2021 IEEE 3rd Global Conference on Life Sciences and Technologies (LifeTech 2021)

Alignment of anatomical flexion axis when using
one-degree-of-freedom ankle stretching exerciser
Ko Suzuki1 , Shogo Okamoto1 , Yasuhiro Akiyama1 , Yuma Shiraishi1 , Naomi Yamada1,2 and Yoji Yamada1
1: Department of Mechanical Systems Engineering, Nagoya, Japan
2: Aichi Medical College for Physical and Occupational Therapy, Kiyosu, Japan

Abstract—Most of the existing ankle stretching machines have
one degree of freedom, whereas the human ankle has threedegrees of freedom of motion. We observed the behavior of
the anatomical flexion axis when the ankle was stretched in thedorsiflexion direction by using an exerciser with one degree of
freedom for flexion. For this purpose, under several levels ofheights and external/internal rotation angles, we monitored thedifference in the angle between the anatomical axis of the ankle
and the flexion axis of the exerciser. When the foot was placed on
the exerciser in a slightly internally rotated state, the inversion
angle of the ankle’s anatomical axis was nearly aligned to that of
the exerciser’s flexion axis in the dorsiflexed posture. In contrast,
when the foot was placed in a slightly externally rotated state,
the ankle’s anatomical axis and exerciser’s flexion axis nearlyaligned in terms of the internal/external rotation.
Index Terms—rehabilitation, ankle, foot, stretching

Fig. 1. Anatomical axis of ankle is defined by a line connecting the lateraland medial malleoli. This axis is slightly inverted and externally rotated.

I. I NTRODUCTION
Stretching is an important exercise for treating ankle contractures. Stretching over a long period of time is expected toimprove the range of motion of the ankle joint and prevent
further development of the contracture. However, for somepatients, effectively performing stretching independently athome is difficult. For such patients, automatic ankle stretchingmachines are effective; nonetheless, such machines are notcommercially available.
The anatomical axis for ankle flexion is defined by the line
connecting the medial and lateral ankles, which is slightly
inverted and externally rotated [1] as shown in Fig. 1. Furthermore, the position and posture of the anatomical flexion axischanges as the ankle flexes in the dorsi/plantar direction [2].
Most of the existing ankle stretching machines have one degreeof freedom, normal to the sagittal plane [3]–[6], whereas thehuman ankle has three degrees of freedom [7]. An ankle
stretching machine with multiple actuated degrees of freedom
was developed in [8], [9]. Few studies have discussed thisdifference in the degrees of freedom of rotation axes between
such machines and the human ankle. There is a common belief
that the flexion axis of an ankle stretching machine and thatof the ankle joint should match for effective stretching [10].
However, few studies have reported the net difference in thepostures of the two flexion axes when the ankle is stretchedusing a one-degree-of-freedom dorsiflexion machine or exerciser.
In this study, we observe the relationships between an
exerciser’s flexion axis and the anatomical flexion axis of thefoot when the ankle joint is stretched by an exerciser with

Fig. 2. Ankle exerciser. The forefoot was placed on an acrylic plate thatrevolves around the exerciser’s flexion axis. The heel was placed on a heel
holder cup.

one degree of freedom. Furthermore, we monitor the changes
in these relationships according to the relative position anddirection between the foot and the exerciser. As a pilot study,
we report the case of one healthy participant.
II. M ETHOD
A. Equipment
Fig. 2 shows the foot exerciser used in the experiment. This
exerciser was composed of aluminum frames and an acrylic
plate that revolves around the exerciser’s flexion axis. A string
was fixed to the tip of the aluminum plate. The user placed
his/her heel on the heel holder and pulled the string to revolve

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978-0-7381-2500-8/21/$31.00 ©2021 IEEEFig. 3. Trajectories of the representative points of calf (blue) and foot (red) and the anatomical center of ankle. The circles indicate the initial position, andthe crosses indicate the dorsiflexed position. Green lines are the anatomical rotation axes for the initial and dorsiflexed positions. These line segments connect
the medial and lateral ankles. (a) y–z plane. (b) x–y plane. (c) x–z plane.

the acrylic plate about the exerciser’s flexion axis, thereby
dorsiflexing the foot. The relative position of the ankle joint
and the exerciser’s axis in the z direction could be adjusted in2-cm increments.
The motions of the foot and calf were monitored using
an optical motion capture system (MAC 3D system, Motion
Analysis Corporation, U.S.). As shown in Fig. 2, we used 10
reflective markers: three each for the calf and the foot, oneeach for the medial and lateral ankles, and one each for thetwo ends of the fixed exerciser’s flexion axis.
B. Task
One male participant, who provided an informed consent,
sat on a chair with the right knee fully extended. He relaxed
his lower limbs with the ankle in the naturally planterflexedstate initially and then slowly pulled the exerciser’s string until
he felt his calf muscles stretched. He repeated this task threetimes for each condition defined by the combination of therelative height and external/internal-rotation levels between thefoot and exerciser. Three levels were set for the relative heightof the foot and exerciser. When the exerciser’s rotation axis
was “high,” the anatomical center of the ankle was nearlycoincident with the flexion axis of the exerciser. Under the“middle” and “low” conditions, the exerciser’s flexion axis waslocated 2 and 4 cm below the anatomical center at the initial
position of the task, respectively. Furthermore, three levels of
the external/internal rotation angle of the foot were adopted.
At each of these three levels, the foot either faced the front,
was slightly externally rotated, or was internally rotated.

anatomical rotation axis and machine’s flexion ankle. Fig. 4
shows these angles at the initial planterflexed position andthe final dorsiflexed position for each condition. When thefoot was externally rotated at the plantarflexed position, theanatomical flexion axis was nearly aligned with the exerciser’sflexion axis in terms of the external/internal rotation angle(middle in Fig. 4 (a)). When the foot was internally rotatedat the plantarflexed position, the anatomical flexion axis wasnearly aligned with the exerciser’s flexion axis in terms of theinversion angle (right in Fig. 4 (b)). The height level exhibited
virtually no effect on the results; however, these results are notconclusive because of the small sample size.
IV. C ONCLUSION
We investigated the alignment of the flexion axes of thehuman ankle and an exerciser when using an ankle stretchingmachine. It was demonstrated that either the rotation or inversion angle could be aligned between the two flexion axes bychanging the relative external/internal rotation angle between
them.

III. R ESULTS
We calculated the central positions of the three markers for the calf and the foot. Fig. 3 shows the exampleof the trajectories of these central positions, the center ofanatomical flexion axis, and the exerciser’s flexion axis. We
investigated the internal rotation angle and inversion angle of
the anatomical rotation axis when the ankle was dorsiflexedusing the exerciser. These angles were defined between the62

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Fig. 4. Rotation angle (a) and inversion angle (b) at the initial planterflexed(blue) and final dorsiflexed (orange) positions for each condition. L, M, andH indicate the relative height of the ankle; they are low, middle, and high,respectively. Error bars represent standard errors.

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