A convenience sample of forty individuals were selected and divided into two groups: surgical (SU) group and non-surgical (NS) group. The estimated sample size was calculated considering a statistic power of 80%, a probability of error type I of 0.05, and a moderate effect size (0.4).13 The SU group consisted of 20 participants with a history of unilateral rupture of the Achilles tendon, at least 12 months previously, with suture of the tendon using the Kessler and Bunnell technique (i.e. without any tendon transfer), and a history of completion of a rehabilitation program. The NS group consisted of 20 asymptomatic individuals without a history of rupture in the Achilles tendon. Exclusion criteria for both groups included current presence of musculoskeletal injuries in the lower limbs and any discomfort that could limit the performance of the tests. None of the participants were excluded. The sample characteristics are shown in Table 1. This study was approved by the Research Ethics Committee of the Mater Dei Hospital, Belo Horizonte, Minas Gerais (Number CAAE 0012.0.170.000-09). All participants signed the informed consent prior to participation.

Initially, anthropometric measures, such as body mass and height using a digital scale with altimeter (Filizola® SA, São Paulo, Brazil), were obtained. Following, clinical testing of passive ankle stiffness14 and evaluations of the plantar flexors performance with the isokinetic dynamometer (Biodex System 3 Pro Biodex Medical Systems Inc., Shirley, USA)15 were then conducted. Testing was done to both limbs of all subjects.

The passive ankle stiffness was evaluated through a clinical test.14 This test had been validated with direct measurement of passive stiffness using the isokinetic dynamometer in a previous study.14 Therefore, it could be used as a clinical test to index the passive stiffness of the ankle.14 To perform this clinical test, the protocol recommended by Araújo et al.14 was used. Each participant was positioned in prone with the knee flexed at 90° for each leg to be tested.14 In this position, markers were placed on the fibular head and lateral malleolus, and then a line was drawn using a ruler joining these two points, which afterwards served to align the fixed arm of the goniometer (Fig. 1A).14 Then, the evaluator conducted a viscoelastic accommodation of the periarticular tissues of the ankle obtained through a series of five sequential movements of dorsiflexion to its maximum amplitude.14 Next, an ankle weight of 2kg was placed 8cm from the lateral malleolus to generate tension in the plantar flexors (Fig. 1A).14 The use of an ankle weight was a procedure recommended by the authors who had validated the test, since the torque produced only by the mass of the foot was insufficient to tension the plantar flexors (i.e. the position of the foot could remain unchanged even with the action the gravity favoring dorsiflexion).14 The position taken by the ankle joint immediately after placing the ankle weight was measured using the goniometer (CARCI®, São Paulo, Brazil). The stationary arm of the goniometer was aligned with a line joining the lateral malleolus to the fibular head, and the moveable arm was parallel to the surface of the foot (Fig. 1A).14 Three measurements were performed on each ankle joint, starting with the participant's preferred lower limb. Each participant was asked to avoid contracting the muscles in the lower limb being investigated so that there was minimal or no influence on the measurement.14 The evaluator palpated the tibialis anterior, triceps surae, and hamstrings muscles during the test to verify the lack of muscular contraction.14 If the participant was contracting any of these muscles, the test was discarded and repeated. This test was performed by a single examiner who showed excellent test–retest reliability16 (intraclass correlation coefficient – ICC3,2=0.84 (95% CI=−0.14 to 0.98) standard error of measurement – SEM=2.73), evaluated in a pilot study with 10 volunteers conducted one week apart.

Figure 1.

(A) Clinical test of ankle passive stiffness: Participant was positioned in prone with the knee flexed at 90°. An ankle weight of 2kg was placed 8cm from the lateral malleolus. The position taken by the ankle was measured using the goniometer (the moveable arm was aligned parallel to the surface of the foot). (B) Evaluation of ankle plantar flexor performance at the isokinetic dynamometer.

(0.15MB).

Initially, all volunteers warmed up by walking at a self-selected fast speed on the ground for five minutes. Next, they seated in the chair of the isokinetic dynamometer, with belts over the trunk, pelvis, and thigh for stabilization. The seat back tilt was set at 70° and each participant's distal thigh was supported over the device's limb-support pad so that the knee remained flexed between 30° and 40°. This range was assured by the evaluator using the goniometer. The dynamometer's axis of rotation was aligned with the lateral malleolus and the barefoot was strapped to the footplate of the ankle attachment of the isokinetic dynamomater so that the plantar surface of the foot was fully supported on this base of support that was attached to the dynamometer (Fig. 1B).

The protocol17,18 consisted of concentric and eccentric evaluations of the plantar flexors muscles, within a range of 10° of dorsiflexion and 20° of plantar flexion, repeated five times at a 30°/s velocity. Initially, the participants were familiarized with the system by performing five repetitions using submaximal contractions.19 The assessment was initiated with the non-surgical lower limb of the participants in the SU group, and with the dominant lower limb (i.e. the limb preferred when kicking a ball) of the participants in the NS group. This order was established to ensure security to the volunteers during the test, especially for the SU group. During the test, participants were instructed to perform with maximal force. Standardized verbal encouragement was provided by the researcher to ensure maximum force generated by the subjects.20

The reliability and validity of the isokinetic dynamometer had been demonstrated in the literature.15 In addition, the test using the isokinetic dynamometer was conducted by only one evaluator with extensive experience with the use of the equipment. The test–retest reliability of the protocol was determined in a pilot study using 10 volunteers, evaluated at an interval of one week apart that showed excellent16 reliability results for the outcome variables studied (ICC3,2=0.92 to 0.94; (95% CI=0.74–0.98) SEM=5.85–12.05).

Initially, to enable data analysis, the output from the lower limbs of the participants from both groups were paired such that: the non-dominant lower limb data of the NS group were considered as corresponding to the surgical lower limb data of the SU group; and the dominant lower limb data of the NS group corresponding to non-surgical lower limb data of the SU group.

The variable passive stiffness of the ankle was determined in degrees from the average of the three values obtained for each lower limb through the clinical passive stiffness test. In addition, the absolute asymmetry of this variable was calculated from the difference between the lower limb side with higher value of passive stiffness and the lower limb side with lower stiffness value.

The performance of the plantar flexor muscles was analyzed using the concentric and eccentric peak torque normalized by body weight, as well as the concentric and eccentric maximum work performed in one repetition, also normalized by body weight. In addition, the absolute asymmetries of the peak torque and maximum work were analyzed. The asymmetry values were obtained from the report of the isokinetic dynamometer software, but without considering the direction (absolute asymmetry).21,22 The asymmetry calculation was performed according to the equation: Asymmetry=((Non-surgical or dominant side−Surgical or non-dominant side)/Non-surgical or dominant side)×100.

The assumptions of normality and homogeneity of variance were checked and confirmed prior to the inferential tests. Mixed analysis of variance (ANOVA) with one independent factor (SU and NS groups) and with one repeated measure factor (non-surgical/dominant lower limb and surgical/non-dominant lower limb) was used to analyze the effects of group, lower limb and the interaction effect of lower limb×group on the passive stiffness of the ankle. Independent t-test was performed to verify if the absolute asymmetry of passive stiffness was different between groups.

Multivariate analysis of variance (MANOVA) with a mixed model was used to evaluate the effect of group, lower limb, and the interaction effect of group×lower limb on the muscle performance variables (i.e. concentric peak torque, eccentric peak torque, maximum concentric work and maximum eccentric work of the plantar flexors of the ankle normalized by body weight). In addition, MANOVA was also conducted to verify the effect of group on the variables of absolute asymmetry of muscle performance. One-way ANOVA were used to identify possible differences observed by both MANOVA. For all the analyses presented, a type I error probability of 0.05 was considered. All data were analyzed using the software IBM SPSS version 20.0 (IBM Corp.).

The ANOVA revealed no main group effect in the passive ankle stiffness (F1,38=1.760, p=0.19; η2=0.044; mean difference=2.190; 95% CI −1.16 to 5.54). However, a main lower limb effect (F1,38=8.129, p=0.01, η2=0.176; mean difference=2.130; 95% CI=0.62–3.64) was observed in which the passive stiffness was reduced (i.e. greater angular displacement) in the surgical/non-dominant lower limb (9.0±0.9°) compared to the non-surgical/dominant lower limb (6.8±1.0°). In addition, there was an interaction effect lower limb×group (F1,38=4.937, p=0.03; η2=0.115). Four contrasts were conducted to investigate this interaction effect. The surgical lower limb of the SU group presented reduced stiffness compared to non-surgical lower limb (t19=3.096, p=0.01; mean difference=3.79, 95% CI=1.23–6.35). The surgical lower limb of the SU group presented also reduced stiffness compared to non-dominant lower limb of the NS group (t38=−2.212; p=0.03; mean difference=−3.86; 95% CI=−7.38 to −0.33). The non-surgical lower limb of the SU group was not different from the dominant lower limb of the NS group (t38=−0.284; p=0.78; mean difference=−0.54; 95% CI=4.35–3.28). In addition, the dominant lower limb of the NS group was not different from the non-dominant lower limb of the NS group (t19=0.548, p=0.59; mean difference=0.47; 95% CI=1.32–.26).

The independent t-test showed a difference between groups in the absolute stiffness asymmetry of the ankle (t38=−2.689; p=0.01; mean difference=−2.630; 95% CI=−4.61 to −0.65). The SU group (5.6±3.6°) was more asymmetric than the NS group (2.9±2.4°).

The MANOVA showed no main group effect (F4,35=1.943, p=0.13, η2=0.182; range of mean differences=2.088–28.345), lower limb effect (F4,35=1.989, p=0.12; η2=0.185; range of mean differences=2.372–10.615), or interaction effect of lower limb×group (F4,35=0.317, p=0.87, η2=0.035) on the muscular performance variables of the plantar flexors. The mean and standard deviation of the muscular performance variables of the plantar flexors, according to the group and lower limb, are shown in Table 2.

In relation to the asymmetry variables of muscle performance, the MANOVA identified differences between the SU and NS groups (F4,35=4.258, p=0.01, η2=0.327). The SU group was more asymmetric than the NS group in the absolute asymmetry variables of concentric work (F1,38=7.010, p=0.01, η2=0.156) and eccentric work (F1,38=7.022, p=0.01, η2=0.156). There was no difference between groups for the absolute asymmetry variables of concentric torque (F1,38=3.478, p=0.07, η2=0.084) and eccentric torque (F1,38=2.003, p=0.17, η2=0.050). The mean, standard deviation, mean difference, and confidence intervals of absolute asymmetry variables are shown in Table 3.