups for all angles. Additionally, the disturbances after the eccentric exercise were relatively larger in the overweight and the underweight than the lean group (Figure 2).

Figure 2
Figure 2:
Position error of the knee joint at 30° (A), 45° (B), and 60° (C) knee flexion in absolute values of angle in the lean (□), the overweight ([Black up-pointing triangle]), and the underweight (•) group after eccentric exercise (mean ±SEM). aSignificantly different from the pre-exercise value (p < 0.05). bSignificantly different between the lean and the overweight group at the same time point (p < 0.05). cSignificantly different between the lean and the underweight group at the same time point (p < 0.05).

Position Sense in Signed Values

There was a main effect for group, angle, and time (p < 0.05). The majority of post-exercise measures for the 3 groups were significantly different from baseline at the angles of 30 and 45° (Figure 3). The effect sizes of position sense in signed values for the lean subjects were 2.52, 2.29, and 1.14, for the overweight subjects were 2.86, 3.11, and 0.94 and for the underweight subjects were 3.64, 2.56, and 1.48 regarding 30, 45, and 60°, respectively. At baseline values, the individuals of the lean group were selected a more accurate (but not significant) position than the overweight and the underweight group (Figure 3). For all 3 groups, the subjects adopted a more extended position for their leg (i.e., produced negative degree values), but the disturbances in the overweight and the underweight groups was larger than in the lean group where the subjects placed their legs at a less extended position.

Figure 3
Figure 3:
Position error of the knee joint at 30° (A), 45° (B), 60° (C) knee flexion in signed values in the lean (□), the overweight ([Black up-pointing triangle]) and the underweight (•) group after eccentric exercise (mean ±SEM). aSignificantly different from the pre-exercise value (p < 0.05). bSignificantly different between the lean and the overweight group at the same time point (p < 0.05). cSignificantly different between the lean and the underweight group at the same time point (p < 0.05).

Force Mismatch

There was a main effect for group, intensity, and time (p < 0.05). At all percentages of MVC, all 3 groups showed significant differences from baseline at almost at all time points of assessment (Figure 4). There was significant interaction among group by intensity by time, between group by time, and intensity by time (p < 0.05). The effect sizes of force mismatch for the lean subjects were 3.59, 10.09, and 14.40, for the overweight subjects were 4.72, 11.19, and 17.15 and for the underweight subjects were 3.62, 6.53, and 11.01 regarding 10, 30, and 50% of MVC, respectively. These interactions indicate that the lean group was more accurate in the strength reproduction than were the overweight and the underweight group at all intensities of MVC. They also indicate that the underweight group exhibited significantly greater force mismatch than the lean group did (Figure 4). Moreover, in almost all measurements, the overweight group exhibited significant greater force mismatch than did the lean group and the recovery of force matching accuracy was delayed more in the overweight group than in the underweight and the lean group. In general, all 3 groups constantly underestimated their torque output in relation to the target intensity.

Figure 4
Figure 4:
Force mismatch of the knee extensors at 10% (A), 30% (B), 50% (C) of MVC in the lean (□), the overweight ([Black up-pointing triangle]), and the underweight (•) group after eccentric exercise (mean ±SEM). aSignificantly different from the pre-exercise value (p < 0.05). bSignificantly different between the lean and the overweight group at the same time point (p < 0.05). cSignificantly different between the lean and the underweight group at the same time point (p < 0.05).

Knee Joint Reaction Angle to Release

There were main effects for group, angle, and time (p = 0.001). There was significant interaction among group by angle by time, between group by time, and between angle by time (p < 0.001). The effect sizes of knee joint reaction angle to release for the lean subjects were −3.35, −3.34, and −5.26, for the overweight subjects were −4.07, −6.51, and −6.34 and for the underweight subjects were −6.90, −5.38, and −6.56 regarding 30, 45, and 60°, respectively. These interactions indicate that the lean group exhibited smaller knee joint reaction angle post-exercise than did the overweight and the underweight group at all angles. Additionally, the knee joint reaction angle to release of the overweight group was smaller than that of the lean and the underweight group at rest. However, the knee joint reaction angle to release was larger at all angles after eccentric exercise in all 3 groups. In this test, the overweight and the underweight group produced greater increases in knee joint reaction angle than the lean group did (Figure 5).

Figure 5
Figure 5:
Knee joint reaction angle from 30° (A), 45° (B), and 60° (C) knee flexion in the lean (□), the overweight ([Black up-pointing triangle]), and the underweight (•) group after eccentric exercise (mean ±SEM). aSignificantly different from the pre-exercise value (p < 0.05). bSignificantly different between the lean and the overweight group at the same time point (p < 0.05). cSignificantly different between the lean and the underweight group at the same time point (p < 0.05).

Discussion

This is the first investigation describing the physiological and neuromuscular characteristics of underweight individuals at rest and after muscle damaging exercise. Additionally, a comparison between individuals being lean, overweight, or underweight was made to reveal their physiological and neuromuscular characteristics at rest and after muscle damaging exercise in more complete dimensions. The present data indicate that deviation from the normal weight in women causes disturbances in proprioception and exercise-induced muscle damage indices.

We found that the overweight individuals suffered more from exercise-induced muscle damage than the lean and the underweight individuals did. This is probably because of the higher content of fast-twitch fibers in the skeletal muscle of overweight individuals compared with that of the lean individuals reported by many (13,24,25,37,39) but not all investigators (10,34,35). Overweight individuals exhibit more sedentary behavior during daily activities than lean and underweight individuals do (36,38), especially for motor tasks containing large eccentric component (such as stair ascending or descending and downhill or uphill walking) (15). The inactive behavior of overweight individuals renders them less adapted to activities involving eccentric actions than lean and underweight individuals. This implies that overweight individuals may need more exercise sessions to familiarize with a new training modality. The possible increased proportion of type II fibers in conjunction with the reduced physical activity of overweight individuals may account for the greater muscle damage observed in these participants compared with lean and underweight participants.

The present results indicated that exercise-induced muscle damage was accompanied by a rightward shift in the length-tension relationship (i.e., the maximum torque appeared at longer muscle length after eccentric exercise than at rest) for all groups. Considering that the assessment performed only at 48 hours posteccentric exercise, the rightward shift in the length-tension relationship probably includes an immediate shift of length-tension curve because of muscle damage and a delayed shift of length-tension curve because of muscle adaptation. This finding agrees with that of Proske and Morgan (33), who also reported a shift at longer lengths of the muscle optimum angle during maximal torque generation. However, the rightward shift in the lean group was smaller than that exhibited by the overweight and the underweight groups. The greater exercise-induced muscle damage detected in the overweight group could explain the greater rightward shift of the optimum angle they exhibited than the lean group. The greater rightward shift of the optimum angle in the underweight group than in the lean group could be attributed to their muscle mass deficiency, which prevented them from performing strenuous activities, rendering them less adaptive to muscle damaging stimuli. It is worth mentioning, that the mechanisms behind these differences (and the other illustrated below) among lean, overweight, and underweight groups are very difficult to be described. This is particularly the case for the underweight group considering that no single study has investigated any aspect of muscle physiology and motor control behavior in this population. The rightward shift in the length tension relationship suggests that maximal torque output appears at longer muscle length (i.e., different joint angle), which may expose subjects to a higher risk for injury.

Baseline measurements revealed that position sense in the overweight and in the underweight group was worse than in the lean group. Even if the causes of these results are unknown it could be suggested that the excessive body mass of overweight participants and the deficient body mass of underweight participants could have led to the development of greater neuromuscular imbalances than the lean group. It is recognized that increased body weight is associated with changes in body geometry and posture (8), because postural stability is mainly controlled by proprioceptive information derived from leg muscles (9). In the underweight individuals, the deficient body mass could have influenced foot morphology, which in turn may have caused musculoskeletal disorders (18). It is clear that reduced position sense in overweight and underweight individuals may lead to increased risk for falls and serious injuries during everyday activities.

The assessment of position sense after eccentric exercise indicated that the participants of all 3 groups believed that their knee extensors were longer than they actually were. Hence, they extended the leg more. This finding is in agreement with a previous investigation from our laboratory (29). Signals from muscle spindles largely contribute to the sense of position and movement of the limbs (31). After eccentric exercise, there is a rise in passive tension that can mechanically unload muscle spindles (4). Unloading of muscle spindles can lower their passive discharge rates, leading subjects to flex their muscles more by extending the knee joint (33). However, there are also exteroceptors, such as the cutaneous receptors (i.e., the Ruffini endings), which detect external stimuli, respond to skin stretch and have been implicated in position and movement sense (7). Given the fact that eccentric exercise increases muscle circumference (22), it could be suggested that increased pressure on these mechanoreceptors is applied. Even if it is unlikely that the mechanoreceptors are directly responsible for the position errors (1), muscle damage in the stressed muscles may lead the subjects to adopt a more extended position during the position sense assessment (i.e., less flexion in the knee extensors). Based on the findings of the present investigation, all individuals participating in unaccustomed exercise should be alert for large disturbances in position sense after strenuous exercise that may increase the risk for injuries.

The pre-exercise force matching assessment indicated that there were no significant differences among the groups. Significant force matching errors compared with baseline were observed in the 3 groups after eccentric exercise. All the individuals constantly underestimated their torque output in relation to the target intensity. Our findings are in line with previous published data where force-matching errors were presented after eccentric exercise (4,32). The novel finding is that eccentric exercise affected more the ability of the overweight participants to match the targeted torque than their underweight and lean counterparts. It has been suggested that after eccentric exercise, the sense of effort is primarily used for estimating muscle force (32). The greater exercise-induced force mismatch observed in the overweight group probably led them to match the amount of sensed effort required to achieve the given level of torque and not the torque itself. Finally, the greater DOMS observed in the overweight than the lean and the underweight participants could have led to a reduced motor cortical excitability. That in turn, could have reduced the motor output, and in this way, lower excitability may have acted as a protective mechanism for the muscle, during the period when damaged muscle was repaired (32). Force mismatch ability was reduced in all individuals after participating in unaccustomed exercise, which renders them susceptible to injuries the days after exercise. However, more attention should be paid by overweight and underweight individuals compared with lean individuals.

The most interesting finding of this study was the smaller knee joint reaction angle to release observed in the overweight group and the larger knee joint reaction angle to release appeared in the underweight group than in the lean group at the resting evaluation. The better reaction angle exhibited by the overweight individuals could be a hint of a higher proportion of fast twitch muscle fibers in their muscles. On the contrary, the weaker muscles of the underweight participants could not contract as fast as the muscles of the other groups and, as a result, more time was required to stop the fall of the leg. The greater reaction angle at rest exhibited by the underweight individuals may increase the risk of falls for these individuals when they stumble or walk on a slippery surface.

Eccentric exercise increased knee joint reaction angle to release at all assessed angles in the 3 groups. The present findings are in line with those of a previous investigation in the same muscles (29). The novel finding of the present investigation was that knee joint reaction angle to release increased more after eccentric exercise in the underweight and the overweight participants. Considering that exercise-induced muscle damage was greater in the overweight participants, it is possible that more sarcomeres were disrupted and became nonfunctional. Consequently, the number of intact sarcomeres in parallel was probably decreased after eccentric exercise, limiting muscle tension during contraction (20). Concerning the underweight group, their decreased muscle mass (6) and their deficit in muscle strength could have delayed their response to the leg release. Special attention should be paid by overweight and underweight individuals the days after unaccustomed exercise because their response to a stimulus (e.g., an object moving toward them) is slower than usual.

Based on the evidence presented in this study, deviating from the normal BMI is associated with significant disturbances in the proprioception of the legs at rest and after participation in activities involving eccentric actions. Future studies should aim to delineate the mechanisms that made overweight, and particularly underweight individuals, to respond differently to a muscle damaging challenge.

Practical Applications

A certain level of functional independence is required for freedom of movement and personal fulfillment. Today, most people follow a sedentary way of living, which renders them more susceptible to injuries during daily activities. For example, gerontologists have postulated that impaired proprioception makes it difficult for older people to detect changes in body position until it is too late for compensatory behavior to prevent falls. Adequate position sense and muscle reaction time are required for skilful and safe movements, whereas unfamiliar exercise can negatively affect the motor control of the human body. Based on this study, it is evident that overweight and underweight individuals may exhibit decreased proprioception of the lower limbs and decreased postural and balance stability during everyday activities. Moreover, the proprioception of overweight and underweight individuals was found to be even more disturbed after participating in exercise activities involving unaccustomed muscle actions. Such impairments may contribute to reduced performance, particularly when individuals are required to execute fine movements.

It has been established that overweight individuals are more sedentary, expend less energy in daily activities, and are more prone to eccentric exercise muscle damage than are lean individuals. On the other hand, the deficient mass in underweight individuals may force them to exhibit greater efforts to maintain the necessary limb position against gravity, resulting in greater alterations in position sense. This is particularly true, for those overweight and underweight individuals participating in sport activities. Therefore, professionals working with overweight and underweight individuals should be aware that participation in everyday life activities may hide increased risks for them.

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Keywords:

obesity; position sense; reaction time; force mismatch; muscle damage; delayed onset muscle soreness

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