Variability in the application of eccentric force using different rotary inertia devices may influence the treatment of tendinopathy

1 Núñez, F. J., Galiano, C., Muñoz-López, A., & Floria, P. (2022). Variability in the application of eccentric force using different rotary inertia devices may influence the treatment of tendinopathy. Journal of Sport Rehabilitation, 31(2), 235–238. https://doi.org/10.1123/jsr.2021-0062 Variability in the application of eccentric force using different rotary inertia devices may influence the treatment of tendinopathy Francisco Javier Núñez, Carlos Galiano, Alejandro Muñoz-López, and Pablo Floría Abstract High intra-set variability has been considered as a potential aid in the treatment of tendinopathy by producing forces in variable and unpredictable contexts that allow the athlete to return to sport pain-free. The aim of this study was to compare the intra-set variability in force profiles between different rotational inertia devices (RIDs) during concentric (CON) and eccentric (ECC) phases of movement, and between different moments of inertia. Thirty-nine men performed a half-squat incremental test on two different RIDs: a horizontal cylinder (YY) and a vertical cone-shaped axis (CP). Intra-set variabilities in vertical force and velocity were analyzed using average coefficients of variation. RID squat exercise produced force intraset variability. The ECC phase of the movement showed more intra-set variability in force output than the CON phase. ECC CP showed a higher intra-set variability in force than ECC YY. This study demonstrated that using an RID to provide resistance in squat-training of athletes produced a high intra-set variability in the application of force. Key words: Strength, muscle activity, knee, Biomechanics 2 Introduction Strength training programs, and particularly those emphasizing eccentric muscle actions, have been considered as potential aids in the treatment of tendinopathy 1, which is why there has been increasing interest in the use of rotational inertia devices (RIDs) in the tendinopathy rehab process 1-4. RIDs are designed to use the moment of inertia of a rotating flywheel to provide a maximal resistance overload during concentric (CON) and eccentric (ECC) phases of movement 5. These systems provide a source of linear resistance via a tether wrapped around a horizontal cylinder-shaped (YY) 6 or a vertical cone-shaped shaft (CP) 7. The kinetic energy from the CON phase of the exercise is transferred to the ECC phase, where an equal impulse is necessary to halt the rotation of the moment of inertia 7. To be efficient using an RID, the athlete tends to apply force at maximal velocity during the CON, and halt the rotation during the ECC to produce force enhancement in the subsequent CON phase 5. When the athlete tries to halt the rotation during the ECC phase, they can do so by increasing the damping time in the eccentric phase (i.e. ECC load < CON load), equalizing the time of the CON and ECC phases (i.e. ECC load = CON load), or reducing the time of the ECC phase with respect to that of the CON phase (i.e. ECC load > CON load) 8. Some authors thus argue that during an RID exercise, the ECC phase of the movement has higher levels of freedom than the CON phase 9. During tendinopathy rehab, the tendon capacity must be gradually loaded to reduce pain and improve function, allowing the athlete to return to sport pain-free 10. Increases in muscle force were noted with the use of RIDs over short periods of training 6, 11, and the use of RIDs showed significantly higher ECC force values compared to free weights 7. Understanding the concept of variability as the normal variations that occur in motor performance across multiple repetitions of a task 12, several studies support the use of RIDs to offer a high variability in force production during an exercise 9, 13. Since repetitions of a sports movement are never identical, 3 some degree of motor variability is beneficial as it allows a more adaptive system with regard to internal and external perturbations of the context 9, 21. However, this variability could made it difficult to offer a gradual loading to the tendon, specially at the beginning of the tendinopathy rehab process 10. Most movements in the field require players to produce forces in variable and unpredictable contexts, with an emphasis on the eccentric (ECC) component 14, so it is possible that a high level of variability could help athletes during the end phase of the tendinopathy rehab process before return to play. However, it is known that very high levels of variability can cause undesired adaptations 15, and despite the widespread use of RIDs in tendinopathy rehab, the level of variability in force and velocity provided during exercise is still unknown. A recent study observed that using YY or CP influences the magnitude of the eccentric peak force, and that using a CP offers less resistance to acceleration in the CON phase and to deceleration in the ECC phase compared to a YY, despite using the same moment of inertia in both devices 16. To the best of our knowledge, no studies have compared the movement variability produced in YY and CP, or determined whether its magnitude remains constant during the CON and ECC phases of the movement, or with different moments of inertia. Consequently, the purpose of this study was to compare the intra-set variability in vertical force profiles between the CON and ECC phases of the movement, in YY and CP devices, and between different moments of inertia. It was hypothesized that intra-set variability is higher in the ECC than the CON phase of the squat exercise in both devices, and the intra-set variability in force values output is higher in CP than YY, with no difference between different moments of inertia. 4 Methods Participants Thirty-nine active men were recruited to participate in this study (mean ± SD; age: 22.9 ± 3.6 years, height: 1.75 ± 0.06 m, and weight: 74 ± 10.4 kg). The study was conducted according to the Declaration of Helsinki, and the protocol was fully approved by the CEI de los Hospitales Universitarios Virgen Macarena y Virgen del Rocío ethics committee (0398-N-17) before recruitment. After a detailed explanation of the aims, benefits, and risks involved in this investigation, all participants gave written informed consent. Procedure The experimental procedure consisted of the assessment of force during a half-squat incremental test performed on a conical inertial device (CP; Costa Mesa, CA, USA) with diameters of the base and vertex of 0.1 m and 0.03 m respectively, and on a cylinder inertial device (YY; YoYo™ Technology AB, Stockholm, Sweden) with a diameter of 0.02 m (see Figure 1). After a 7- to 8-min standardized general warm-up, including jogging, joint mobility exercises and two sub-maximal sets of 7 reps of a half-squat exercise in each RID, 3 sets of 7 maximal repetitions with three different moments of inertia were performed on the CP (CPi0.11, CPi0.22 and CPi0.33 kg/m2) and on the YY (YYi0.11, YYi0.22, YYi0.33 kg/m2), with 3 min of recovery between sets. The exercise required the participants to wear a harness fastened at both shoulders and waist. To offer the same resistance to movement in both systems, the same numbers of pulleys were used, except in the CP, which because of its arrangement needed an extra pulley to move the participant away from the 0.40 m array. The execution of the squat was identical in both systems (see Figure 1). The tension of the tether was adjusted while maintaining both legs in extension. The movement was initiated by winding the tether until reaching 90° of knee flexion. The participant used two repetitions to increase the velocity. Thereafter participants were requested to apply force throughout the course of the full upward 5 movement at maximal velocity and then resist during the final downward movement, aiming to bring the device to a stop at a ~90º knee angle before initiating the next cycle 17. The tests were performed on two different days with a minimum separation of 72 h, and the order of the tests was randomized for each subject. Figure 1. Half squat with CP (A) and with YY (B) Vertical ground reaction force data were recorded using two force platforms (SmartCoach Europe AB, Stockholm, Sweden), with the participant standing with one leg on each platform. Vertical velocity was measured with a linear encoder (Smartcoach Power Encoder, SmartCoach Europe AB, Stockholm, Sweden) attached to the harness worn by the participant. Vertical velocity and vertical force data were sampled at 100 Hz and synchronized using an analog-to- digital converter (SmartCoach Europe AB, Stockholm, Sweden). The vertical displacement was derived by integrating the vertical velocity. 6 To calculate the variability, continuous methods were used. Using the time series technique, the intra-set variability of the force-time curves from all participants and sets were calculated. This technique was applied as follows. Vertical ground reaction force time-series data for each set were segmented into 7 maximal repetitions based on the time course of the vertical velocity. A maximal repetition was defined as the event between the initiation of upward movement (concentric phase) and the end of downward movement (eccentric phase) during half squat execution. A positive vertical velocity indicated an upward movement while a negative vertical velocity indicated a downward movement. Vertical force data for each maximal repetition were normalized to 101 points using a cubic spline interpolation technique. This enabled all forcetime waveforms to be expressed over normalized periods (percentage time), so that individual data could be aligned. Subsequently, an ensemble average curve from the 7 reps for each participant and set, as well as the mean and standard deviation of each data point on the average curve, were calculated. Finally, the average standard deviation and average coefficient of variation for each concentric and eccentric phase were calculated separately using Equations 1 and 2. 𝑆𝐷𝑎𝑣𝑔 =(Σ𝑖=1 𝑘𝑆𝐷𝑖2 𝑘)1/2(1) 𝐶𝑉𝑎𝑣𝑔 =𝑆𝐷𝑎𝑣𝑔 ∑ |𝑀𝑖| 𝑘 𝑖=1 𝑘·100 (2) For the equations, SDavg is the average of individual point-by-point standard deviation values, i indicates the specific value for the ith sample, SD is the standard deviation value for the ith sample, and k is the number of samples. CVavg is the average of individual point-by-point coefficient of variation values, and Mi is the mean for the ith sample Statistical analyses 7 We show the data as mean ± SD. Before conducting any analyses, we tested the data distribution using a Shapiro-Wilk test of normality. We tested the possible differences in all the variables using a repeated-measures design, using a three-way within-subjects ANOVA test and all the possible interactions resulting from combining the RIDs (CP-YY), the movement phase (CONECC) and the moments of inertia used (Resistance). Also, we tested the main effects of those factors. Finally, we performed a post-hoc analysis for pairwise comparisons with a Bonferroni adjustment. We set the significance level at p<0.05. Finally, we calculated the eta partial squared (η²p) and the Cohen’s d as effect sizes, for interactions and pairwise comparisons, respectively. For all the analyses, we used the statistical JASP software (JASP Team, 2020, v.0.13.1) for Windows. 8 Results The results of the intra-set variability in force analysis are shown in Figure 2. Only RID x Phase showed a significant interaction (η²p= 0.309; p< 0.001). As main effects, Phase (η²p= 0.785; p< 0.001) and Resistance (η²p= 0.161; p= 0.001) showed significant differences. Post-hoc analyses showed a significantly higher variability in ECC compared to CON for both devices (CP mean difference= 6.87 %, d= 1.903, p<0.001; YY mean difference= 3.99%, d= 1.104, p<0.001). Finally, a higher variability was found in ECC for CP compared to YY (mean difference = 2.25 %, d= 0.599; p= 0.002) Figure 2. Intra-set variability in application of force during a squat exercise in vertical coneshaped shaft conical pulley (CP) and horizontal cylinder-shaped (YY) with different moments of inertia. White box correspond to concentric phase, grey box correspond to eccentric phase of the movement. Discussion The purpose of this study was to compare the intra-set variability in vertical force profiles between the CON and ECC phases of the movement, YY and CP, and between different moments of inertia. It was hypothesized that the intra-set variability is higher in the ECC than the CON phase of squat exercise in both devices, and the intra-set variability in force values output are higher in CP than YY, with no differences between different moments of inertia. The main findings of this study were: a) the RID squat exercise produced intra-set variability in force; b) the ECC phase of the movement showed higher intra-set variability than the CON phase in vertical force values output; and c) during the ECC phase, CP showed a higher intraset variability in force than YY. 9 To the best of our knowledge no studies to date have described the intra-set variability in vertical force using CP and YY. However, our results are in agreement with previous research where the intra-trial variability of force production was analyzed using non-linear methods. The authors observed a high force variability during the CON phase in CP squat exercise with a moment of inertia of 0.27 kg/m2 9, 18. In our study, to offer the same resistance to movement in CP as in YY, the athlete accelerated-decelerated the inertial load from a pulley in both systems (see Figure 1) and the force was recorded by a force platform, while Moras et al. 18 moved the CP inertial load from a tether and the force was recorded by a strain gauge. This implies that in our study the resistance to moving the same moment of inertia was significantly greater, but we also found a high variability in both devices. Moras et al. 18 concluded that increases in CP CON force did not accompany increases in variability, and we found no difference between CP CON and YY CON with regard to intra-set variability in force. Moras et al. 9, in another study, showed there was acceleration variability during the CON-ECC phases in CP exercise with a moment of inertia of 0.27 kg/m2. Moras et al. 9 found unclear differences between the variability of forward movements (i.e. CON) and backward movements (i.e. ECC). These studies are not comparable since they used different exercises (unilateral three steps horizontal vs bilateral squat), in a different plane of movement (horizontal vs. vertical), different resistance to the pull of the inertial load (tether vs. pulley), different variables analyzed (acceleration vs force) and the use of different instruments (accelerometer vs force platform). Disagreeing with the results of Moras et al. 9, our study confirmed our first hypothesis and indicated that the ECC phase of the movement involves more intra-set variability than the CON phase in force output during a basic tendinopathy rehab exercise like the squat. This result may