Fortunately, muscles also have large stores of a carbohydrate, called glycogen, which can be used to make ATP from glucose. But this takes about 12 chemical reactions so it supplies energy more slowly than from creatine phosphate. Oxygen is not needed — this is great, because it takes the heart and lungs some time to get increased oxygen supply to the muscles.
A by-product of making ATP without using oxygen is lactic acid. You know when your muscles are building up lactic acid because it causes tiredness and soreness — the stitch. Within two minutes of exercise, the body starts to supply working muscles with oxygen. When oxygen is present, aerobic respiration can take place to break down the glucose for ATP.
When muscles contract, they break down ATP in a reaction that provides energy. However, muscle cells only store enough ATP to fuel a few seconds of maximal contraction. Once muscle contraction starts, the making of ATP must start quickly. Since ATP production is so important, muscle cells have several different ways to make it. These systems work together in phases. The three biochemical systems for producing ATP are, in order:. To continue working, muscle cells must replenish their ATP supply.
All muscle cells contain a high-energy compound, creatine phosphate, which is quickly broken down to make ATP. Because stores of creatine phosphate are also limited, this energy system can only sustain maximal muscle output for about 10 seconds. The phosphagen system is the primary energy source during very short, rapid bursts of activity, such as sprints.
To sustain exercise for more than 10 seconds, muscles must break down fuel sources such as carbohydrates and fats to provide the energy to re-synthesize ATP. Carbohydrate metabolism is faster than fat metabolism. Because muscle force scales with cross-sectional area, and muscle length change scales with optimal length, larger muscles will have higher absolute mechanical work per cycle than smaller muscles due to their larger cross-sectional areas and longer optimal lengths. However, since muscle force scales with cross-sectional area that has dimensions of length-squared, and the length change scales with optimal length which has dimensions of length, the work scales with the volume which has dimensions of length-cubed.
Muscle tissue density is often considered to be constant across muscles, so the work per cycle also scales with muscle mass. Thus, based on these assumptions, two muscles with the same geometric proportions of different sizes or length scales would, in theory, generate the same mass-specific mechanical work per cycle. However, as muscle force and work are difficult to measure, particularly in larger animals, this theory has not been directly tested.
These earlier comparative studies, as with most studies in biomechanics, ignored the effects of muscle mass on contractile behaviour. Thus, the loads due to muscle mass increase faster than the muscle force available to accelerate the tissue loads as muscles increase in size. As a consequence, the larger of the two muscles with the same geometric proportions but different sizes or length scales will have lower mass-specific mechanical work per cycle due to its greater muscle mass Ross et al.
Submaximal fibre activation can amplify the effects of muscle mass, as evidenced by studies that have shown lower maximum shortening speeds during submaximal compared to maximal contractions of rat-sized muscle Holt et al. In this study, we found that the reduction in mass-specific work due to muscle mass with greater muscle size is amplified with submaximal activation Figure 5B , which is consistent with the results of previous 1D model simulations Ross et al.
Therefore, muscle mass is an important determinant of whole muscle behaviour, particularly for larger muscles and during submaximal contractions.
Larger muscles may mitigate the effects of their greater mass by having different geometric proportions and architecture than smaller muscles. In this study, we found that greater initial pennation angle resulted in smaller reductions in mass-specific work per cycle with greater muscle size Figure 5B.
While data on scaling of muscle pennation angle with body sizes are limited, positive allometry for pennation angle has been shown for monitor lizard muscles across a range of body sizes 8 g short-tailed monitors to 40 kg Komodo dragons , in that larger animals have higher muscle pennation angles than smaller animals Dick and Clemente, While we varied fibre length with the initial pennation angle of the muscle model, we geometrically scaled the model to larger sizes such that the initial fibre lengths were the same relative length across models of different sizes.
However, muscles in larger animals tend to have relatively shorter fibres compared to muscles in smaller animals Alexander et al. While we found greater maximum fibre strains with greater initial pennation angles, this did not vary with scale, unlike pennation angle over time Figure 6 , so scaling fibre length with muscle size likely would not have altered our results.
Thus, scaling of fibre length with body mass in living animals may be due to space constraints within a limb compartment rather than as a means to minimise the contractile consequences of muscle mass.
We found that the mean fibre pennation angle increased when both the muscle and fibres were shortening Figure 6 , which is consistent with previous in vivo ultrasound measures of human muscles during cyclic contractions Kurokawa et al. Rotating to higher angles during shortening allows the fibres to shorten slower than the muscle belly, and as a consequence, generate greater forces.
However, this increase in force with higher fibre rotation is balanced by a force reduction due to the fibres no longer being oriented along the longitudinal axis of the muscle.
We found smaller fibre rotations over a contraction cycle for larger muscles; however, this difference in fibre rotation with greater muscle size was smaller with higher initial pennation angle Figure 6.
Higher fibre angles during shortening may allow larger muscles to overcome the effects of their mass and achieve higher mass-specific work per cycle, although whether this relationship is causal cannot be determined from our simulations.
We may have seen different fibre strains and rotations had we constrained movement of the model aponeuroses. The bulging of muscle and movement of aponeuroses in transverse directions in vivo is limited by resistive forces applied by neighbouring muscles, connective tissue, and skin, which can alter muscle fibre strains and rotations Wakeling et al.
In our simulations, we did not apply transverse compressive tractions to the muscle to mimic the effects of loads from surrounding tissue, as these loads acting on muscle during dynamic contractions are poorly understood. As a result, our muscle model and aponeuroses were free to bulge and rotate in ways that may not entirely reflect the behaviour of muscle in vivo. While the lack of transverse loads in our simulations may have altered the fibre strains and rotations, it is unlikely that they would have substantially influenced our reported effects of muscle mass, given that we previously found a similar pattern of mass effects across scales for a mass-enhanced 1D model.
Studies have shown that the properties of aponeurosis can alter fibre strains and rotations Rahemi et al. To model the aponeuroses, we used two sheets of tissue that were uniform in thickness and composed of a base material with embedded collagen-like fibres.
These fibres were unidirectional and oriented along the length of each aponeurosis at rest so that the overall behaviour of the tissue was anisotropic, consistent with previous experimental studies Azizi and Roberts, ; Azizi and Roberts, We selected the thickness and material properties of the aponeuroses so that the longitudinal and transverse aponeurosis strains during maximal fixed-end contractions matched in situ measures of intact aponeurosis during the same contractile conditions Azizi and Roberts, However, the utility of these in situ aponeurosis measures is limited, as only muscle forces acting externally and not forces applied to the aponeurosis can be measured or controlled.
Thus, it is not clear to what extent our modelled aponeuroses reproduced the behaviour of in vivo aponeuroses, and so further work is needed to quantify the structural and material properties of aponeurosis and determine its role in constraining fibre strains and rotations during contraction. The properties of aponeurosis may also vary with muscle scale, and this may have influenced our reported effects of muscle mass.
Aponeurosis tissue, as well as tendon, likely plays an important role in energy storage and return during locomotion Wager and Challis, ; Arellano et al.
In our simulations, we assumed that the aponeurosis had the same relative effect on the model across different length scales or sizes, in that the stress-strain properties were constant, and the thickness, length, and width of the aponeuroses scaled with the length scale.
While there is some evidence that tendon cross-sectional area relative to that of muscle varies with animal body mass Alexander et al. It may be that the energetic role of these elastic structures varies with body size and alters the contractile effects of muscle mass. In reality, whole muscles are composed of thousands of sarcomeres grouped into myofibrils at the microscopic level, which constitute muscle fibres at the cellular level that are then bundled into fascicles at the tissue level.
Experiments on in situ and in vivo muscle during dynamic contractions have shown that fascicles within different regions of whole muscle can have different strains and be at different positions on their force-length curves at a given time Pappas et al.
While the non-uniformity in muscle behaviour at the tissue level could be due to regional variations in activation Monti et al. Muscle mass may also have implications for non-uniformity at the microscopic level. Consider a massless ideal spring with uniform structural and material properties along its length.
Initially, the strains will be high near the applied load and low or zero near the fixed end. Then, a wave of strains will propagate along the length of the spring, causing a time delay between when the material near the free and fixed ends experience extension. This pattern of behaviour in a spring with mass, which has also been shown in simulations of a mass-enhanced 1D Hill-type model Ross and Wakeling, and the 3D model in this study, is consistent with experimental findings of sarcomere strains across different regions of intact whole muscle.
Moo et al. These strains become less uniform when muscle is activated during fixed-end contractions Moo et al. This pattern of sarcomere non-uniformity may be in part due to muscle mass and may vary depending on the size of the muscle, much like the response of the spring with mass and 1D and 3D muscle models that account for muscle mass.
Over the last decade there has been growing interest in using 3D muscle models to explain the mechanisms that underly whole muscle contraction. As these models become more refined, they are increasingly able to replicate features of in situ and in vivo muscle behaviour. Rahemi et al. More recently, we showed similar magnitudes and patterns of muscle bulging during submaximal contractions for the 3D model Wakeling et al.
We also showed a similar reduction in muscle thickness and pennation angle with transverse compression Ryan et al. In this study, in which we used a model formulation that accounts for distributed muscle mass and fibre force-velocity effects, we were again able to show similarities in behaviour between the simulated 3D model and experimental muscle.
We found that greater muscle size, and therefore mass, for the simulations and greater added mass for the in situ experiments, as well as higher maximum strain amplitude, led to lower maximum and higher minimum acceleration in the longitudinal direction near the middle of the muscle compared to at the non-fixed end.
Greater muscle size and higher maximum strain also led to greater reductions in mass-specific work per cycle, consistent with previous results from experiments on in situ rat plantaris muscle Ross et al. This reduction in mass-specific work with larger muscle size was lower for simulated muscles with higher initial pennation angles.
We also found that larger muscle size resulted in higher relative kinetic energy per cycle, relatively more energy stored in the aponeurosis, and less stored in the base material that represented the intra and extracellular tissue components apart from the myofibrils.
While these results highlight that muscle mass can substantially decrease muscle performance, higher initial pennation angle and greater energy storage in elastic tissues may mitigate some of these performance losses.
Activation specifically refers to the active state of the contractile elements muscle fibres and is used to scale the active force that they can develop. In muscle physiology, excitation refers to the electrical potentials on the membrane of the muscle fibre that are typically recorded using EMG.
Muscle contraction is the process of muscle developing forces when its activation level is greater than zero. In muscle physiology, contraction does not necessarily mean shortening because tension can be developed without a change in length.
The longitudinal direction is the major x -axis of the muscle. Transverse direction is used to describe directions in the y—z plane, and thus is perpendicular to the longitudinal direction of the muscle. This is sometimes called the radial direction in other studies. Muscle mass refers to the total mass in kilograms of the muscle belly, including the muscle fibres, aponeuroses, and connective tissue.
Note that this muscle mass is distributed throughout the tissue. Scale refers to the length scale factor that multiplied the lengths of the muscle geometries to geometrically scale the muscle model to larger sizes. Both increasing the added mass and scale increase the total mass of the muscle, although the effective mass in the in situ experiments was added only to a single location whereas increasing the model scale increased the mass distributed throughout the muscle and aponeurosis tissue.
SR and JW contributed to the study design. SR ran the model simulations, collected the experimental data, analysed the results, and wrote and revised the first draft of the manuscript. All authors contributed to the article and approved of the submitted version.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. We would like to thank Barbora Rimkus, Nicolai Konow, and Andrew Biewener for their assistance with experimental data collection. Supplementary Figure 1 Local velocity of quadrature points over time.
Ahn, A. Different segments within vertebrate muscles can operate on different regions of their force—length relationships. In vivo and in vitro heterogeneity of segment length changes in the semimembranosus muscle of the toad.
Alexander, R. Allometry of the leg muscles of mammals. Arellano, C. Evidence of a tunable biological spring: elastic energy storage in aponeuroses varies with transverse strain in vivo. Arndt, D. The deal. Mechanical work can also have a negative value. It is so when a body is displaced against the direction of the acting force. Ice hockey goal keeper performs negative work when catching a puck into his catching glove, gymnast when he does crucifix on rings, weight lifter when he drops the barbell from the top to the bottom position, the pad does it when gymnast lands on it, friction force of skis is also negative.
Breaking forces generally perform negative work. Positive work is performed when the body is displaced along the same line as the force is acting. Muscles can also perform mechanical work. Muscle contractions are divided into:. The muscle shortens. The muscle lengthens.
In mechanics energy is defined as an ability of a body to perform work. We know many forms of energy: acoustic energy, light energy, chemical energy, nuclear energy, etc.
In biomechanics we are most of all interested in mechanical energy, which can have two forms: kinetic energy and potential energy.
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