Change in Achilles Tendon Length after Walking on Treadmill with Gradient

ABSTRACT

Objectives: Improving ankle joint contracture is important because stiffness in ankle dorsiflexion can lead to pain, especially when weight-bearing during walking, which tends to concentrate on the forefoot. We hypothesized that the contraction of the gastrocnemius muscle in ankle dorsiflexion would increase the Achilles tendon length and improve the dorsiflexion range of motion. We evaluated the effects of walking with and without a gradient on Achilles tendon length.

Methods: This study included 23 men who underwent ultrasound imaging to measure the Achilles tendon length while they stood on an inclined table adjusted according to the dorsiflexion angle. Treadmill walking was performed for 10 min with a 10° incline (gradient condition) or without gradient (level condition). The measurements were compared using a paired t-test.

Results: In the gradient condition, the range of motion for ankle dorsiflexion was significantly increased after the intervention. In the gradient condition, the Achilles tendon length while standing on an inclined surface was significantly increased after the intervention.

Conclusions: Walking under gradient conditions led to the extension of the Achilles tendon in the ankle dorsiflexion position. This was accompanied by contraction of the gastrocnemius muscle, resulting in lengthening of the Achilles tendon. This finding suggests that such interventions may have clinical applications.

INTRODUCTION

Ankle dorsiflexion stiffness is associated with musculoskeletal disorders, such as plantar fasciitis, metatarsalgia, and hallux valgus. Patients with forefoot and midfoot pain have an average ankle dorsiflexion angle of 11.6° compared to 17.2° in individuals without pain.¹⁾ This stiffness shifts the weight-bearing area from the forefoot to the midfoot, leading to pain, in a condition called midfoot–forefoot overload syndrome.²⁾ In addition, common conditions such as diabetes and hypercholesterolemia contribute to reduced Achilles function, potentially increasing the risk of injury.^3,4) Therefore, there is significant need to understand factors affecting the flexibility of the Achilles tendon.

During walking, the lengths of the gastrocnemius muscle fibers do not change when pushing off the ground, because the power of the muscle is generated by the elasticity of the stretched Achilles tendon. The same principle is used during vertical jumping and running. Furthermore, flexibility of the Achilles tendon is important for ankle dorsiflexion during loading. Although tendon flexibility does not change before and after static stretching, dynamic stretching increases Achilles tendon flexibility.⁵⁾ When heel-raising exercises are performed on a single leg, the highest extension load on the Achilles tendon is obtained when the heel is descending.⁶⁾ Furthermore, hopping in the anterior–posterior direction and gastrocnemius contraction are known to have a lengthening effect on the Achilles tendon.⁷⁾ These aforementioned studies differ from those using conventional static stretching methods because they used gastrocnemius contraction to improve Achilles tendon flexibility. However, it is difficult for older people to perform stretching and hopping in dynamic situations; therefore, it is important to develop a variety of simple methods that can be tailored to each individual’s needs.

We hypothesized that walking on a graded treadmill could effectively improve the flexibility of the Achilles tendon. In this particular scenario, the gastrocnemius muscles contract during ankle dorsiflexion to improve the flexibility of the Achilles tendon, including the myotendinous transition zone. The purpose of this study was to examine whether walking in a graded environment would alter the Achilles tendon length.

MATERIALS AND METHODS

Participants

This study included 23 healthy adult male university students (mean age, 20.7 ± 0.4 years; mean height, 173.8 ± 4.8 cm; mean weight, 69.0 ± 7.5 kg) with no history of orthopedic disease in the lower extremities. Before participation, all participants read and signed an informed consent form that was approved by the Ethics Committee of the Kansai University of Health Sciences (approval No. 20–19). The measurements were performed in the university laboratory between April 10 and May 30, 2022. Using G-Power 3.19.7 software, the sample size was calculated as 23 participants, based on an effect size of 0.8, alpha error of 0.05, and power of 0.95.

Measurements

Range of Motion Test

Ankle joint range of motion was performed against resistance applied by a physical therapist to reach maximum dorsiflexion (knee joint extension and 30° flexion) and plantar flexion of the ankle joint. The ankles were photographed using a Video Analysis App (OnForm, USA) on a tablet (iPad; Apple, Cupertino, CA, USA), and the dorsiflexion and plantar flexion angles were calculated. The measurements were performed in the dominant leg. The foot used to kick balls was considered the dominant foot. The angle of the ankle joint was determined by using the line perpendicular to the fibula as the basic axis and the angle formed by the fifth metatarsal as the movement axis. The range of joint motion was measured in increments of 5° and the measured values were rounded to the adopted value.

Measurement of Achilles Tendon Length

Participants were instructed to stand on level ground with their feet set shoulder-width apart, toes pointing straight ahead, and one foot on one of the two scales. This stance was adopted to ensure equal load on the lower limbs. Next, participants were asked to grasp the wall bars in front of them at the level of their shoulders to prevent loss of balance. An ultrasound imaging system (SONIMAGE HS1; Konica Minolta, Japan) was used to measure the length of the Achilles tendon based on the ultrasound-tape (US-Tape) method.⁸⁾ The myotendinous transition area of the Achilles tendon and the medial head of the gastrocnemius was evaluated via B mode ultrasound imaging by positioning it on the center of the screen (Figs. 1, 2). A round sticker (diameter 0.5 cm) was placed on the skin at the center of the linear-type probe (frequency: 7.5 MHz), which was placed along the long axis of the lower leg. A measuring tape was used to measure the Achilles tendon length from the calcaneal ridge to the myotendinous transition of the medial head of the gastrocnemius muscle.⁸⁾ Then, the tendon length was measured while the individual was in the standing posture on an inclined table. The angle of the table was adjusted to match the maximum dorsiflexion angle of the ankle joint, as determined before the intervention. Measurements were taken in the standing position on level ground and in the standing position on the inclined table (Fig. 3). Each measurement was performed three times, and the average value was used for analysis.

Fig. 1.

Use of ultrasonography in measurement of Achilles tendon length on an inclined table. The muscle–tendon transition area of the medial head of the gastrocnemius muscle was positioned at the center of the screen and marked with a sticker at the height of the center of the probe on the lower leg.

Fig. 2.

Ultrasound image of the Achilles tendon. Hash symbol, gastrocnemius medial head; asterisk, Achilles tendon and gastrocnemius muscle-tendon transition zone.

Fig. 3.

Measurement of Achilles tendon length on the inclined table. A measuring tape was used to measure the Achilles tendon length from the calcaneal ridge to the myotendinous transition of the medial head of the gastrocnemius muscle.

Interventions

The intervention consisted of a 10-min walk on a treadmill with a 10° gradient (gradient condition) (Fig. 4). Prior to the task, the walking speed was adjusted to an arbitrary free walking speed for the first minute and gradually increased to a 10° gradient. In the comparison condition, walking was performed on a treadmill without a gradient. Prior to the study, the gradient and level conditions were randomly allocated based on a random number table. A washout period of at least 2 weeks was allowed to elapse between measurements. Following each treadmill walking intervention, the ranges of motion of the ankle joint in dorsiflexion and plantar flexion were measured using the methods used before the intervention. The Achilles tendon length was measured while the participants stood on an incline set at the same angle as that measured before the task. All measurements were obtained without informing the investigator regarding the treadmill walking conditions.

Fig. 4.

Treadmill walking interventions. Left, level condition; right, gradient condition.

Assessment

After each treadmill walking intervention, the range of motion of the ankle joint in dorsiflexion and plantar flexion was measured in the same manner as that before the intervention. The measurements were taken by assessing the Achilles tendon length while the participants were standing on an incline, which was adjusted to match the angle measured before the task. We measured the joint range of motion, including the maximum plantar flexion angle and the maximum dorsiflexion angle, as well as the Achilles tendon length before and after the intervention in both gradient and horizontal conditions. To measure the Achilles tendon length on the inclined table, we adjusted the dorsiflexion angle in increments of 5° to match the maximum dorsiflexion angle before the intervention.

Statistical Analysis

Measurements were compared between the gradient and level conditions. The measurements were evaluated for normality of distribution using the Shapiro–Wilk test and were compared using the paired t-test. In addition, the effect size (d) of the intervention was calculated. An intraclass correlation coefficient (ICC=1.1) was used to examine intraclass reliability. SPSS Statistics version 28 (IBM, Armonk, NY, USA) was used for data analysis. Statistical significance was set at 5%.

RESULTS

Walking Speed

The treadmill walking speed was 3.8 ± 0.2 m/s in the gradient condition and 3.9 ± 0.2 m/s in the level condition.

Range of Motion

Before intervention in the gradient condition, the range of motion for ankle dorsiflexion was 13.5 ± 5.1° in knee extension and 17.9 ± 4.8° in knee flexion, whereas that for plantar flexion was 41.8 ± 5.8°. After the intervention, the range of motion for ankle dorsiflexion was 20.6 ± 5.1° in knee extension and 26.3 ± 5.0° in knee flexion, whereas that for plantar flexion was 41.9 ± 6.8° (Fig. 5). Before intervention in the level ground condition, the range of motion for ankle dorsiflexion was 13.7 ± 5.3° in knee extension and 18.5 ± 5.9° in knee flexion, whereas that for plantar flexion was 43.4 ± 6.1°. After the intervention, the range of motion for ankle dorsiflexion was 14.5. ± 6.1° in knee extension and 20.1 ± 8.0° in knee flexion, whereas that for plantar flexion was 43.8 ± 5.2° (Fig. 6). After the gradient condition intervention, the range of motion for ankle dorsiflexion in both knee extension and flexion were significantly higher than before the intervention (knee extension: P<0.01, d=1.48; knee flexion: P<0.01, d=1.96).

Fig. 5.

Comparison of ankle joint range of motion (ROM) before and after the gradient condition intervention. The dorsiflexion range of motion of the ankle joint was significantly improved after the gradient walking intervention. Asterisk indicates significant difference (P<0.05).

Fig. 6.

Comparison of ankle joint range of motion (ROM) before and after the level condition intervention. There was no change in the range of motion of the ankle joint after the level walking intervention.

Achilles Tendon Length

Before the intervention in the gradient condition, the Achilles tendon length was 20.6 ± 1.5 cm while standing on a flat surface and 20.6 ± 1.6 cm while standing on an inclined surface. After the intervention in the gradient condition, the Achilles tendon length was 20.9 ± 1.6 cm while standing on a flat surface and 21.7 ± 1.5 cm while standing on an inclined surface. Before the intervention in the level condition, the Achilles tendon length was 20.5 ± 1.6 cm while standing on a flat surface and 20.1 ± 1.4 cm while standing on an inclined surface. After the intervention in the level condition, the Achilles tendon length was 20.6 ± 1.7 cm while standing on a flat surface and 20.6 ± 1.7 cm while standing on an inclined surface. Following the intervention in the gradient condition, the Achilles tendon was significantly longer than before the intervention (P<0.01, d=0.88) (Figs. 7, 8). The reliability of the ultrasound measurement of Achilles tendon length was confirmed by the relatively high values of ICC (0.84–0.92) (see Table 1).

Fig. 7.

Comparison of Achilles tendon length before and after the gradient condition intervention. The length of the Achilles tendon was significantly extended after the intervention when standing on an inclined surface. Asterisk indicates significant difference (P<0.05).

Fig. 8.

Comparison of Achilles tendon length before and after the level condition intervention. The length of the Achilles tendon did not change between standing on a flat surface and standing on an inclined surface.

Table 1. ICCs for ultrasound measurements

Circumstance	Standing on flat surface		Standing on inclined surface
	ICC (1.1)	95% CI	ICC (1.1)	95% CI
Before task in level condition	0.84	0.79–0.87	0.88	0.85–0.93
After task in level condition	0.92	0.9–0.94	0.87	0.84–0.92
Before task in gradient condition	0.89	0.86–0.94	0.91	0.86–0.94
After task in gradient condition	0.88	0.85–0.94	0.91	0.85–0.94

ICC determined for one-way random effects by single measure.

CI, confidence interval.

DISCUSSION

Restriction of dorsiflexion of the ankle joint is a known sequela post sprain or fracture of the ankle joint and surrounding foot joints. Limited dorsiflexion of the ankle joint may induce forefoot pain during movement or have adverse effects on other parts of the body, such as low back pain.⁹⁾ In this study, we focused on the gastrocnemius muscle, which is considered to be a factor of influence in the limitation of ankle joint dorsiflexion, and measured Achilles tendon length via ultrasound imaging. In a study comparing groups with and without forefoot pain, there was no significant difference in ankle dorsiflexion angle during knee flexion; however, a significant decrease in ankle dorsiflexion angle was noted in the group with forefoot symptoms during knee extension.¹⁰⁾ These findings suggest that forefoot pain may be strongly influenced by the gastrocnemius muscle, which is a polyarticular muscle. A study examining the limiting factors during lateral dorsiflexion of the ankle joint using shear wave elastography and ultrasonography reported that the medial head of the gastrocnemius muscle had the strongest limiting effect.¹¹⁾ Therefore, in the present study, we measured the Achilles tendon length from the myotendinous transition area of the medial head of the gastrocnemius muscle to the calcaneal ridge. Our results showed that walking on a graded treadmill extended the Achilles tendon length and improved the dorsiflexion range of motion of the ankle joint, while plantar flexion of the ankle joint was unchanged. In a previous study that evaluated the dynamics of muscle fibers and tendons during walking, it was reported that during the stance phase of walking, the gastrocnemius muscle maintained its length and acted isometrically, whereas the Achilles tendon was stretched when the gastrocnemius muscle contracted.¹²⁾ In another related study, when the ankle joint was passively dorsiflexed, both the gastrocnemius muscle belly and the Achilles tendon were extended, whereas the Achilles tendon was extended during isometric contraction of the gastrocnemius muscle in the dorsiflexed position, with the ankle joint directed toward plantar flexion.¹³⁾ Elongation of the Achilles tendon requires stretching of the muscle–tendon complex with increased muscle stiffness over the Achilles tendon. In addition, on walking, the contraction of the gastrocnemius muscles is reported to act more strongly on a treadmill with a gradient of 10° than with no gradient.¹⁴⁾ However, the activity of the medial head of the gastrocnemius muscle when walking on a level surface is no different from that at a gradient of about 4°, so it is considered that a gradient of about 10° is necessary.¹⁵⁾ Our results suggest that walking on a treadmill with a gradient may have increased the Achilles tendon length because of stretching of the Achilles tendon during ankle dorsiflexion, the force exerted while kicking off during the stance phase, and the traction exerted by the gastrocnemius muscle on the Achilles tendon. Consequently, there was no significant change in the range of motion for ankle plantar flexion. Previous studies have found that elongation for 2 min is required to increase Achilles tendon and muscle flexibility.¹⁶⁾ In this study, treadmill walking was performed for 10 min, with the ratio of stance phase to swing phase of 6:4. Assuming 5 min of walking on one leg, approximately 3 min of loading was performed for each lower limb. No significant difference was observed between the level and gradient walking conditions in terms of the stance time, stride length, or step distance.¹⁷⁾ Therefore, we speculate that temporal extension loads on the muscle–tendon transition complex, including the Achilles tendon, were obtained.

Achilles tendon length was recorded using the US-Tape method, which is a simple, reliable, and validated method based on three-dimensional ultrasound images.⁸⁾ The high intra-examiner reliability of our measurements confirmed that intra-examiner variability was not a significant concern. In a previous study of an autopsy specimen, a 1-cm extension of the Achilles tendon was associated with an average increase in the range of motion for ankle dorsiflexion of 12°.¹⁸⁾ This is in line with the results of the present study, in which elongation of the Achilles tendon by an average of 1.1 cm was associated with an average improvement in the range of motion for ankle dorsiflexion of 7.1°. While considering the differences between living organisms and specimens, these findings suggest that factors other than the Achilles tendon should also be evaluated for their potential effects on ankle dorsiflexion. The effect on the range of motion for ankle dorsiflexion was greater in the knee flexion position than in the knee extension position. The soleus muscle, a monoarticular muscle connected to the Achilles tendon, and the gastrocnemius muscle, a polyarticular muscle, have different muscle bundle behaviors and functional characteristics during physical exercise.^19,20) Treadmill walking in uphill conditions increased hip and knee flexion angles during the stance phase and increased the forward tilt of the thorax during the entire gait cycle.²¹⁾ These findings suggest that the gradient condition can increase the extensibility of single-joint muscles, including the soleus muscle.

The main limitation of this study is that we focused on the immediate posterior effect of treadmill walking on the extension of the Achilles tendon. In addition, the effects of the intervention on patients with ankle dorsiflexion limitation were not examined. Future studies should examine the long-term effects of interventions in older adults with ankle dorsiflexion limitation to determine whether ankle joint function can be improved.

CONCLUSIONS

Walking on a graded treadmill extends Achilles tendon length during loading and improves ankle joint range of motion.

ACKNOWLEDGMENTS

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

CONFLICTS OF INTEREST

The authors declare no conflict of interest.

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