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|a Muscle structure and function are important to quality of life and physical performance. With eccentric and isometric resistance training established for improving muscle size and strength, eccentric quasi-isometric (EQI) contractions, defined as "holding a position until isometric failure and maximally resisting the subsequent eccentric phase", are the focus of this thesis. The primary aims were to answer the overarching research question: "What are the acute, and long-term effects of EQI loading on muscle form and function?". Systematic and narrative reviews were conducted, followed by the evaluation and optimization of testing methods, culminating in acute and short-term experimental studies. Reviews of the literature established that isometric training at longer muscle lengths produced greater hypertrophy than volume-equated shorter muscle length training (0.1-1.0%·week-1, effect size (ES) = 0.05-0.2·week-1), and transferred better to full range of motion (ROM) performance. Ballistic intent resulted in greater increases in rate of torque development (RTD) and neuromuscular activation (-1.5-3.6%·week-1, ES = 0.03-0.28·week-1). Hypertrophy and strength improvements were not related to isometric training intensity, however, contractions ≥ 70% were likely required to improve tendon qualities. While there is a lack of studies directly examining EQIs, they may provide a practical means of increasing metabolic and hormonal factors, while safely applying large quantities of mechanical tension. EQI training appears to be effective for improving musculotendinous morphological and performance variables with low injury risk. To be confident in the primary findings of the PhD, it was important to test and determine an optimized assessment battery for the acute and long-term effects and adaptations to EQI loading. Repeated between-day testing determined that ultrasound derived muscle thickness (MT) and subcutaneous fat corrected echo intensity (EI) had low variability in all quadriceps muscles and regions. Pennation angle (PA) and extended field-of-view fascicle length (FL) could only be reliably assessed in the vastus lateralis. Concentric torque and impulse were reliable between 90-20° of knee-flexion. Maximal voluntary isometric torque (MVIT), and RTD and impulse from 0-200 ms can be confidently assessed regardless of joint angle. Correlational analysis revealed that the isometric length-tension relationship was minimally associated with regional architecture and that the middle and distal architecture were the strongest predictors of MVIT. When comparing impulse-equated bouts of EQI and isokinetic eccentric loading (ECC), physiological responses were similar in 21/56 variables. EQIs resulted in greater vastus intermedius swelling (7.1-8.8%, ES = 0.20-0.29), whereas ECC resulted in greater soreness at the distal and middle vastus lateralis and distal rectus femoris (16.5-30.4%, ES = 0.32-0.54) and larger echogenicity increases at the distal rectus femoris and lateral vastus intermedius (11.9-15.1%, ES = 0.26-0.54). Furthermore, ECC led to larger reductions in concentric (8.3-19.7%, ES = 0.45-0.62) and isometric (6.3-32.3%, ES = 0.18-0.70) torque and RTD at medium-long muscle lengths. There were substantial differences in the number of contractions required to impulse-match the conditions (ECC: 100.8 ± 54 vs EQI: 3.85 ± 1.1). Mean contraction velocity over four contractions was 1.34º·s-1 with most (62.5 ± 4.9%) impulse produced between 40-70º. Most between-contraction changes in total angular impulse, contraction velocity, and time-under-tension occurred between 30-50º (ES = 0.53 ± 0.31, 60 ± 52%), while kinetics and kinematics relatively constant between 50-100º (ES = 0.10 ± 0.26, 14.3 ± 24.6%). Findings suggest that EQI loading could be an alternative to traditional resistance-training, possibly for individuals suffering from, or susceptible to musculoskeletal injury. Practitioners could shift the loading distribution to longer muscle lengths by prescribing a greater number of contractions, reducing rest periods, or implementing EQI contractions towards the end of a traditional training session where fatigue may be present. Although extensive future research is required to understand underlying mechanisms and long-term adaptations, the thesis provided novel and original information on the biomechanics and physiological effects of EQI loading. With the benefits of time-efficiency and minimal negative effects, EQI training is likely best applied in rehabilitation, general preparatory, unloading, or transition periods of the periodized plan.
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