**How Deep Should You Squat to Maximise a Holistic Training Response? Electromyographic, Energetic, Cardiovascular, Hypertrophic and Mechanical Evidence**

Gerard E. McMahon, Gladys L. Onambélé-Pearson, Christopher I. Morse, Adrian M. Burden and Keith Winwood

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/56386

**1. Introduction**

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Skeletal muscle possesses the ability to change its structural and mechanical characteristics in response to its external environment (i.e. it is adaptable). The exact nature of such adaptations is manipulated by, amongst other things, the mechanical stimulus provided to the said muscle. Resistance exercise is an example of one such stimulus, and is used in a variety of settings, such as athletic performance, general health and fitness, injury prevention and rehabilitation. It is also now commonplace for resistance exercise to be used to offset the debilitating effects of illness, disease and sarcopenia (the latter being a term used to describe the age-related loss in muscle mass, which is also accompanied by increased fatty tissue infiltration and the ensuing decrement in muscle 'quality'). The objectives of the resistance exercise protocol therefore, will vary due to the unique nature of each setting, and therefore should be optimised in order to bring about a specific and desirable set of adaptations. Frequent adaptations that are sought from resistance exercise regimes include an increase in muscle cross-sectional area (CSA) and strength [1], alterations to muscle architecture (spatial arrangement of muscle fibres within a muscle [2]), and greater maximal activation of the musculature [3].

Muscle activation has been widely assessed using surface electromyography (SEMG), and in many cases is expressed as a relative level (%) of maximal voluntary contraction (or MVC). It comprises the sum of the electrical contributions made by the active motor units in proximity to the measurement site. The global characteristics of the surface EMG, such as its amplitude and power spectrum, depend on the membrane properties of the muscle fibres as well as on

© 2013 McMahon et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 McMahon et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

the timing of the single fibre action potentials. Thus the surface EMG reflects both peripheral and central properties of the neuromuscular system [4]. For many muscles, optimal firing rate, which is that elicited by a maximal voluntary effort, is sufficient to generate a fused tetanus in individual motor units. In predominantly fast-twitch muscles (e.g. biceps brachii), this firing rate is ~30Hz whereas in predominantly slow-twitch muscles (e.g. soleus), this firing rate is ~10Hz [5]. This electromyographic signature is warranted in order for the muscle to express its maximal force generating capabilities, and there have been many studies carried out that have reported a significant increase in agonist SEMG recordings following a resistance training program in both males and females, and in the young as well as the elderly [3, 6-12]. As mentioned previously, muscle adapts in a specific manner to the stimulus provided, and in the case of the aforementioned studies, increases in agonist muscle activation has been shown to be specific to the mode of muscular contraction employed during the resistance training period, and has been fairly well characterised. It is however unclear whether chronic changes in the magnitude of the EMG signal occur with training.

Previous investigations have also identified the link between muscle length (or joint-angle) and gains in strength and/ or levels of muscle activation following more extended periods of resistance training [17-21]. Briefly, these studies have shown that significantly greater increases in isometric strength are attained when tested at the training length or position, and that these changes in strength are accompanied by significantly greater activation of the muscle at the training position. Furthermore, several studies have outlined that at shorter muscle lengths, the phenomenon of length-specific adaptations are more marked compared to those at longer muscle lengths [19, 21]. For example, performing resistance training at a shorter muscle length results in increases in strength at, and close to the training muscle length, whereas training at longer muscle lengths results in strength increases at, and around a larger range of muscle lengths. However, all of the above data is provided via controlled isometric (static) contrac‐ tions, when resistance training programs for most individuals are predominantly of a dynamic nature, and therefore warrants further research to extend the knowledge in this area. Therefore

How Deep Should You Squat to Maximise a Holistic Training Response?...

http://dx.doi.org/10.5772/56386

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1. To describe the acute differences in activation of the Vastus Lateralis (VL) muscle whilst performing dynamic resistance exercise over relatively short muscle lengths compared to long muscle lengths; here comparisons were carried out a) where the external 'perceived' workload is matched, and b) when the internal workload is systematically matched between the two training modalities. 2. To describe the changes in oxygen consumption and cardiovascular responses during these exercise protocols. 3. To identify any link between the acute responses to loading at shorter vs longer muscle lengths; and the more chronic adaptations on VL muscle activation following 8 weeks of length-specific resistance training and 4 weeks detraining.

Ten males (23±3 years, 1.79±0.06m, 73.4±8.4Kg) gave written informed consent to take part in the study. All procedures and experimental protocols were approved by the Ethics Committee at the Manchester Metropolitan University. Exclusion criteria for participation in the study were the presence of any known musculoskeletal, neurological, inflammatory or metabolic disorders or injury. Participants were physically active, involved in recreational activities such as team sports, and had either never taken part in intensive (more than two hours a week) lower limb resistance training or not within the previous 12 months. Participants attended the laboratory for a total of five occasions. The first visit included demonstration of the appropriate squat technique for a standard barbell back squat, and familiarisation of the exercise protocol and testing equipment. The following week participants returned on four occasions, firstly to record their one repetition maximum over each range-of-motion, which was defined as the maximum amount of external weight (Kg) that could be lifted in a controlled manner through the entire range-of-motion, and their MVC on an isokinetic dynamometer ( Cybex, Phoenix

follows: Day 1; 1RM & MVC, Day 2; Rest, Day 3; Protocol 1, Day 4; Rest, Day 5; Protocol 2, Day

and 90o of knee flexion. The time-line of the sessions was as

the aims of the body of work presented for the first time here were:

**2. Methods**

**2.1. Acute study**

Healthcare Products, UK) at 50o

One aspect of resistance training that is scarcely reported in the literature is the acute (and/ or chronic) responses to resistance training programs whereby the length of the muscle when it is loaded is being manipulated. Acutely, it has been demonstrated that there are significantly different responses to exercising at different joint-angles (and thus different muscle lengths). De Ruiter et al. [13] showed that during isometric MVC exercise at 30o , 60o , and 90o of knee flexion, maximal activation of the knee extensors was significantly greater at 90o than the other two angles, despite having identical torque production as 30o (90o ; 199±22Nm, 30o ; 199±29Nm) and significantly lower torque production than 60o (298±41Nm). A subsequent study [14] found that maximal muscle oxygen consumption was reached significantly later, and was on average ~60% less at 30o compared to 60o and 90o knee flexion. Furthermore, Hisaeda et al.[15] found that when performing isometric contractions at 50% MVC to failure at either 50o or 90o of knee flexion, endurance time was significantly shorter at 90o than 50o . This effect was present both when the exercise was performed with the local circulation occluded and not occluded, thereby highlighting local events as being key to the performance of the musculature at discrete knee angles (or muscle lengths). In addition to this, the slope of the iEMG-time to fatigue regression was significantly greater in the 90o condition compared to 50o . It is proposed that one of the reasons for an increase in oxygen consumption at longer muscle lengths (or more flexed joint angles) is that to produce the same external torque, the internal mechanical stress must be higher at more flexed angles (90o ) compared to extended angles (30o or 50o ) because the moment arm of the in-series elastic component (i.e. the distance between the tendon and the joint centre of rotation, a factor which impacts on the forces required at the muscle) is shorter [16] at more flexed angles. The above studies provide compelling evidence of the link between the muscle length during a bout of resistance exercise and the acute impact on muscle activation levels, energetic provision, fatigability, as well as torque production. It has therefore been important to determine the nature of the acute effects of length-specific training because it is the accumulation of the acute responses that ultimately are reflected in the chronic muscle adaptations (known as the repeated bout effect).

Previous investigations have also identified the link between muscle length (or joint-angle) and gains in strength and/ or levels of muscle activation following more extended periods of resistance training [17-21]. Briefly, these studies have shown that significantly greater increases in isometric strength are attained when tested at the training length or position, and that these changes in strength are accompanied by significantly greater activation of the muscle at the training position. Furthermore, several studies have outlined that at shorter muscle lengths, the phenomenon of length-specific adaptations are more marked compared to those at longer muscle lengths [19, 21]. For example, performing resistance training at a shorter muscle length results in increases in strength at, and close to the training muscle length, whereas training at longer muscle lengths results in strength increases at, and around a larger range of muscle lengths. However, all of the above data is provided via controlled isometric (static) contrac‐ tions, when resistance training programs for most individuals are predominantly of a dynamic nature, and therefore warrants further research to extend the knowledge in this area. Therefore the aims of the body of work presented for the first time here were:

1. To describe the acute differences in activation of the Vastus Lateralis (VL) muscle whilst performing dynamic resistance exercise over relatively short muscle lengths compared to long muscle lengths; here comparisons were carried out a) where the external 'perceived' workload is matched, and b) when the internal workload is systematically matched between the two training modalities. 2. To describe the changes in oxygen consumption and cardiovascular responses during these exercise protocols. 3. To identify any link between the acute responses to loading at shorter vs longer muscle lengths; and the more chronic adaptations on VL muscle activation following 8 weeks of length-specific resistance training and 4 weeks detraining.
