Experimental Study of Military Crawl As a Special Type of Human Quadripedal Automatic Locomotion

applied sciences

Article Experimental Study of Military Crawl as a Special Type of Human Quadripedal Automatic Locomotion

Dmitry Skvortsov 1,2,* , Victor Anisimov 2,3,* and Alina Aizenshtein 2,4

1 Federal Scientiﬁc and Clinical Center for Specialized Medical Assistance and Medical Technologies of the Federal Medical Biological Agency, 115682 Moscow, Russia 2 Dmitry Rogachev National Medical Research Center of Pediatric Hematology, Oncology and Immunology, 117997 Moscow, Russia; [email protected] 3 Faculty of Biology, Lomonosov Moscow State University, 119991 Moscow, Russia 4 Federal Center of Brain and Neurotechnology, 117997 Moscow, Russia * Correspondence: [email protected] (D.S.); [email protected] (V.A.); Tel.: +7-916-692-5419 (D.S.); +7-926-519-6801 (V.A.)

Featured Application: The results of this study can be useful for further research into features related to the development of military crawl locomotion in groups of different ages, and for un- derstanding the development of this type of locomotion in ontogenesis. In rehabilitation clinics, it is possible to compare patients’ indicators with the norm registered in this study (which could be further expanded), to assess the degree of violations in the motor control system.

Abstract: The biomechanics of military crawl locomotion is poorly covered in scientiﬁc literature so far. Crawl locomotion may be used as a testing procedure which allows for the detection of not only obvious, but also hidden locomotor dysfunctions. The aim of the study was to investigate the biomechanics of crawling among healthy adult participants. Eight healthy adults aged 15–31 Citation: Skvortsov, D.; Anisimov, V.; (four women and four men) were examined by means of a 3D kinematic analysis with Optitrack Aizenshtein, A. Experimental Study optical motion-capture system which consists of 12 Flex 13 cameras. The movements of the shoulder, of Military Crawl as a Special Type of elbow, knee, and hip joints were recorded. A person was asked to crawl 4 m on his/her belly. The Human Quadripedal Automatic Locomotion. Appl. Sci. 2021, 11, 7666. obtained results including space-time data let us characterize military crawling in terms of pelvic https://doi.org/10.3390/ and lower limb motions as a movement similar to walking but at a more primitive level. Progressive app11167666 and propulsive motions are characterized as normal; additional right–left side motions—with high degree of reciprocity. It was found that variability of the left-side motions is signiﬁcantly lower than Academic Editor: Mark King that of the right side (Z = 4.49, p < 0.0001). The given normative data may be used as a standard to estimate the test results for patients with various pathologies of motor control (ataxia, abasia, etc.). Received: 1 June 2021 Accepted: 18 August 2021 Keywords: crawling; biomechanics; kinematics of motion; 3D kinematic analysis Published: 20 August 2021

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in 1. Introduction published maps and institutional afﬁl- For a human being crawling is a particular stage of phylo- and onto-genesis. This iations. locomotion consists of phylogenetically old and well-automated motions, which human beings acquire at an early stage of their ontogeny and could be described as a type of quadripedal automatic locomotion [1,2]. The locomotion itself is used in a number of physical therapy systems, for instance, in the Klapp system [3]. Nowadays, crawling is Copyright: © 2021 by the authors. mostly applied for a military purpose and also as a ﬁtness exercise. Crawling distinguishes Licensee MDPI, Basel, Switzerland. several types: on one’s side, belly crawl (military crawl) and on hands and knees (bear This article is an open access article crawl). The term crawling itself is used to denote several different locomotion types. distributed under the terms and Normally it refers to moving on hands, legs and belly, which would be more correct to call conditions of the Creative Commons military crawling (Figure1). Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ A more detailed literature search has shown that the term crawling may be used 4.0/). for other locomotion types as well. In the English literature, there are two terms used

Appl. Sci. 2021, 11, 7666. https://doi.org/10.3390/app11167666 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 7666 2 of 10

to describe this movement. The common word for this is crawl. Like any word in the English language, this one has plenty of meanings as well. It is used to describe all crawl types and some styles in swimming. Each connotation is interpreted based on the context. Another term is creeping, which is used for the same purpose but may have a limited Appl. Sci. 2021, 11, x FOR PEER REVIEW 2 of 10 context. This term is used to describe moving on hands and knees, and sometimes by way of military crawling.

FigureFigure 1. 1.Example Example of of military military crawling. crawling.

InA themore modern detailed literature, literature there search are has the shown following that typesthe term of crawling crawling usedmay be by used babies for andother toddlers locomotion at different types as developmental well. In the Englis stages:h literature, moving onthere hands are two and terms knees used (diagonal to de- limbscribe functioning), this movement. moving The on common hands and word feet, for moving this is oncrawl belly,. Like scooting any word (moving in the in English sitting positionlanguage, with this the one help has of plenty one’s hands), of meanings which as is awell. mix It of is the used first to two describe types (oneall crawl hand types and footand on some one styles side and in swimming. one hand and Each knee connotation on the other is sideinterpreted [3,4]), and based a mixed on the way context. of using An- threeother limbs term asis creeping the main, which support. is used for the same purpose but may have a limited context. ThisNevertheless, term is used to after describe analyzing moving different on hands or slightlyand knees, similar and samplessometimes of by babies’ way crawlof mili- theretary crawling comes a. conclusion that, despite the high variability of the applied crawl techniques, all of themIn the have modern certain literature, constant there values are inside the following the interlimb types coordination of crawling [used4]. A by pace babies such and as trottoddlers prevails at evendifferent if only developmental three limbs are st usedages: [ 4moving,5]. It may on behands confined and toknees the synchronized(diagonal limb functioningfunctioning), of moving limbs, whichon hands is possible and feet, only moving when on the belly, nervous scooting system (moving is mature. in sitting posi- tionIn with study the help [6], the of one’s above-mentioned hands), which group is a mix of of healthy the first participants two types (one showed hand that and thefoot troton paceone side used and when one one hand moves and onknee hands on the and other knees side turned [3,4]), out and to bea mixed more constantway of using and frequent,three limbs but as at the a lowermain support. speed. At the same time, while the body is gradually moving forward,Nevertheless, it experiences after minimal analyzing fluctuations. different or slightly similar samples of babies’ crawl thereIn comes paper a [ 1conclusion], the authors that, came despite to athe conclusion high variability about theof the mechanisms applied crawl of moving techniques, on allall four of them limbs have using certain patterns constant characteristic values inside of both the interlimb apes and coordination other animals. [4]. Meanwhile, A pace such babiesas trot feel prevails much even more if constrained only three limbs when are they used start [4,5]. moving It may this be way confined (using to all the four synchro- limbs) becausenized functioning their central of nervous limbs, which system is ispossible less flexible. only when the nervous system is mature. AnotherIn study study [6], the [7 above-ment] examinedioned military group crawling of healthy biomechanics participants using showed IMU that sensors the trot in 33pace healthy used adults. when one The moves parameters on hands studied and were knees crawl turned speed, out crawlto be stridemore constant time, ipsilateral and fre- limbquent, coordination, but at a lower and speed. contralateral At the limb same coordination. time, while Itthe was body found is gradually that the group moving could for- beward, divided it experiences into two subgroups minimal fluctuations. according to the level of performance: good and low. Good performanceIn paper is characterized[1], the authors by ca fasterme to execution a conclusion and better about coordination the mechanisms between of moving the limbs. on all fourIn fact, limbs it is using a rare patterns occasion characteristic that crawling isof studiedboth apes in termsand other of its biomechanics.animals. Meanwhile, From the literature available, we can only know about studies performed based on hands and babies feel much more constrained when they start moving this way (using all four limbs) knees crawl [8,9]. Meanwhile there has been no examination of the biomechanics of military because their central nervous system is less flexible. crawling. The difference between these types is significant and quite obvious. In the first Another study [7] examined military crawling biomechanics using IMU sensors in 33 case, it is walking on hands and knees [10,11] in real dimensional space. In the second case, healthy adults. The parameters studied were crawl speed, crawl stride time, ipsilateral it is moving on a horizontal surface. The English term for both cases is crawl, which also limb coordination, and contralateral limb coordination. It was found that the group could refers to the locomotion of swimming in the crawl style—its biomechanics is being studied be divided into two subgroups according to the level of performance: good and low. Good as well [12]. performance is characterized by faster execution and better coordination between the limbs. In fact, it is a rare occasion that crawling is studied in terms of its biomechanics. From the literature available, we can only know about studies performed based on hands and knees crawl [8,9]. Meanwhile there has been no examination of the biomechanics of military crawling. The difference between these types is significant and quite obvious. In the first case, it is walking on hands and knees [10,11] in real dimensional space. In the second Appl. Sci. 2021, 11, 7666 3 of 10

We consider the locomotion of military crawling based on a specific motor proficiency test which has the following peculiarities: It is a phylogenetic ancient locomotion which has its own automatic behaviors for both lower and upper limbs and trunk. This type of movement is relevant to a certain stage of ontogenesis during the first year of its existence. However, further on in life, it has no particular importance and is mostly not applied. These peculiarities let us consider the locomotion of military crawling as a potentially useful test which makes it possible to detect hidden forms of motor pathology. We assume that the high and low degree of performance, found in [7], even in healthy subjects, demonstrates the high sensitivity of this test and its biomechanical parameters for detecting early forms of motor control disturbances. Additionally, the process of military crawling on a formal level may be defined as moving mostly in two dimensions (the third vertical dimension is not obvious). This makes the locomotor analysis technically less complicated and may be considered as a 2D one. The research hypothesis is as follows: We assumed that crawling locomotion, being phylogenetically ancient in humans, remains well automated. This is also true for adulthood and in the case when this type of locomotion is not used in later life in any way (except for the first year, where this is one of the stages of motor development). Symmetry of parameters (left-right) and reciprocity should be relevant to automatic quadripedal locomotion. In the available literature [7–9], specific questions of the biomechanics of movement during crawling were studied separately. Nevertheless, military crawling as an integral quadripedal process in its kinematic part, has not been studied sufficiently. The aim of this research is to study the locomotor kinematics of military crawling in accordance with the fixed normal parameters.

2. Materials and Methods 2.1. Participants There were 8 healthy participants tested, aged between 15 and 31 (average age 23 years old)—4 men and 4 women without any pre-existing locomotor traumas or dysfunctions. The average height was 178 cm (ranging between 170 and 184 cm) and all subjects were a normal weight (average BMI—22.7, ranging between 21 and 24). In this examination, one of the participants was left handed. All participants had no professional training experience. All the participants had normal physical development. Three participants had no sports training experience. Three others had trained in the past, but by the time the study starting the training break was more than 1 year. Two of the participants trained systematically. All the participants signed informed consent forms (they conﬁrmed their voluntary intentions of participation in the study and consent of wearing registered markers on the body). All the participants had a good state of health.

2.2. Military Crawling A biomechanical analysis of motions used in military crawling was performed. Before the recording started the subject was laid face-down on a special carpet surface with legs joined, arms along the body. Then, the participant had to adopt the initial position: lie down on the floor (the floor has a soft surface), legs straight, arms bent and placed in front of the subject. The task for the person is to crawl 4 m on his/her belly by means of legs and arms at any pace. Before placing reflective markers, each subject tried to crawl 2–3 times. This was done in order to make sure that the subject would perform exactly what was needed, and not other options, for example, walking on fours. That was a common mistake. However, we deliberately did not allow crawling to be trained for the reason that we were interested in how automatic, symmetrical, and reciprocal the crawling locomotion, which has not been used for many years, would be. We made 2–3 recordings of the crawling process. The recording with the highest quality of registration was selected for subsequent analysis. Appl. Sci. 2021, 11, x FOR PEER REVIEW 4 of 10

Appl. Sci. 2021, 11, x FOR PEER REVIEW 4 of 10

Appl. Sci. 2021, 11, 7666 how automatic, symmetrical, and reciprocal the crawling locomotion, which has not4 been of 10 used for many years, would be. We made 2–3 recordings of the crawling process. The recordinghow automatic, with the symmetrical, highest quality and reciprocal of registra thetion crawling was selected locomotion, for subsequent which has not analysis. been usedEquipment for many years, would be. We made 2–3 recordings of the crawling process. The 2.3. Equipment recordingThe kinematics with the highest were recorded quality of by registra the Optitiontrack was opticalselected system for subsequent (“Motion analysis. Capture Sys- The kinematics were recorded by the Optitrack optical system (“Motion Capture tems”.Equipment OptiTrack, http://www.optitrack.com/index.html, accessed on 3 May 2021) which Systems”. OptiTrack, http://www.optitrack.com/index.html, accessed on 3 May 2021) consistsThe of kinematics 12 Flex 13 were cameras, recorded 100 by Hz the each. Opti trackThe cameras optical system are placed (“Motion at a Captureheight of Sys- 2.2 m which consists of 12 Flex 13 cameras, 100 Hz each. The cameras are placed at a height evenlytems”. aroundOptiTrack, the http://www. perimeter ofoptitrack.com/index.html, the room, which was 5 accessed × 8 m. Theon 3 examination May 2021) which was ar- of 2.2 m evenly around the perimeter of the room, which was 5 × 8 m. The examination rangedconsists based of 12 onFlex the 13 standard cameras, procedure:100 Hz each. calib Theration cameras and are marker placed fixation. at a height For ofthe 2.2 partici- m was arranged based on the standard procedure: calibration and marker fixation. For pantsevenly there around was the a set perimeter of 13 light-reflecting of the room, whichmarkers was fixed 5 × on8 m. the The following examination joints was (1 marker ar- the participants there was a set of 13 light-reflecting markers fixed on the following joints onranged each basedjoint): on shoulder the standard, elbow ,procedure: radiocarpal calib, kneeration and andankle marker joints. fixation.The remaining For the partici-3 markers (1pants marker there on was each a joint):set of 13 shoulder, light-reflecting elbow, radiocarpal,markers fixed knee on the and following ankle joints. joints The (1 remainingmarker 3were markers fixed were on the fixed subject’s on the back: subject’s 2 markers back: 2on markers the left onand the on left the and right on at the the right level atof thethe sacroiliacon each joint): joints shoulderand 1 on, elbow the 10th, radiocarpal spinous, kneeprocess and ofankle vertebra joints. (Figure The remaining 2). 3 markers levelwere of fixed the sacroiliacon the subject’s joints andback: 1 2 on markers the 10th on spinous the left processand on the of vertebraright at the (Figure level 2of). the sacroiliac joints and 1 on the 10th spinous process of vertebra (Figure 2).

FigureFigure 2.2. PointsPoints wherewhere markersmarkers werewere ﬁxed:fixed: acromion,acromion, laterallateral elbowelbow epicondyle,epicondyle, externalexternal placeplace ofof wristwristFigure (watches(watches 2. Points wristwrist where placement),placement), markers were spinaspina fixed: iliacailiaca acromion, posteriorposterior superior,lateralsuperior, elbow trochantertrochanter epicondyle,major, major, external lateral lateral epicondyleplace epicondyle of wrist (watches wrist placement), spina iliaca posterior superior, trochanter major, lateral epicondyle ofof thethe knee,knee, laterallateral malleolus.malleolus. of the knee, lateral malleolus. TwoTwo particular particular markers markers are are connected connected with with a straight a straight line line thus thus forming forming the linksthe links of the of Two particular markers are connected with a straight line thus forming the links of model.the model. The linksThe links form joints.form joints. Thereby Thereby the angle the inangle thejoint in the is determinedjoint is determined by three markersby three the model. The links form joints. Thereby the angle in the joint is determined by three (Figuremarkers2). (Figure 2). markers (Figure 2). TheThe crawlingcrawling cyclecycle isis consideredconsidered toto bebe fromfrom the the moment moment the the leg leg is is bent bent in in the the knee knee The crawling cycle is considered to be from the moment the leg is bent in the knee and hip joints until the next knee and hip ﬂexion of the same leg (Figure3). andand hip hip jointsjoints until thethe nextnext knee knee and and hip hip flexion flexion of of the the same same leg leg (Figure (Figure 3). 3).

FigureFigureFigure 3.3. 3. SequentialSequential Sequential phasesphases phases ofof of a aa full fullfull cyclecycle ofof motionsmotionsmotions for for the thethe right rightright side—upper side—upperside—upper row rowrow andand and leftleft left side—lower side—lower side—lower row. row. row.

Since the model is somewhat simpliﬁed, the obtained goniograms only partly reﬂect the joint movements. These movements are studied only in the projection of a horizontal surface. However, to make the description less complicated, we will keep calling them hip, knee, shoulder and elbow joints. Appl. Sci. 2021, 11, x FOR PEER REVIEW 5 of 10

Appl. Sci. 2021, 11, 7666 5 of 10 Since the model is somewhat simplified, the obtained goniograms only partly reflect the joint movements. These movements are studied only in the projection of a horizontal surface. However, to make the description less complicated, we will keep calling them hip, kneeAfter, shoulder the recordsand elbow made joints. with the Optitrack system have been processed in the standard way, furtherAfter the processing records made was with performed the Optitrack based system on Visual have 3Dbeen (C-Motion processed Inc.,in the USA). stand- For theard biomechanical way, further processing analysis ofwas the performed military crawlbased task,on Visual there 3D has (C-Motion been a special Inc., USA). simplified For multilinkthe biomechanical model of analysis a human ofbody the military developed, crawl whichtask, there consists has been of eight a special links simplified which form eightmultilink joints model in the of horizontal a human plane.body developed, which consists of eight links which form eightThe joints beginning in the horizontal and end plane. of each motion cycle on each side (right and left) are de- terminedThe manuallybeginning basedand end on of the each data motion about cycle the shifton each of theside markers (right and fixed left) on are the deter- lateral malleolus.mined manually After that based a reporton the containingdata about the one-cycle shift of kinematicsthe markers parameters fixed on the was lateral generated. malleolus.The After report that includes a report the containing following one- standardcycle kinematics biomechanics parameters information: was generated. a movement patternThe of report the marker includes fixed the following at the level standard of the biomechanics 10th spinous information: process of vertebra,a movement cycle duration,pattern of proportion the marker between fixed at the the level beginning of the of10th one spinous cycle andprocess beginning of vertebra, of the cycle next du- cycle (inration, percentage), proportion diagrams between of the angle beginning variance of withinone cycle a movement and beginning cycle of (meters the next per cycle second), (in space-timepercentage), parameters: diagrams of movement angle variance cycle wi durationthin a movement (CD) in seconds, cycle (meters start of per a cycle second), on the oppositespace-time side parameters: (SOpS) versus movement the one cycle on duration the given (CD) side in in seconds, percentage start of a CD, cycle length on the of a motionopposite cycle side (LMC,(SOpS) meters) versus the and one speed on the of given the 10th side spinous in percentage marker’s of CD, shift—V length (meters of a permotion second). cycle (LMC, meters) and speed of the 10th spinous marker’s shift—V (meters per second).To describe the shoulder girdle—pelvis arrangement, a special value shoulders–pelvis To describe the shoulder girdle—pelvis arrangement, a special value shoulders–pelvis was used. This value is an adjoining angle formed by the lines going through the markers was used. This value is an adjoining angle formed by the lines going through the markers placed on the acromial extremities of the clavicles and markers on the pelvis. placed on the acromial extremities of the clavicles and markers on the pelvis. The data set includes the following parameters: range of motion—an amplitude The data set includes the following parameters: range of motion—an amplitude for for each joint from minimum to maximum and extremum phase (maximal flexion of a each joint from minimum to maximum and extremum phase (maximal flexion of a joint)— joint)—Tmax% (percent of CD). Tmax% (percent of CD). Statistical processing was performed based on Statistica 10 with the use of variation Statistical processing was performed based on Statistica 10 with the use of variation statisticsstatistics methods methods and and variance variance analysis. analysis. Evaluation Evaluation was was conducted conducted according according to Wilcoxon–to Wil- Mann–Whitneycoxon–Mann–Whitney criteria. criteria. These criteriaThese criteria was used was becauseused because the distribution the distribution of parameters of param- in theeters study in the was study not was parametric. not parametric. This is This partly is partly due to due the to genesis the genesis of the of valuesthe values themselves, them- andselves, partly and to partly the sample to the sample size (8 participants).size (8 participants). 3. Results 3. Results TheThe typicaltypical motion pattern pattern for for 10th 10th spinous spinous process process of vertebra of vertebra marker marker is shown is shown be- belowlow (Figure (Figure 4).4 ).This This path path looks looks like likea saw-tooth a saw-tooth curve. curve. Meanwhile Meanwhile “to the “toleft-to the the left-to right” the right”motions motions are symmetric, are symmetric, and the andforward the movement forward movement is progredient is progredient and non-stop. and Thereby, non-stop. Thereby,the speed the does speed not doeschange not significantly. change significantly.

FigureFigure 4. Typical4. Typical motion motion path path of of 10th 10th spinous spinous process process ofof vertebravertebra marker. Vertically—motions Vertically—motions to tothe the left left (up) (up) and and to the to the rightright (down), (down), scale scale in in meters. meters. Horizontally—motions Horizontally—motions fromfrom thethe left to the right right (scale (scale in in meters). meters).

Results of the motion cycle test of space-time parameters are shown in Table1. The above ﬁndings show that the parameters for the right and left side are relatively symmetrical in terms of both the cycle duration and the covered distance and speed. The shift of motion phases in one cycle versus another ﬂuctuates within 50%, that is a half-period shift. Motion angles test results for healthy participants are shown in Figure5. Appl. Sci. 2021, 11, x FOR PEER REVIEW 6 of 10

Appl. Sci. 2021, 11, 7666 6 of 10 Results of the motion cycle test of space-time parameters are shown in Table 1.

Su Table 1. Space–time parameters of the tested group, p > 0.05. The above findings show that the parameters for the right and left side are relatively Opposite Side Start Length of Motion symmetricalParameter in Motion terms Cycle of both (CD c.)the cycle duration and the covered distanceSpeed and speed. (m/s) The (SOpS, Percent of CD) Cycle (LMC, m) shift of motion phases in one cycle versus another fluctuates within 50%, that is a half- Left side 2.2 ± 0.8 47.8 ± 4.9 1.2 ± 0.1 0.5 ± 0.1 periodRight sideshift. Motion 2.2 angles± 0.8 test results for 52.3 healthy± 4.4 participants 1.2 ± are0.8 shown in 0.5 Figure± 0.1 5.

Figure 5. Diagrams of activity in joints and body motions on the left and right side in crawling. Horizontally—motion cycle Figure 5. Diagrams of activity in joints and body motions on the left and right side in crawling. Horizontally—motion in percentage, vertically—angle in degrees. cycle in percentage, vertically—angle in degrees. Appl. Sci. 2021, 11, 7666 7 of 10

The left and right sides are functioning in a relatively symmetric way, but the motions in the right elbow joint have a slightly lower ﬂexion amplitude. Amplitude phase characteristics of motions are given in Table2.

Table 2. Amplitude-phase characteristics of motions for the left and right side. Means and standard deviations are presented. Comparison was executed by Mann–Whitney nonparametric criterium.

T %, Mann–Whitney, Parameter Motion Cycle Max, Grad. max Percent of CD Z, p Shoulders– Left 56.0 ± 14.4 78.5 ± 7.8 6.4, <0.0001 pelvis Right 57.1 ± 14.2 31.6 ± 12.0 Left 78.4 ± 45.0 56.9 ± 3.3 0.08, <0.4 Shoulders Right 75.1 ± 37.1 51.3 ± 16.9 Left 89.8 ± 38.3 59.4 ± 3.7 0.58, <0.56 Elbows Right 89.3 ± 45.7 56.0 ± 16.8 Left 79.3 ± 8.3 33.6 ± 5.0 0.06, <0.95 Pelvis and hips Right 79.0 ± 12.3 36.1 ± 5.0 Left 116.9 ± 15.4 32.9 ± 7.1 0.48, <0.63 Knees Right 117.0 ± 7.7 35.1 ± 6.1

Here, it is shown (Table2) that the amplitude and phase of their occurrence for the left and right side are relatively symmetric. The curve shoulders–pelvis has two obvious extremums at 32 and 79 percent of the motion cycle. At these moments the angle between the shoulder-girdle and pelvic-girdle achieves a peak value. However, in one case the angle opens to the left, in the other case it opens to the right. The ﬂexion amplitudes of hip joints and knee joints reach the peak values approximately at the same time, which is 34–36 percent of the motion cycle, while the knee joints’ amplitude is 27–28 degrees higher than that in hip joints. The peak amplitudes of shoulder and elbow joints are also almost synchronized and occur at an interval of 51–59 percent of the motion cycle. It is obvious that for them the maximum phase variability is higher, moreover it occurs much later than that of the lower limbs’ joints.

4. Discussion An objective, descriptive analysis of military crawling was conducted in the study using the method based on Optitrack motion capture system. In fact, a motion cycle has a different variability because it is arbitrary. If in the case of walking it is evaluated by resonant characteristics of a pendulum represented by lower limbs, in the former case this influence is excluded. The Motion Cycle Start parameter is almost symmetric and fluctuates at about 50 percent of the motion cycle, which is predictable. These are the values characteristic of two-sided cyclic locomotion and walk, particularly in the case of studies [11]. Motion cycle duration is symmetric for both sides too, though the value for average squared difference for the left side is higher. Meanwhile the motion cycle duration is almost similar to a normal cycle length of a step [12]. It sounds quite unexpected, but if the cycle length of a step while walking is an automatic index that normally cannot be controlled on purpose, we may suppose that the same mechanism of control is used in this case as well. However, this phenomenon might be occasional and requires further studies. The speed parameter is also expected to be symmetric, as long as it is measured by the 10th spinous process of vertebra, and normally there must be no significant asymmetry of the left and right sides for this parameter. When analyzing the motion path of the 10th spinous process of vertebra marker, we may find that the norm implies a propulsive progredient motion forward. This is the direction that prevails. Furthermore, there are additional symmetric right–left motions with high reciprocity. The nearest analogue for these additional motions may be found in biomechanics of a normal walk. The center of gravity of the body is also a curve similar to a sinusoid [13]. The Appl. Sci. 2021, 11, x FOR PEER REVIEW 8 of 10

10th spinous process of vertebra, and normally there must be no significant asymmetry of the left and right sides for this parameter. When analyzing the motion path of the 10th spinous process of vertebra marker, we may find that the norm implies a propulsive progredient motion forward. This is the direction that prevails. Furthermore, there are addi- Appl. Sci. 2021, 11, 7666 8 of 10 tional symmetric right–left motions with high reciprocity. The nearest analogue for these additional motions may be found in biomechanics of a normal walk. The center of gravity of the body is also a curve similar to a sinusoid [13]. The differences are quantitative—the differencesproportion are of quantitative—the sideways motions proportion to the forward of sideways motion motions in crawling to the is forward close to motion 1:5, but in in crawlingwalking is the close step to length 1:5, but is in much walking bigger the than step fluctuations length is much in the bigger bodies’ than center ﬂuctuations of gravity in thefor bodies’ an adult, center and ofthe gravity proportion for an is adult, approximately and the proportion 1:14. For more is approximately information, 1:14. the speed For moreof the information, 10th spinous the process speed of of the vertebra 10th spinous marker process was analyzed. of vertebra For marker all the was tested analyzed. partici- Forpants, all the the tested diagram participants, looks like thea two-phase diagram lookssinusoid. like Meanwhile, a two-phase the sinusoid. speed varied Meanwhile, by 0.2– the1.0 speed meters varied per second by 0.2–1.0 and metersnever reduced per second to nil, and which never relates reduced to toa full nil, stop. which It relatesis noticeable to a fullthat stop. speed–time It is noticeable curves that are speed–timein fact similar curves to those are in in fact walking similar both to those in terms in walking of amplitude both inand terms phase of amplitude described andin this phase work described [13]. Normally, in this work shoulder–girdles [13]. Normally, and shoulder–girdleship–girdles change andthe hip–girdles angle with respect change to the each angle other with by respect 30 degr toees, each thus other forming by 30 an degrees, angle open thus to forming the right anor angle to the open left depending to the right on or the to themotion left depending phase. Respectively on the motion for the phase. left and Respectively right cycle formo- thetions left will and be right reverse-phased, cycle motions because will be the reverse-phased, body represents because an integral the body structure represents that andis- integraltributes structure its motions that among distributes the limbs its motions on both amongsides. Therefore, the limbs the on bothgoniogram sides. Therefore,of the angle thebetween goniogram the hip–girdles of the angle and betweenshoulder–girdles the hip–girdles looks like and a smooth shoulder–girdles sinusoid with looks breakdown like a smoothby left sinusoidand right with side breakdowncycles. The biggest by left andchallenge right sidewas cycles.caused Theby the biggest phenomenon challenge of was high causedvariability by the of phenomenon motions in upper of high and variability lower limbs’ of motions joints induring upper military and lower crawl limbs’. To find joints out duringthe consistency, military crawl. we have To ﬁndanalyzed out the the consistency, variability ratio we have for goniograms. analyzed the Variations variability for ratio each forof goniograms.the goniograms Variations are shown for eachin Figure of the 6. goniograms are shown in Figure6.

FigureFigure 6. 6. VariationVariation values (average (average absolute absolute deviation) deviation) for for each each goniogram goniogram in the in format the format of a motion of a motioncycle. cycle.

MinimumMinimum values values ofof variationvariation were obtained obtained for for shoulders shoulders–pelvis–pelvis and andhip joints hip joints gonio- goniograms.grams. The values The values are higher are higher for knee for joints knee jointsespecially especially in the inphase the phaseprior to prior full toflexion, full flexion,that is thatto say is tothe say flexion the flexion itself is itself variab isle. variable. There is There an even is an higher even highervariation variation for shoulder for shoulderjoints and joints the and peak the one peak in oneelbow in joints. elbow joints.At the Atsame the time same the time locomotor the locomotor strategy strategy at the atinterval the interval of 50–80 of 50–80 percent percent motion motion cycle cycle is sign is significantlyificantly different different (bef (before,ore, during during and and after afterthe theamplitude amplitude maximum). maximum). For For shoulder shoulder joints joints the thevalues values vary vary down down at first, at first, then then go go up; up;and and for for elbow elbow joints—vice joints—vice versa. versa. Furthermore, Furthermore, we estimatedestimated averageaverage variation variation values values (based(based on on the the step step cycle). cycle). TheThe result confirms confirms the the graphic graphic one. one. Average Average values values come come in in the thesame same ascending ascending order: order: shoulders shoulders–pelvis—5.9,–pelvis—5.9, hips—6.0, hips—6.0, knees—9.3, knees—9.3, shoulders shoulders—16.0,—16.0, elbows— elbows—17.9.17.9. A similar A calculation similar calculation was performed was performed for the neighboring for the neighboring joints on joints the left on and the leftright andside. right The side. result The shows result that shows goniogram that goniogram variation variation values valuesof the ofleft the side left are side much are much lower lower than those of the right side which is significantly high (Z = 4.49, p < 0.0001). The only examined left-handed participant had an inverse relationship. More variability was presented in parameters on the left and less on the right. As we suppose, in the phylogenetic ancient locomotor cycle such as crawling the automation of motions has a greater impact on the left side that is less trained. For the more trained right side of the body, voluntary control turns out to be higher. However, this phenomenon requires further study. There are certain physiological pre-conditions for the observed data, namely availability of foot–walk Appl. Sci. 2021, 11, 7666 9 of 10

motions generator [14]. The fact that the activity of this structure influences this locomotion as well (though in a modified way) is proven by a high correlation (Pearson coefficient = 0.98, p < 0.0001) of hip and knee joints’ performance. The maximum flexion for both is at 36–37 percent motion cycle. In terms of kinematics, it is not even a motion of one and the other joint, but a single action (knee and lower leg moved forward) distributed equally among two joints. Control and coordination processes for upper and lower limbs have some peculiarities as far as upper limbs are concerned. Considering the high variability, the maximum amplitudes for both shoulder and elbow joints are close to 56–59 percent motion cycle. Their variability is much higher than that of lower limbs. Additionally, the control process is organized in such a way that the main motions of an upper limb occur much later than those of a lower limb (on average by 20 percent). The latter is related to the fact that upper limbs play a supportive role, while the leading role is played by lower limbs. First, the main motion is executed by a lower limb, then a supportive motion is performed by an upper limb of the same side. The obtained data demonstrate the presence of a number of quantitative and qualitative symmetries in healthy subjects, as well as stable reciprocity. Prior to this study, we could only assume that this is the case. Thus, the very presence of symmetry of the left and right sides with a high degree of reciprocity and a decrease in variability from the periphery to the center makes it possible to have benchmarks for assessing the performance of this test in patients with CNS lesions.

5. Conclusions The above-mentioned data, though it comes from limited resources, may be used as a guide to estimate results obtained by means of this test for patients with different pathologies of central nervous system and locomotor dysfunctions [15]. Military crawl execution could be an interesting model and a test in clinics for the estimation of motion abilities compared to norm ranges. An approach for collecting data for such norm ranges and a ﬁrst case is described in this article. The obtained results including space–time data let us characterize military crawling in terms of pelvic and lower-limb motions in a way similar to walking but at a more primitive level. At the same time, the upper limbs’ activity is shifted to the second half on a motion cycle—as opposed to walking, when upper limbs’ motions are steady and symmetric with reference to the step cycle [15,16]. However, as any reciprocal locomotion, crawling of a healthy adult who does not practice this style of locomotion is characterized by the precise synchronization of movements of the limbs and trunk. Since the data variability descends from periphery to center, an express estimation can use data obtained from a fewer number of markers or even from only one, the 10th spinous process of vertebra marker, as well as by means of slow-response sensors. This is an important conclusion that can assist in the organization of experimental routine in studies based on technology of motion capture systems.

6. Limitations of the Study The capabilities of the method are limited by potentially solvable diagnostic tasks. The study involved a small number of participants. We consider our work as a case study, and the main message of the work is to offer an interesting model of human locomotion, which is not widely covered in the literature. At the same time, the application of a modern objective numeric method for estimation of a special type of human locomotion was applied. In further studies, it is possible to increase the sample of participants and reveal more stable and reliable statistics. It is also possible to compare this type of crawling with other well-known types. The article is positioned as descriptive, escalates the attention to poorly studied type of human locomotion military crawl and shows the application of new objective methods (motion capture system) for the clariﬁcation of hidden parameters of complex motion. Appl. Sci. 2021, 11, 7666 10 of 10

Author Contributions: Conceptualization, D.S.; methodology, D.S. and A.A.; software, V.A.; validation, D.S., A.A. and V.A.; formal analysis, D.S., A.A. and V.A.; investigation, D.S., A.A. and V.A.; resources, D.S.; data curation, V.A. and D.S.; writing—original draft preparation, V.A. and D.S.; writing—review and editing, D.S. and V.A.; visualization, D.S. and A.A.; supervision, D.S.; project administration, D.S. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: The study was conducted according to the guidelines of the Declaration of Helsinki. Ethical protocol of the study is №8э/15-17 from 27 October 2017. Informed Consent Statement: Informed consent was obtained from all subjects involved in the study. Acknowledgments: We thank Shipilov A.V. for his assistance during data collection. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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