Differences between Mental and Physical Preparation of Muscular

Contraction: A Pilot Study

Yosra Saidane

, Soﬁa Ben Jebara

Tarak Driss

and Giovanni de Marco

University of Carthage, Higher School of Communications of Tunis, LR11TIC01, COSIM Lab,

Route de Raoued KM 3.5, Cite El Ghazala Ariana 2083, Tunisia

Universit´e Paris Nanterre, Centre de Recherches sur le Sport et le Mouvement, 92000 Nanterre, France

Keywords:

Mental Preparation, Physical Preparation, EMG Signal, Normalized Mutual Information, Mean Normalized

Preparation Power.

Abstract:

This paper studies some differences between mental and physical preparation of muscular contraction from a

signal processing point of view. Mental preparation is a cognitive process prior to performance while physical

preparation is a bodily movement produced by skeletal muscles. Two features are selected. The ﬁrst indicator,

called Mean Normalized Preparation Power (MNPP), represents the amount of muscular activity produced

during preparation. The second feature, called Normalized Mutual Information (NMI), studies the functional

connectivity between a pair of agonist/antagonist muscles entering in action. Results showed that connectivity

is more important in Mental Preparation Activity (MPA) than in Physical Preparation Activity (PPA) while

muscles power is more important in PPA than in MPA. When classifying the preparation according to muscular

activity importance (low and large preparation), previous conclusions are valid for large preparation. In case

of small preparation, there is no differences between MPA and PPA in term of MNPP while MPA has higher

NMI. Finally, the study of the correlation between MNPP and NMI showed a moderate dependence with the

agonist muscle and an independence with the antagonist muscle.

1 INTRODUCTION

Finding the best preparation of physical activity is al-

ways a daily issue especially during the instants pre-

ceding the activity beginning. This activity, called

motor preparation (Henry and Rogers, 1960), is use-

ful in many ﬁelds like sports and rehabilitation. In

sports, some studies showed that motor preparation

is effective for performance enhancement in basket-

ball free throw shooting (Lonsdale and Tam, 2008),

golf putting (Bell et al., 2010), water polo penalty

(Marlow et al., 1998), rugby goal kicking (Jackson

and Baker, 2001) and volleyball serving(Lidor and

Mayan, 2005). In rehabilitation, one study demon-

strated that motor preparation promote the relearning

of motor strategies post-stroke on patients with stroke

(Malouin et al., 2009). It was also shown beneﬁcial

effects in patients with Parkinson’s disease especially

for reducing bradykinesia (Tamir et al., 2007).

Motor preparation is classiﬁed into two types:

mental and physical preparation. Physical Prepara-

tion Activity (PPA) is deﬁned as any bodily move-

ment produced by skeletal muscles that results in en-

ergy expenditure (Caspersen et al., 1985) while Men-

tal Preparation Activity (MPA) is the cognitive re-

hearsal of a task prior to performance (Driskell et al.,

1994).

In this work, we aim studying the differences be-

tween these two ways of preparation by considering a

handgrip exercise. To study the differences between

PPA and MPA, the mean power of preparation activ-

ity that appears in EMG signals (Tabar et al., 2017)

and the functional connectivity between the two con-

sidered muscles during the preparation stage are ad-

dressed. The functional connectivity is described

through the Normalized Mutual Information (NMI)

feature (Arjunan and Kumar, 2015; Kawczy´nski et al.,

2015; Madeleine et al., 2011; Saidane and BenJebara,

2017; Johansen et al., 2013). In a previous work, we

showed that the functional connectivity between ag-

onist and antagonist muscles increases during move-

ment’s initiation and then decreases during effective

contraction (Saidane and BenJebara, 2017). It means

that agonist/antagonist muscles put their effort to-

gether to initiate the movement. We also showed

that when subjects prepare their movement, the inter-

muscular dependence in greater than the one without

preparation at movement’s initiation. When dealing

Saidane, Y., Ben Jebara, S., Driss, T. and de Marco, G.

Differences between Mental and Physical Preparation of Muscular Contraction: A Pilot Study.

DOI: 10.5220/0007256301010108

In Proceedings of the 12th International Joint Conference on Biomedical Engineering Systems and Technologies (BIOSTEC 2019), pages 101-108

ISBN: 978-989-758-353-7

101

with preparation power in muscles, we showed in pre-

vious work (Saidane and BenJebara, 2016) that prepa-

ration occurs in both cases of presence and absence

of warning of preparation. When it exists, it is higher

when a warning of preparation is given.

In order to enrich previous works, we aim in this

work to study the differences between mental and

physical preparation. To do it, MPA and PPA are ﬁrst

deﬁned and their percentages of occurence along the

experience are announced (section 2). Then, section 3

(resp. 4) analyzes the differences between mental and

physical preparation in terms of quantity of prepa-

ration expressed using the feature Mean Normalized

Preparation Power MNPP (resp. inter-muscular de-

pendence described by Normalized Mutual Informa-

tion). Section 5 is devoted to the classiﬁcation of

preparation stage into low/large preparation in order

to reﬁne the analysis of the differences between MPA

and PPA. Finally, the correlation between the two

features is given and some concluding remarks are

drawn.

2 MATERIALS AND METHODS

4 women and 8 men volunteers recruited for this pur-

pose are young people (aged 25.1 ± 5.9 years). They

were healthy and had no known neurological or neu-

romuscular disorders at the time of the study and did

not take any medication interfering with central ner-

vous system and muscles. Each volunteer executed

15 contractions (5 in self-initiated tasks and 10 in

externally-triggered tasks). Hence, a total amount of

180 contractions are studied. In externally-triggered

task,tThey were asked to mentally prepare the action

during 6.6 seconds and execute the exercice (squeez-

ing a handgrip) during 4.4 seconds, and afterwards

rest during 44 seconds. The mental preparation is

simulated by a preparation warning (verbal instruc-

tion). During this period, they had to ﬁrmly motivate

themselves and to strongly concentrate on the move-

ment that they had to execute. In self-initiated task,

they were also asked to do the same handgrip exercice

without mental preparation. Two muscles are consid-

ered: the Flexor Digitorum Superﬁcialis (FDS) is the

agonist muscle while the Extensor Digitorum Com-

munis (EDC) is the antagonist muscle.

Electromyographic signals were digitized and

recorded on a personal computer placed in the control

room and equipped with an MP 150 data acquisition

system (Biopac MP150, System Inc., Santa Barbara,

CA). The system software AcqKnowledge(Version

4.2) was conﬁgured to display the EMG signals. Sur-

face EMG signals were recorded using a sampling

rate of 1000 Hz during the handgrip preparation and

contraction periods from the FDS and EDC muscles

of the right arm. The overlying skin was shaved and

rubbed with an alcohol wipes to remove dead cells,

dirt or skin oils. Bipolar electrodes (ADD208, 8-mm

recording diameter) were ﬁrmly attached on skin sur-

face overlying each of the two muscles. The muscles

were identiﬁed by palpating the skin when subjects

ﬂexed and extended the ﬁngers. A reference elec-

trode was placed on the skin overlying the lateral epi-

condyle near the elbow joint of the right arm. The

electrode wires were effectively shielded by multi-

ple layers of shielding and connected to the ampliﬁers

(EMG 100C). EMG signal was ampliﬁed (gain range

at 2000).

Simultaneously with EMG, the force was measured

by a pressure captor (hand dynamometer TSD121B-

MRI).

3 MENTAL VERSUS PHYSICAL

PREPARATION

3.1 Deﬁnition

In order to deﬁne MPA and PPA from a signal pro-

cessing point of view, let’s illustrate the FDS muscu-

lar activity during the contraction and the time inter-

val preceding it (see Fig. 1). The ﬁrst case (a) illus-

trates the absence of preparation and the two others

concern the presence of preparation. One can notice a

small activity preceding the movement that we qual-

ify as preparatory activity (it was checked and veriﬁed

that it does not correspond to any kind of noise, inter-

ference or crosstalk). Two types of preparation are

differentiated. In the ﬁrst one (Fig. 1.b), the force sig-

nal which is acquired simultaneously is equal to zero

during preparation. It means that the subject did not

apply an action on the grip. We called this activity:

controlled or Mental Preparation Activity (MPA). In

the second case (Fig. 1.c), the force is different from

zero during preparation. It means that the subject un-

intentionally exerteda pressure on the grip. It is called

involuntary or Physical Preparation Activity (PPA).

3.2 Preparation Stage in Numbers

To have an idea on when and how much MPA and

PPA occurs, the contractions are ﬁrstly categorized

into two main classes: preparation presence (denoted

Yes) and preparation absence (denoted No). Next and

in parallel, two kinds of sub-classes are identiﬁed:

self-initiated/externally-triggered and MPA/PPA (see

BIOSIGNALS 2019 - 12th International Conference on Bio-inspired Systems and Signal Processing

102

Time (s)

0 2 4 6 8 10 12

-0.5

0.5

Normalized force

EMG signal

Motor activity

Without preparation

(a)

Time (s)

1 3 5 7

-1

-0.5

0.5

EMG signal

Normalized force

MPA

Motor activity

(b)

Time (s)

1 5 10 15

-1

-0.5

0.5

EMG signal

Normalized force

PPA

(c)

Figure 1: Temporal evolution of EMG and force signals.

(a): without preparation, (b): with mental preparation and

(c): with physical preparation.

Fig. 2). The ﬁrst one depends on the use or not of

preparation warning to simulate preparation while the

second one looks if a muscular preparation occurs or

not.

Figure 2: Preparation arboresence.

Tab. 1 resumes the frequency of occurence of the

two main classes (with and without preparation) and

the frequency of occurence of the sub-classes. Note

that each muscle (FDS and EDC) is ﬁrstly studied

alone and next both muscles are considered together

(denoted FDS/EDC). In the last case, both muscles

prepare at the same time while the other cases con-

sider each muscle separately without regard to the

other (if it prepares or not). One can notice that par-

ticipants prepare the activity in 70.7% of cases for

FDS muscle and 67.85% for EDC muscle. When

dealing with preparation mode, one can notice that

the percentage of preparation is more important in

externally-triggered than in self-initiated for FDS,

EDC and FDS/EDC muscles in case of with prepa-

ration. When dealing with preparation type, one can

notice that the preparation percentage is higher in PPA

than MPA when a preparation activity occurs (with).

However, we found the opposite result in without

preparation case. It conﬁrms that PPA is related to

physical preparation while MPA can be considered as

a mental preparation. Moreover, results showed that

the all percentage are slightly more important for FDS

than EDC muscle in the case of ”with” preparation. It

can be explained by the fact that FDS is the agonist

muscle responsible for of handgrip exercise.

Table 1: Preparation percentages for FDS, EDC and

FDS/EDC muscles.

Preparation FDS EDC FDS/EDC

Yes (%) Overall 70.7 67.85 58.57

Externally 54.28 48.57 43.57

triggered

initiated

Self 16.42 19.28 15

MPA 31.42 28.57 20.71

PPA 39.28 38.57 37.85

No (%) Overall 29.28 32.13 19.99

Externally 20 25.71 15.71

triggered

Self 9.28 6.42 4.28

initiated

MPA 27.14 30 18.57

PPA 2.14 2.8 1.42

4 POWER PREPARATION

DURING MPA AND PPA

The objective of this section is to analyze the power

during MPA and PPA. The feature characterizing the

power is called Mean Normalized Preparation Power

(MNPP). It represents the mean of normalized Root

Mean Square (RMS). This latter is normalized to 80%

of maximal voluntary contraction. In fact, above this

level, it was proven that the EMG signal and the force

are exceptionally unstable and cannot provide a suit-

able reference point (De Luca, 1997). We recall that

the RMS is a smoothed version of EMG signal during

Differences between Mental and Physical Preparation of Muscular Contraction: A Pilot Study

103

preparation activity. It is obtained as follows:

RMS(n) =

N + 1

N/2

∑

k=−N/2

(n+k), (1)

where N is the window length. Its duration is chosen

equal to 512 (the equivalent duration is 512 ms since

the sampling frequency is 1 kHz). x(n) is the EMG

signal.

774 776 778 780

0.17

0.34

0.51

0.68

0.85

1.02

Normalized RMS signal

MNPP

Motor activity

Preparation offset=Motor onset

Preparation warning

Preparation onset

Time (s)

Figure 3: MNPP and the relevant instants illustation on

RMS signal.

Fig. 3 illustrates the RMS of an example of prepa-

ration activity. MNPP is the mean of the hashed re-

gion which appears between the preparation onset and

the preparation offset. These instants are detected

automatically according to the method described in

(Ben Jebara, 2015). This graph is enriched with

two other instants: the preparation warning instant at

which the subject receives the warning instruction in

case of externally-triggered and the motor onset is the

instant of movement initiation.

Fig. 4 shows the MNPP boxplots during MPA and

PPA for FDS and EDC muscles. One can notice

that MNPP is more important in PPA than in MPA

for both muscles. One way ANalysis Of VAriance

(ANOVA) gives a signiﬁcant difference between the

two modes: P

value=0.003 in case of FDS muscle

and P value<0.001 in case of EDC muscle. These

results can be explained by the fact that in physi-

cal preparation, MNPP is higher because subjects ex-

erced a mechanical reponse (non null force) while the

force is null in MPA.

5 FUNCTIONAL CONNECTIVITY

BETWEEN FDS AND EDC

MUSCLES

To study the functional connectivity between FDS

and EDC muscles, we used the Normalized Mu-

tual Information (NMI)(Arjunan and Kumar, 2015;

Kawczy´nski et al., 2015; Madeleine et al., 2011;

Saidane and BenJebara, 2017; Johansen et al., 2013).

MPA PPA

MNPP

0.02

0.04

0.06

(a)

MPA PPA

MNPP

0.05

0.1

0.15

0.2

0.25

(b)

Figure 4: MNPP boxplots for FDS muscle (a), EDC muscle

(b).

It is calculated during preparation activity starting

from preparation onset to preparation offset instant. It

is based on the entropy H which measures the uncer-

tainty about an event given probability distribution. It

is written as follows:

H(X) = −

∑

)log(P

)), (2)

where P

) is the probability of the i

outcome of a

random variable X. Here, it is an EMG signal obser-

vation.

Mutual Information (MI) detects the dependence of

FDS and EDC signals. It is deﬁned as follows:

X,Y

∑

X,Y

)log(

X,Y

)

), (3)

where P

X,Y

) is their joint probability density.

The NMI is used to study the functional connectivity

between muscles. It is deﬁned as follows:

NMI =

X,Y

min(H(X),H(Y))

, (4)

where min denotes the minimum of quantities.

Fig. 5 illustrates NMI boxplots during MPA and PPA.

From this ﬁgure, one can notice that NMI values are

more important in MPA than in PPA. Statistical re-

sults (ANOVA) show a signiﬁcant difference between

MPA and PPA (P

value=0.003). The fact that NMI

is higher in MPA than in PPA means that the inter-

muscular dependence is more important between the

ﬂexor/extensor muscles during mental preparation.

This result can be explained by the fact that motor

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104

units are more activated during PPA than MPA be-

cause the force is non null and there is a mechanical

response in PPA. So, the connectivity does not need

to be higher to initiate the action. This result joins an-

other study which demonstrates that the reduction of

number of recruited motor units increases the depen-

dency of functional connectivity between the remain-

ing motor units(Arjunan and Kumar, 2015).

MPA PPA

NMI

0.02

0.04

0.06

0.08

0.1

0.12

Figure 5: NMI boxplots during MPA and PPA.

6 POWER AND NMI ANALYSIS

ACCORDING TO LOW AND

LARGE PREPARATION

The objective of this section is to study MNPP and

NMI differences between MPA and PPA according to

preparation importance and the correlation between

them.

6.1 Classiﬁcation Process

In a previous work (Saidane and BenJebara, 2016),

we found that preparation duration is a pertinent fea-

ture to characterize the motor preparation. In this

study, we used preparation duration and MNPP fea-

tures in order to classify the muscular activity during

preparation phase according to the quantity of prepa-

ration produced. More precisely, we divide prepa-

ration activity into low and large preparation. Low

preparation has a short duration and a weak power

while large preparation is the opposite case.

Supervised classiﬁcation is used to deﬁne low and

large preparation for FDS and EDC muscle sepa-

rately. Tab. 2 shows the repartition of preparation

percentage according to low and large preparation for

each muscle. For FDS muscle, one can notice that, in

PPA type, the percentage of large preparation repre-

sents 83.01% of total preparation. In MPA type, low

preparation presents 72.41% of total preparation. It

is explained by the fact that mental preparation with-

out pressure on the handgrip leads to low muscular

activity. However, in physical preparation, exerting a

pressure on the hand leads to a relatively high muscu-

lar activity.

Dealing with EDC muscle, results are equivalent to

those of FDS muscle. The difference is that the per-

centage of preparation is slightly higher for FDS than

EDC muscle.

Table 2: Low/large preparation percentage for FDS and

EDC muscles.

MPA PPA

Low Large Low Large

FDS 72.41% 27.59% 16.99% 83.01%

EDC 62.06% 37.94% 22.65% 77.35%

6.2 MNPP Boxplots During Low and

Large Preparation

After specifying low and large preparation, MNPP

is drawn in Low/MPA, Low/PPA, Large/MPA and

Large/PPA. Fig. 6 illustrates the MNPP boxplots us-

ing the results of the classiﬁcation of FDS muscle

(Fig. 6.a) and EDC muscle (Fig. 6.b).

During low preparation, we show that MNPP values

are slightly higher in PPA than in MPA using the two

classiﬁcations. Statistical tests give no signiﬁcant dif-

ference between MPA and PPA during low prepara-

tion (P

value=0.231) (resp. P value=0.1253) using

FDS (resp. EDC) classiﬁcation.

Dealing with large preparation which is the case of

interest since preparation is meaningful (high level

of muscle activity), MNPP is more important in

PPA than in MPA in both ﬁgures. Statistical re-

sults give a signiﬁcant difference between the two

modes: P value=0.039 using the FDS classiﬁcation

and P value= 0.005 using the EDC classiﬁcation.

Hence, we can conclude that there is no difference

between preparing physically or mentally the prepa-

ration when the generated muscular activity is weak.

However, it is different when the muscular activity is

important (large).

This result is useful in rehabilitation. In fact, patient

with musculoskeletal disease can replace physical

preparation with mental preparation, even though this

later is less powerful. This conclusion conﬁrms the

ones of other studies which demonstrated that mental

preparation can be successfully substituted to physi-

cal preparationto improvemotor performance(Gentili

et al., 2010)(Allami et al., 2008). In fact, Allami et

al. have found that a combination of extensive mental

practice (75% of the training trials) followed by phys-

ical practice (25% of the training trials) is sufﬁcient

to give similar result as physical practice alone.

Differences between Mental and Physical Preparation of Muscular Contraction: A Pilot Study

105

Low/MPA Low/PPA Large/MPALarge/PPA

MNPP

0.05

0.1

0.15

(a)

Low/MPA Low/PPA Large/MPA Large/PPA

MNPP

0.05

0.1

0.15

0.2

0.25

(b)

Figure 6: MNPP boxplots using FDS classiﬁcation (a),

EDC classiﬁcation (b).

6.3 NMI boxplots During Low and

Large Preparation

Fig. 7 illustrates NMI boxplots of low/MPA,

low/PPA, large/MPA and large/PPA. The two cases

are also given according to the used classiﬁcation.

Fig. 7.a (resp. Fig. 7.b) uses the classiﬁcation of FDS

(resp. EDC) preparation activity into low and large.

During low preparation, one can notice that NMI is

higher in MPA than in PPA. It conﬁrms the results an-

alyzing overall MPA and PPA. Statistical results show

a signiﬁcant difference between MPA and PPA us-

ing the FDS classiﬁcation (P

value=0.036). However,

there is no difference between the two modes using

the EDC classiﬁcation (P

value=0.249). During large

preparation, there is no signiﬁcant difference between

two types of preparation (P

value=0.4) using the FDS

classiﬁcation while NMI is a discriminant feature be-

tween MPA and PPA (P

value=0.027) using the EDC

classiﬁcation.

6.4 Correlation between NMI and

MNPP during MPA and PPA

In this subsection, we aim to study the degree of

ressemblance between the two features selected to

study the preparation stage. To this end, we used

the pearson’s correlation coefﬁcient r between NMI

and MNPP. Its mathematical expression is as follows

(Benesty et al., 2009):

r(NMI,MNPP) =

Cov(NMI,MNPP)

NMI

MNPP

, (5)

Low/MPA Low/PPA Large/MPA Large/PPA

NMI

0.05

0.1

0.15

(a)

Low/MPA Low/PPA Large/MPA Large/PPA

NMI

0.05

0.1

0.15

(b)

Figure 7: NMI boxplots using (a): FDS classiﬁcation, (b):

EDC classiﬁcation.

where Cov is the covariancebetween NMI and MNPP.

MNPP

and σ

NMI

are the standard deviation of NMI

and MNPP respectively. Tab. 3 shows the pearson’s

correlation coefﬁcient r between NMI and MNPP in

case of large preparation only. In fact data leading to

low preparation are discarded because of the insigniﬁ-

cance of preparation. From this table, we show a very

low correlation between the NMI and MNPP for EDC

muscle in both MPA and PPA. So, these two features

are independent and complementary for analyzing the

preparation. In fact, the power of preparationseems to

not depend on the functional connectivity since EDC

muscle permits to hold the hand, whatever the method

of preparation and whatever the degree of connectiv-

ity between muscle. Dealing with FDS muscle, re-

sults show a moderate correlation in both cases of

mental and physical preparation. It means that the two

features share a common information. Moreover, this

correlation is positive which means that when con-

nectivity increases, the mean power of preparation in-

creases too.

Table 3: correlation coefﬁcients between NMI and MNPP.

Large Large

MPA PPA

FDS 0.4183 0.5555

EDC -0.0885 0.1787

BIOSIGNALS 2019 - 12th International Conference on Bio-inspired Systems and Signal Processing

106

7 CONCLUSION

This work deals with the mean power and the func-

tional connectivity between FDS and EDC muscles

as features to differentiate between mental and phys-

ical preparation. Results showed that the increase of

functional connectivity of ﬂexor/extensor muscles de-

pends on the mode of preparation. Mental prepa-

ration is characterized by an important connectivity

and a weak power of muscular activity while physical

preparation is characterized by an important power

of muscular activity with low connectivity. Finally,

a relatively moderated correlation between the mean

power and the functional connectivity is observed

during large preparation in physical and mental prepa-

ration.

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