Amount and structure of force variability during short,
ramp and sustained contractions in males and females
Jacob H. Svendsen, Pascal Madeleine
To cite this version:
Jacob H. Svendsen, Pascal Madeleine. Amount and structure of force variability during short,
ramp and sustained contractions in males and females. Human Movement Science, Elsevier,
2010, 29 (1), pp.35. .
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Accepted Manuscript
Amount and structure of force variability during short, ramp and sustained con‐
tractions in males and females
Jacob H. Svendsen, Pascal Madeleine
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10.1016/j.humov.2009.09.001
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Please cite this article as: Svendsen, J.H., Madeleine, P., Amount and structure of force variability during short,
ramp and sustained contractions in males and females, Human Movement Science (2009), doi: 10.1016/j.humov.
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ACCEPTED MANUSCRIPT
Gender differences in force variability
1 Amount and structure of force variability during short, ramp
2 and sustained contractions in males and females
3 4 Jacob H. Svendsen, Pascal Madeleine*
5 6 Laboratory for Ergonomics and Work-related Disorders, Center for Sensory-Motor
7 Interaction (SMI), Department of Health Science and Technology, Aalborg University,
8 Denmark
9 10 *
11 Disorders, Center for Sensory-Motor Interaction (SMI), Dept. of Health Science and
12 Technology, Aalborg University, Fredrik Bajers Vej 7, Bldg. D-3, DK-9220 Aalborg,
13 Denmark.
14 Tel:
(+45) 99 40 88 33
15 Fax:
(+45) 98 15 40 08
16 E-mail:
[email protected]
Corresponding author: Pascal Madeleine, Laboratory for Ergonomics and Work-related
17 1
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Gender differences in force variability
1 Abstract
2 The aim of this study was to investigate the effect of gender differences on force
3 variability assessed by means of linear and nonlinear estimators during short duration,
4 ramp and sustained isometric elbow flexions. Ten males and 10 females performed elbow
5 flexion receiving visual feedback from the direction of force exertion. Isometric elbow
6 flexions were performed during: maximum voluntary contraction (MVC before and after
7 endurance test), short contraction at 10-90 %MVC with 10 % increment for 5 s, ramp
8 contraction from 5 to 50 %MVC over 30 s, and endurance contraction at 20 %MVC.
9 Standard deviation (SD), coefficient of variations (CV), and sample entropy (SaEn) were
10 computed from the force signals recorded in 3D. During short and ramp contraction, SD
11 increased with contraction level while SaEn followed an inverted U-shape function (p <
12 .01). During endurance test, SD and CV increased with contraction time (p < .01). SD and
13 SaEn were consistently higher in males than females while it was opposite for CV (p <
14 .05). Separate control and compensatory mechanisms could be responsible for the
15 observed changes in the amount and structure of task-related and tangential forces
16 variability. Moreover, gender differences most likely point towards gender-dependent
17 force control mechanisms. The lower magnitude and structure of variability observed in
18 females may increase the risk for muscle overload and damage.
19 20 Key-words: Force variability, motor control, gender differences, endurance contraction,
21 sample entropy.
22 2
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Gender differences in force variability
1 1. Introduction
2 The study of gender differences with respect to for instance pain and motor control has
3 attracted a lot of interest within the last decade (Greenspan et al., 2007; Semmler,
4 Kutzscher, & Enoka, 1999). Differences in factors like muscle mass, muscle morphology,
5 substrate utilization, and neuromuscular activation have been reported among males and
6 females (Hicks, Kent-Braun, & Ditor 2001; Krogh-Lund & Jørgensen, 1991; Sejersted et
7 al., 1984). A larger muscle mass concomitant to the increased strength (Miller,
8 MacDougall, Tarnopolski, & Sale, 1993) could result in larger increased intramuscular
9 pressure in males compared with females. Thus, metabolic imbalance between supply and
10 demand may occur, resulting in a greater rate of fatigue development in males (Béliveau et
11 al., 1992; Sahlin, Cizinsky, Warholm, & Höberg, 1992). Further, females are found to be
12 more fatigue-resistant than males during relative endurance tasks at sub-maximal force
13 level (Maughan, Harmon, Leiper, Sale, & Delman, 1986; Semmler, Kutzscher, & Enoka,
14 1999). Contradictory results are reported in studies testing gender difference in strength-
15 matched groups. For instance, no difference between genders is found in line with the
16 hypothesis that intramuscular pressure and muscle mass play a role (Clark, Manini, Thé,
17 Doldo, & Ploutz-Snyder, 2003; Hunter & Enoka, 2001), while other studies have shown
18 opposite results arguing for a less potent effect of metabolic imbalance (Hunter,
19 Critchlow, Shin, & Enoka, 2004b; Larivière et al., 2006).
20 In addition to muscle mass and morphology, neuromuscular activation, and coordination
21 have been suggested as important factors but only a few studies have examined this issue
22 (Beck et al., 2005; Ge, Arendt-Nielsen, Farina, & Madeleine, 2005; Sung, Zurcher, &
23 Kaufman, 2008). An altered neuromuscular activity is reported to lead to improved
3
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Gender differences in force variability
1 recovery period for the deactivated motor units during endurance contraction (Fallentin,
2 Jørgensen, & Simonsen, 1993; Hunter & Enoka, 2003), and this mechanism could be more
3 potent for females compared with males explaining difference in an endurance task
4 (Semmler, Kutzscher, & Enoka, 2000). Finally, both peripheral and central mechanisms
5 are likely to explain gender differences during sustained contraction.
6 Fluctuation in a recorded signal has in general been perceived as noise and disturbance to
7 the signal of interest (Fitts, 1954; Schmidt, Zelanik, Hawkins, Frank, & Quinn, 1979). In
8 studies investigating force control, force fluctuation or variability can be a way to assess
9 important aspects of the motor control (Latash et al., 2002; Jordan & Newell, 2004). As
10 previous studies have indicated, variability is not random like noise, but exhibit a degree
11 of order that can be attributed to the operation of an adaptive control system (Slifkin &
12 Newell, 1999). Nonlinear analysis derived from nonlinear dynamics and chaos theory, is a
13 valuable approach to understand the underlying aspects of neurophysiological signals
14 (Heffernan et al., 2009; Richman & Moorman, 2000; Slifkin & Newell, 1999; van
15 Emmerik & van Wegen, 2002). The structure of variability is usually measured by
16 computing for instance the approximate entropy of the force signal (Heffernan et al., 2009;
17 Slifkin & Newell, 1999; Sosnoff, Valantine, & Newell, 2006).
18 Hong, Lee, and Newell (2007) have recently shown during short duration contractions that
19 compensatory mechanisms are governing the amount and structure of variability between
20 dimensions, i.e., task-related force and tangential forces. To our knowledge, no studies
21 have investigated the amount and structure of force variability in task-related and
22 tangential forces during discrete and continuous force production. Moreover, linear
23 methods have mainly been used to assess variability in genders during, for instance,
4
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Gender differences in force variability
1 sustained contraction (Hunter & Enoka, 2001) and only one study has reported unexpected
2 higher entropy values for males than for females (Sung et al., 2008). This calls for further
3 studies describing the structural changes of force variability in task-related and tangential
4 forces in relation with respect to gender difference during short and continuous force
5 production.
6 In the present study, we hypothesized gender differences in the magnitude and structure of
7 force variability in task-related and tangential forces. For this purpose, we examined the
8 effect of gender on force variability in 3D during short duration, ramp and endurance
9 contractions. Gender comparison was assessed by means of linear and nonlinear methods
10 during discrete and continuous isometric elbow flexion.
11 12 2. Materials and methods
13 2.1. Participants
14 Ten males and 10 females participated in the study after having given their informed
15 consent. The participants’ anthropometric information is shown in Table 1. All
16 participants were healthy and without any known neurological disorders. The study was
17 approved by the local ethics committee (N-20070004MCH) and conducted in conformity
18 with the Declaration of Helsinki.
19 Table 1, here
20 21 2.2. Experimental procedure
5
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Gender differences in force variability
1 The participants performed isometric elbow flexion consisting of short duration
2 contractions, ramp contraction, and endurance contraction. The forces were recorded in
3 3D: 1) Task-related force: Fx: mainly elbow flexion and wrist flexion. 2) Tangential
4 forces: Fy: mainly shoulder flexion, and Fz: mainly external shoulder rotation and radial
5 deviation of the wrist. During the recordings, participants were sitting on a chair and
6 holding the force sensor with the palm towards the ceiling and the elbow flexed at 90°
7 (Fig. 1). The wrist and the shoulder were maintained in neutral position and the arm was
8 in contact with the chest (slight touch). The experimenter controlled the participant
9 position throughout the experiment.
10 11 Fig. 1, here
The experiment consisted of:
12 1. Maximal voluntary contraction (MVCinit) of Fx was recorded three times to
13 determine the participants’ maximum elbow flexion force. Each trial lasted 3 s and
14 was separated by a 2 min pause. MVCinit of Fx was used as reference contraction
15 to set sub-maximal levels. Verbal encouragement was given during MVC tests.
16 Participants rested for 2 min after MVC recordings.
17 2. Short duration contractions consisted of nine contractions ranged from 10% to
18 90% of MVCinit of Fx with a 10% increment in between (Fig. 2). The order of the
19 contractions was randomized to prevent a priori knowledge of the following
20 contraction level. Each contraction lasted 5 s with a 30 s pause in between. The
21 participants were asked to reach the desired contraction level as fast as possible.
22 Participants rested for 2 min after short duration contractions.
6
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Gender differences in force variability
1 3. Ramp contraction lasted 30 s starting at 5% and reaching 50 %MVCinit of Fx
2 (slope of 1.5 %MVC/s; Fig. 2). Participants rested for 2 min after ramp
3 contraction.
4 4. Endurance contraction: The participants maintained a level corresponding to 20
5 %MVCinit of Fx until task failure (Fig. 2). Task failure occurred when the
6 participants were not able to maintain force at 20 ± 2 %MVC of Fx for more than 5
7 s. Verbal encouragement was also given during endurance test.
8 5. MVCfinal of Fx was recorded 30 s after endurance test to assess changes in
9 maximum force with respect to MVCinit of Fx.
10 For the short duration, ramp and endurance contraction, the task-related force Fx was
11 visually fed back to the participants. The pixel/N ratio was 2.4±0.8 pixel/N.
12 13 Fig. 2, here
14 15 2.3. Data analysis
16 The applied forces were measured in 3D (FS-6, AMTI, Watertown, MA, USA). Force
17 signals were low-pass filtered (10.5 Hz) and amplified 1000 times. The signals were AD-
18 converted (12 bits A/D converter, Nidaq 6024, National Instruments, Austin, TX, USA)
19 and recorded through a custom made program in Labview 8.2 (National Instruments,
20 Austin, TX, USA), which also provided force feedback to the participant. All signals were
21 sampled at 500 Hz and saved for further analysis in MATLAB R2007a (The MathWorks
22 Inc, Natick, MA, USA).
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1 For MVC trials, the maximum force was computed over 1 s with 100 ms overlapping. The
2 highest force was then considered as MVC of Fx.
3 The contributions of tangential forces (Fy and Fz) to the task-related force (Fx) were
4 calculated as the percentage contribution of each tangential force compared to Fx.
5 The error of performance was calculated as the difference between the exerted force signal
6 and the required force level in percent.
7 Linear analysis of the force signals was performed to quantify the amount of variability.
8 Standard deviation (SD) and coefficient of variation (CV) were calculated. SD reflects the
9 amount of the variability, and CV is the relative variability. CV was derived from the
10 absolute value of the mean and SD of the signal.
11 Nonlinear analysis of the force signals was performed to assess the structure of variability.
12 Time dependent structure of force variability reveals deterministic and stochastic
13 organization of the neurophysiological system and the degree of complexity of a signal is
14 typically associated with the number of system elements and their functional interactions
15 (Jordan & Newell, 2004; Heffernan et al., 2009; van Emmerik & van Wegen, 2002).
16 Sample entropy (SaEn) was computed for this purpose. SaEn expresses the complexity of
17 the recorded signal (Kuusela, Jartti, Tahvanainen, & Kaila, 2002; Richman & Moorman,
18 2000). SaEn is the negative logarithm of the relationship between the probabilities that
19 two sequences coincide for m+1 and for m points, where larger value indicates more
20 complex structure or lower predictability. The embedding dimension, m, and the tolerance
21 distance, r, were set to m = 2 and r = 0.2× SD of the force channel.
22 For the short duration contractions, SD, CV, and SaEn were calculated over 3 s epoch
23 (discarding the 1st and last s) for each contraction level. Similarly for the ramp contraction,
8
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Gender differences in force variability
1 linear and nonlinear parameters were calculated over 3 s epochs through the 30 s
2 contraction. Similarly, for the endurance contraction, seven epochs of 3 s from 0 to 100%
3 of contraction time (steps of 16.7%) were used.
4 5 2.4. Statistical analysis
6 A one-way analysis of variance (ANOVA) with factor gender and dependent variable
7 endurance time, a two-way ANOVA with factors gender and time (before/after endurance
8 test) and dependent variable MVC level/tangential force contribution, and a three-way
9 ANOVA with factors gender, contraction level/contraction time, and force direction and
10 dependent variables error of performance, SD, CV and SaEn, were performed using SPSS
11 16.0 (SPSS Inc., Chicago, Illinois, USA). A post-hoc test of Least Significant Difference
12 was used for pair wise comparisons. Mean ± SD is reported. The level of significance was
13 set at p < .05.
14 15 3. Results
16 Males had higher MVC of Fx than females (F = 70.9, p < .01), respectively 245.6 ± 61.2
17 vs. 134.6 ± 25.8 N. MVC of Fx decreased from MVCinit to MVCfinal (F = 11.5, p < .01),
18 respectively from 212.5 ± 76.1 to 167.8 ± 63.6 N.
19 Fig. 3, here
20 3.1. Contribution from tangential forces to the task-related force Fx
21 For the short duration contractions (Fig. 3a), the two-way ANOVA showed gender
22 differences, males had larger contribution from tangential forces to Fx than females (F =
9
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Gender differences in force variability
1 14.2, p < .01), respectively 19.2 ± 7.9 % vs. 11 ± 5.5 %. Fy contributed significantly more
2 than Fz to Fx (F = 12, p < .01), respectively 18.8 ± 9.2 % vs. 11.3 ± 6.8 %.
3 For the ramp contraction (Fig. 3b), the two-way ANOVA showed gender differences,
4 males had larger contribution from tangential forces to Fx than females (F = 5.2, p = .02),
5 respectively 30.1 ± 35.1 % vs. 10.2 ± 7.6 %. For the endurance contraction (Fig. 3c), Fy
6 contributed significantly more than Fz to Fx (F = 17, p < .01), respectively 31.2 ± 212 %
7 vs. 10.7 ± 5.2 %.
8 9 Fig. 4, here
3.2. Short duration contractions
10 The error of performance increased with increasing contraction level for short duration
11 contractions (Fig. 4a; F = 5.9, p < .01) but did not change with gender (F = 0.1, p = .76).
12 The three-way ANOVA showed gender differences or tendency towards differences, SD
13 and SaEn were larger for males compared with females while it was the opposite for CV
14 (Fig. 5; respectively F = 93.0, p < .01, F = 5.2, p = .05 and F = 12.1, p < .01). SD
15 increased with increasing contraction level (F = 48.7, p < .01). SaEn also tended to
16 increase and decrease with contraction level (F = 5.2, p = .05). SD was higher for Fx
17 compared with Fy and Fz while it was the opposite for CV and SaEn (respectively, F =
18 54.5, p < .01, F = 101, p < .01 and F = 25.9, p < .01). Moreover, there was a significant
19 Gender × Contraction Level interaction for SD (F = 10.8, p < .01). SD was higher for
20 males compared with females at 10-20-30-40-50-70-80-90 %MVC of Fx (p < .01). There
21 was a significant Gender × Force Direction interaction for CV (F = 7.5, p < .01). CV was
22 higher for females compared with males in Fy (p < .01). Finally, there was significant
23 Contraction Level × Force Direction interaction for SaEn (F = 1.9, p = .02). SaEn was
10
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Gender differences in force variability
1 higher for Fz than Fx at 10- 20-30-60-70-80-90 %MVC of Fx (p < .05). SaEn was also
2 higher for Fz than Fy at 10-20 %MVC of Fx (p < .02) and for Fy than Fx at 60-70-80-90
3 %MVC of Fx (p < .01).
4 Fig. 5, here
5 3.3. Ramp contraction
6 The error of performance increased with increasing contraction level during the ramp
7 contraction (Fig. 4b; F = 4.0, p < .01) but did not change with gender (F = 0.6, p = .44).
8 The three-way ANOVA showed gender differences, SD and SaEn were larger for males
9 compared with females while this pattern of results was the opposite for CV (Fig. 6;
10 respectively, F = 217.4, p < .01, F = 12.6, p < .01, and F = 26.2, p < .01). SD and SaEn
11 increased with increasing contraction level, while CV decreased (respectively, F = 15.9, p
12 < .01, F = 31.2, p < .01, and F = 3.4, p < .01). SD was higher for Fx compared with Fy
13 and Fz while it was the opposite for CV and SaEn (respectively, F = 61.2, p < .01, F =
14 61.1, p < .01 and F = 136, p < .01). There was a significant Gender × Contraction Level
15 interaction for SD (F = 2.5, p < .01). SD was higher for males compared with females for
16 all contraction levels (p < .01).
17 There was also a significant Gender × Force Direction interaction for CV (F = 6.5, p <
18 .01). CV was higher for females compared with males in Fy (p < .01). There was a
19 significant Contraction Level × Force Direction interaction for SaEn (F = 2.0, p = .01).
20 SaEn was higher for Fy than Fx at 5-15-20-25-30-35-40-45-50 %MVC of Fx (p < .05).
21 SaEn was higher for Fz than Fx at all contraction levels (p < .01) and than Fy at 5-10-15-
22 20 %MVC of Fx (p < .01).
23 Fig. 6, here
11
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1 3.4. Endurance contraction
2 The error of performance did not change during the sustained contraction (Fig. 4c; F =
3 0.9, p = .50) and nor with gender (F = 0.01, p = .91). Females had longer endurance
4 duration than males, respectively 682.8 vs. 344.8 s (F = 7.5, p = .01). The three-way
5 ANOVA showed gender differences, SD and SaEn were higher for males than females
6 while it was the opposite for CV (Fig. 7; respectively F = 186.2, p < .01, F = 5.0, p = .02
7 and F = 4.5, p = .03). Both SD and CV increased with contraction time (respectively, F =
8 34.1, p < .01 and F = 8.5, p < .01). SD was higher for Fx compared with Fy and Fz while
9 it was the opposite for CV and SaEn (respectively, F = 30.0, p < .01, F = 141, p < .01 and
10 F = 23.4, p < .01). Moreover, there were significant Gender × Contraction Time, Gender
11 × Force Direction and Contraction Time × Force Direction interactions for SD
12 (respectively, F = 4.1, p < .01, F = 3.3, p = .04, and F = 2.5, p < .01). SD was higher for
13 males compared with females throughout the contraction (p < .01). SD was also higher for
14 males compared with females in all force directions (p < .01). SD was higher in Fx and Fy
15 compared to Fz at 50-66.7-88.3-100 % of contraction time. There was also a significant
16 Contraction Time × Force Direction interaction for CV (F = 2.2, p = .01). CV was higher
17 for Fy and Fz compared with Fx throughout the contraction time (p < .01) and CV was
18 higher for Fz compared with Fy at 50-66.7-83.3-100 % contraction time (p < .01).
19 Fig. 7, here
20 21 4. Discussion
22 The primary goal of this study was to investigate the effect of gender differences in
23 variability of task-related and tangential forces by means of linear (amount of variability)
12
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Gender differences in force variability
1 and nonlinear (structure of variability) analysis during voluntary short duration, ramp and
2 endurance isometric elbow flexions. The findings revealed for the first time that gender
3 played a role in force variability during short, ramp and sustained contractions. Females
4 were characterized by lower amount (SD) and structural complexity (SaEn) than males.
5 Further, the amount of variability increased with contraction level while the structure of
6 variability changed according to an inverted U-shape function in the task-related force and
7 tangential force Fy during discrete contractions. Finally, the amount of variability also
8 increased with contraction time in the task-related force and tangential force Fy.
9 10 4.1. Effect of contraction level and gender on force variability
11 For short duration and contractions, the observed changes in the amount and structure of
12 variability are in line with previous results reporting an exponential increase in SD and a
13 decrease in CV in the direction of force exertion (Hong et al., 2007; Slifkin & Newell,
14 1999; Tracy, Mehoudar, & Ortega, 2007). Similar to that, SD increased for both task-
15 related and tangential forces during short and continuous contractions. CV remained low
16 most likely due to failure in reaching the required level for the high sub-maximal
17 contractions as depicted by the increase in error of performance. Moreover, CV decreased
18 slightly during ramp contraction as expected due to the exponential increase in SD with
19 linearly increasing forces (Hong et al., 2007; Sosnoff et al., 2006).
20 SD was larger for males than for females in the direction of force exertion but also in the
21 tangential directions even though there were no differences in the error of performance. As
22 expected, the results were opposite for CV. This confirmed that the variability in discrete
23 force production is non proportional to force level (Carlton & Newell, 1993). Moreover,
13
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1 the contributions of tangential forces to the task-related force were larger for males than
2 for females during short duration and ramp contractions due to higher absolute force in
3 males. This can in turn have influenced the magnitude of force variability (Hong &
4 Newell, 2008; Vaillancourt, Larsson, & Newell 2002) as there were gender differences in
5 the pixel/N ratio. Furthermore, the larger amount of variability (SD) found in males
6 compared with females could also indicate that males had an elevated activity in the
7 biceps brachii muscle compared with females due to higher force level (Hunter & Enoka,
8 2001) and that muscle activation pattern could be different among genders (Ge et al.,
9 2005; Yoon, Delap, Griffith, & Hunter, 2007).
10 With respect to the changes in the structure of variability, our results in the direction of
11 force exertion are also in line with Slifkin and Newell (1999) and Hong et al., (2007). The
12 sample entropy increased up to approximately 50 %MVC of Fx during short duration
13 contractions and then decreased, following an inverted U-shape (Hong et al., 2007; Slifkin
14 & Newell, 1999). Compensatory synergistic mechanisms are likely to explain the similar
15 changes in the task-related force and tangential force (Fy) seen during both discrete and
16 continuous force production in line with a recent study (Hong et al., 2007). The sample
17 entropy was higher for tangential forces than for task-related force. This confirmed recent
18 results showing that the changes in approximate entropy values differ between task-related
19 and tangential forces (Hong et al., 2007). Further, the present study emphasizes that the
20 amount and structure of force variability during discrete and continuous contractions could
21 be governed by separate control processes (Sosnoff et al., 2006).
22 Similar to the magnitude of variability, the complexity of the task-related and tangential
23 forces were also higher for males than for females arguing for potential gender dependent
14
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1 force control strategies. This would imply that the forces generated by males arise from a
2 higher number of attractors or neural oscillators than females (Heffernan et al., 2009;
3 Lipsitz, 2002; Slifkin & Newell, 1999; Sosnoff, Vaillancourt, & Newell, 2004; van
4 Emmerik & van Wegen, 2002). Finally, the observed changes in the amount and structure
5 of variability occurring in the task-related and tangential forces can be explained by a lack
6 of control of the participants’ dominant arm position and/or compensatory mechanisms
7 like co-contraction and changes in agonist/antagonist relationship aiming at maintaining
8 the same force output (Ervilha, Arendt-Nielsen, Duarte, & Graven-Nielsen, 2004; Hong et
9 al., 2007; Reeves, Cholewicki, Milner, & Lee, 2008; Rudroff, Staudenmann, & Enoka,
10 2008). More studies assessing force variability in 3D are warranted to investigate
11 neuromotor function.
12 13 4.2. Effect of endurance time and gender on force variability
14 The present study confirmed the presence of gender differences in endurance time (Hunter
15 et al., 2004b; Maughan et al., 1986; Sato & Ohashi, 1989) even though similar endurance
16 time have also been reported in strength-matched groups (Hunter & Enoka, 2001; Hunter,
17 Critchlow, Shin, & Enoka, 2004a). The amount of force variability increased in task-
18 related and tangential forces with contraction time while the level in error of performance
19 did not change. The tangential force Fy contributed more than Fz to the task-related force
20 showing that the exerted force was not kept purely mono-directional during the endurance
21 test. Similar to what we observed during discrete and continuous contractions, the amount
22 (SD) and structure (SaEn) were larger for males than for females during fatiguing
23 isometric contraction while opposite results were obtained for CV. The results concerning
15
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1 gender effects are corroborated by recent studies from Yoon et al. (2007) and Sung et al.
2 (2008).
3 Muscle fatigue is found to alter biomechanical movement patterns (Selen, Beek, & van
4 Diëen, 2007). The present gender-dependency in the amount and structure of force
5 variability during endurance contraction could be due to the discrepancies in muscle
6 activation pattern (Ge et al., 2005; Hunter et al., 2004a,b; Yoon et al., 2007). The level of
7 muscular fatigue assessed by the decrease in MVC of Fx, immediately following
8 endurance test was similar for males and females in line with Yoon et al. (2007). During
9 sustained contraction, the CNS compensate for the decrease in the force generated by
10 individual motor units by additional recruitment of motor units, changes in motor unit
11 discharge rate and motor unit substitution (Gandevia, 2001). These mechanisms are likely
12 to account for the difference in size and structure of force variability among genders
13 during an endurance test. Similarly to our findings during short duration and ramp
14 contractions, the control mechanisms influencing the amount and structure of force
15 variability could be gender dependent.
16 Moreover, the lower complexity or higher predictability found in females compared with
17 males corresponding to a higher degree of regularity may not be favourable. Indeed, it
18 may not allow appropriate adaptation to changes in sensory afferent feedback and may
19 increase the risk for muscle overload and damage in females (Ge et al., 2005; Lipsitz,
20 2002; Madeleine, Mathiassen, & Arendt-Nielsen, 2008; Slifkin & Newell 1999). While
21 among males, the CNS may be better to take benefit of the redundancy of the motor
22 system (Latash & Anson, 1996).
23 16
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Gender differences in force variability
1 In summary, the analysis variability of task-related and tangential forces showed that the
2 changes in the amount and structure of force variability differed with increasing
3 contraction level and increased similarly with contraction time. This could be due to
4 separate control processes and compensatory mechanisms influencing variability in task-
5 related and tangential forces. Furthermore, the amount and structure of force variability
6 was higher in males compared with females arguing for possible gender-dependent force
7 control mechanisms.
8 9 Acknowledgements
10 The authors want to thank Afshin Samani for his contribution to data collection. This work
11 was financially supported by the Finale project (grant from Arbejdsmiljøforskningsfond),
12 Det Obelske Familiefond and the Danish National Research Foundation.
13 17
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23
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1 Legends
2 Fig. 1: Experimental setup depicts a participant performing an isometric elbow flexion
3 (short duration contractions, ramp contraction and endurance contraction). Forces were
4 recorded in 3D. Task-related force (Fx: mainly of elbow flexion and wrist flexion) and
5 tangential forces (Fy: mainly of shoulder flexion and Fz: mainly shoulder rotation and
6 radial deviation of the wrist).
7 Fig. 2: Examples of 3D force traces for one male and one female. Task-related forces (Fx;
8 2a, b, c) and tangential forces (Fy; 2d, e, f and Fz; 2g, h, i) are shown for short duration
9 (10, 40 and 80 % MVC of Fx), ramp (from 5 to 50 % MVC of Fx), and endurance
10 contractions (20 % MVC of Fx).
11 Fig. 3: Mean + SE force (N) for task-related force (Fx) and tangential forces (Fy and Fz)
12 during a) short duration contractions, b) ramp contraction and c) endurance contraction. *
13 significant difference between genders (p < .05).
14 Fig. 4: Mean + SE error of performance (%) during a) short duration contractions, b) ramp
15 contraction and c) endurance contraction.
16 Fig. 5: Mean + SE standard deviation (SD, N), coefficient of variation (CV, %) and sample
17 entropy (SaEn) for task-related force (Fx; 5a, b, c) and tangential forces (Fy; 5d, e, f and
18 Fz; 5g, h, i) during short duration contractions.
19 Fig. 6: Mean + SE standard deviation (SD, N), coefficient of variation (CV, %) and sample
20 entropy (SaEn) for task-related force (Fx; 6a, b, c) and tangential forces (Fy; 6d, e, f and
21 Fz; 6g, h, i) during ramp contraction.
25
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1 Fig. 7: Mean + SE standard deviation (SD, N), coefficient of variation (CV, %) and sample
2 entropy (SaEn) for task-related force (Fx; 7a, b, c) and tangential forces (Fy; 7d, e, f and
3 Fz; 7g, h, i) during endurance contraction.
26
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1 2 Table 1: Participants information (mean ± SD).
Females
Males
Age (yrs)
24.7 ± 3.9
25.8 ± 2.5
Weight (kg)
65.8 ± 9.3
80.2 ± 7.0
Height (cm)
170 ± 7.7
188.9 ± 7.2
Body Mass Index (kg/m2)
22.6 ± 2.4
22.5 ± 1.2
3 27
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Gender differences in force variability
Fx
Fy
Fz
1 28
ACCEPTED MANUSCRIPT
Task−related force (Fx)
2.5 a
d
Tangential force (Fy)
g
Tangential force (Fz)
Females
SD (N)
2
Males
1.5
1
0.5
CV (%)
0
b
e
h
c
f
i
20
10
0
SaEn
0.3
0.2
0.1
0
0
50
100
% of endurance time
0
50
100
% of endurance time
0
50
100
% of endurance time
ACCEPTED MANUSCRIPT
Task−related force (Fx)
Tangential force (Fy)
a
Tangential force (Fz)
d
g
Females
SD (N)
3
Males
2
1
CV (%)
0
b
e
h
c
f
i
20
10
0
SaEn
0.3
0.2
0.1
0
10
20
30
40
%MVC of Fx
50
10
20
30
40
%MVC of Fx
50
10
20
30
40
%MVC of Fx
50
ACCEPTED MANUSCRIPT
Task−related force (Fx)
a
Tangential force (Fy)
Tangential force (Fz)
d
g
Females
SD (N)
15
Males
10
5
0
CV (%)
30 b
e
h
f
i
20
10
0
SaEn
c
0.2
0.1
0
10
30
50
70
%MVC of Fx
90
10
30
50
70
%MVC of Fx
90
10
30
50
70
%MVC of Fx
90
ACCEPTED MANUSCRIPT
15
a
10
5
0
10
30
50
70
90
Error in force production (%)
% MVC of Fx
15
b
10
5
0
10
20
30
40
50
% MVC of Fx
10
c
5
0
0
50
% of endurance time
100
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250 a
Task−related force (Fx)
*
200
*
*
d
Tangential force (Fy)
Tangential force (Fz)
Females
*
Males
*
150
*
100
*
50 *
0
10
*
*
*
30
50
70
90
10
30
50
*
0
*
10
*
*
*
*
*
*
*
*
30
50
70
90
10
30
40
50
*
*
*
*
*
*
*
*
*
20
30
40
50
*
10
%MVC of Fx
90
*
*
20
*
*
30
*
*
*
*
40
50
%MVC of Fx
i
f
c
70
h
10
%MVC of Fx
50
%MVC of Fx
e
*
20
*
*
%MVC of Fx
150 b
100
*
*
*
*
*
*
*
%MVC of Fx
Absolute force (N)
g
60
40
20
0
0
50
100
% of endurance time
0
50
100
% of endurance time
0
50
100
% of endurance time
ACCEPTED MANUSCRIPT
Short
contractions (N)
Task−related force (Fx)
200
Tangential force (Fy)
a
d
g
20
0
−20
100
0
Male
Female
−40
0
10
40
80
10
%MVC of Fx
Ramp
contraction (N)
Tangential force (Fz)
b
80
10
40
80
%MVC of Fx
e
h
0
100
20
−20
50
0
−40
0
5
50
5
50
%MVC of Fx
Endurance
contraction (N)
40
%MVC of Fx
%MVC of Fx
60 c
%MVC of Fx
0
20
0
50
20 i
f
40
0
5
0
−20
200
400
Time (s)
600
0
200
400
Time (s)
600
0
200
400
Time (s)
600