title: The Biomechanical Underpinnings and Subsequent Physiological Adaptations of Accentuated-Eccentric Loading in Strength-Trained Males
Creators: name: Armstrong, R
Publisher: name: Liverpool John Moores University
Year: 2024

description: Resistance training is a well-documented practice that has been shown to increase the physical performance of athletic populations. Vast bodies of research, which have resulted in hundreds of publications, have explored the concentric phase of resistance training, for which programming recommendations for applied practice have been generated. However, there are far fewer studies which have examined the effects of the eccentric phase of resistance training, particularly in multi-jointed movement patterns such as the squat. To date, few recommendations exist on how to programme eccentric resistance training, which is further hindered by the dearth of research which have examined the underpinning biomechanical properties of the eccentric phase of squatting. To this end, the primary aim of this thesis was to examine the movement dynamics of the eccentric phase of the squat, and to establish whether modifying the loading parameters during the eccentric phase of the squat elicited superior adaptations compared to traditional loading paradigms. In a collection of initial exploratory studies presented in this thesis, a novel ‘smart-resistance training’ device (Kineo Training System) that facilitates loading modification of the eccentric phase of the squat was evaluated. Several exploratory studies were undertaken to assess different components of the Kineo including; the assessment of movement dynamics compared to barbell squatting, the assessment of load and velocity compared to barbell squatting, and the assessment of the reliability and validity of the Kineo isovelocity mode. Data revealed no significant differences for squatting kinematics between squats performed on the Kineo or with a barbell. However, greater loads were capable of being lifted with the barbell (~9%). Assessment of the isovelocity mode identified a small but consistent bias in the prescribed velocity (~0.01 m·s-1), but high reliability (coefficient of variation 1.6 to 4%; intraclass coefficient 0.99), with a mean variance of 0.01 m·s-1 between repetition to repetition. Therefore, it was concluded that the Kineo can accurately and reliably facilitate squatting in a similar manner to that of a barbell, but with the added benefit of safely modifying the load in the eccentric phase, thus making the Kineo ideal to assess the primary aims of this thesis. In the first primary study of this thesis, the force-velocity relationship as expressed during squatting was assessed. 15 strength-trained males performed maximal effort isovelocity squats at three concentric, and three eccentric velocities, whilst ground reaction forces were measured. The force-velocity relationship conformed in shape to pre-existing single-joint relationships. However, the magnitude of eccentric force in relation to estimated isometric force (1.1 times) was lower than what is typically seen in single-joints. There was also a large variance between individuals in this magnitude, which was not influenced by the participants concentric performance. This novel data is the first to explore the force-velocity relationship in the squat for both the concentric and eccentric phase. In the second primary study of this thesis, movement dynamics and muscle activity of the lower limbs were assessed during squatting. Nine strength-trained males performed squats with concentric loads of 20 to 100% of 1RM, and with eccentric loads of 20 to 150% 1RM. When equal load was used in the concentric and eccentric phase, concentric joint moments were always greater than eccentric joint moments, with the 80% concentric trial (2.19 N·m·kg-1) producing a significantly greater knee joint moment than the 80% eccentric trial (1.85 N·m·kg-1). Only with the use of accentuated-eccentric loading could this difference be negated, with no further significant increases in eccentric knee joint moment past an eccentric load of 120% (2.16 N·m·kg-1). This increase in joint moment was not accompanied by any increase in vastus lateralis muscle activity. There was no effect of accentuated-eccentric loading on hip or ankle joint moments. In the final study of this thesis, a 6-week resistance training intervention was conducted which compared traditional resistance training loading paradigms to accentuated-eccentric loading, with the protocol based off the data collected in the previous study. 22 strength-trained males partook in the intervention. Findings indicated that all participants improved their isometric and concentric knee extensor strength (~6-12%) but only those who performed accentuated-eccentric loading displayed improvement in eccentric knee extensor strength (~8-16%). This improvement was accompanied by an increase in eccentric vastus lateralis muscle activity. The accentuated-eccentric loading groups also saw a significantly greater improvement in squat 1RM (~15kg) comparted to the traditional group (~9kg). Small, but statistically significant increases in vastus lateralis muscle thickness was observed across all three groups, with no differences between groups. Although DOMS and RPE were greater initially following accentuated-eccentric loading, these were attenuated to the level of traditional resistance training following eight training sessions. These novel data indicate that accentuated-eccentric loading can be utilised to elicit eccentric-specific strength adaptations, whilst enabling superior/equivalent improvements in strength compared to traditional methods. This thesis has provided novel data that has expanded resistance training research, with a particular focus on the eccentric phase of the squat. Two studies have been conducted which will help practitioners and researchers better understand the underpinning biomechanical factors that influence eccentric resistance training. Whilst the final study has provided a scientifically justified approach to accentuated-eccentric squatting that has been shown to elicit adaptations which will be of benefit to certain athletic populations.