Effects of posture during strength training
Strength training exercises can be done in various postures. Importantly, posture has at least three key physiological effects, which are very rarely discussed.
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#1. Posture and the oxygen conforming response
Observations regarding the oxygen conforming response have shown us that when oxygen delivery is reduced during an exercise, the slow twitch muscle fibers fatigue more quickly such that muscle activation must be increased to maintain the same muscle force output. Importantly, oxygen delivery is reduced anytime that muscle perfusion pressure is reduced. Elevating a muscle above the level of the heart (such as when lifting overhead) reduces muscle perfusion pressure, while lowering a muscle below the level of the heart decreases it (read more).
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#2. Posture and sympathetic nervous system activation
Observations regarding the sympathetic nervous system have shown us that when sympathetic nervous system activity is increased during an exercise, there is an increase in the activity of persistent inward currents (PICs) in the motor neurons of the spinal cord, which leads to an increase in corticospinal excitability and this may in turn cause an increase in the levels of motor unit recruitment (read more).
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#3. Posture and coordination demands
Finally but perhaps most importantly, performing movement tasks while standing is more difficult that performing the same movement tasks while seated. This is because there is a finite amount of attentional resources that can be devoted to a given task and standing itself requires some attentional resources to maintain, especially when an upper body movement task is superimposed upon it. In the context of strength training, this means that we should expect lower levels of motor unit recruitment to be achieved at maximum effort when an exercise is performed in a standing position compared to when it is performed while seated.
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Conclusions
Strength training exercises can be done in various postures. Importantly, posture has at least three physiological effects. Firstly, the height of the working muscle relative to the heart affects its muscle perfusion pressure, which affects the rate of oxygen delivery, which in turn affects the rate of fatigue of slow twitch fibers. This is known as the oxygen conforming response. In practice, it means that when a muscle exercises above the height of the heart, its slow twitch fibers fatigue more rapidly because the lower muscle perfusion pressure reduces the rate of oxygen delivery. And this in turn leads to the more rapid recruitment of high-threshold motor units, potentially reducing the negative effects of central nervous system fatigue mechanisms. Secondly, the height of the working muscle relative to the heart affects the activation of the sympathetic nervous system (SNS). When a working muscle is above the height of the heart, SNS activity increases, and this increases persistent inward currents (PICs) in the spinal motor neurons, thereby increasing corticospinal excitability. In practice, this means that muscles may well achieve higher levels of motor unit recruitment at task failure when the exercise involves them working above the height of the heart. Thirdly, greater postural demand (such as when standing versus sitting) increases mental effort. Mental effort is known to suppress the level of central motor command that can be generated at the maximum tolerable level of perceived effort that occurs at task failure. In practice, this means that reducing the postural demand in a strength training exercise may allow higher levels of motor unit recruitment to be achieved at task failure.
