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Let's say I go to the gym and lift some weights an hour. During this time my arms will grow due to the "pump" -- the extra blood rushing in to feed the muscles. For example, I've measured about 2-3 centimeters increase just in the diameter of the upper arm (bicep+tricep).
But where did this blood come from? Also, if my arms got bigger, since I'm the same weight that means some part of my body must have gotten smaller, right?
The blood comes from the body's reservoirs:
- spleen (mostly erythrocytes) 
- liver 
- veins (probably the most important blood resevoir as they contain 50-60 % of the volume) 
In pathological situations, if hypovolemia occurs, blood can also come from:
- splachnic vascular bed 
But what attracts the blood into the muscle? The phenomenon is called active hyperemia:
Active hyperemia is the increase in organ blood flow (hyperemia) that is associated with increased metabolic activity of an organ or tissue. An example of active hyperemia is the increase in blood flow that accompanies muscle contraction, which is also called exercise or functional hyperemia in skeletal muscle. Blood flow increases because the increased oxygen consumption of during muscle contraction stimulates the production of vasoactive substances that dilate the resistance vessels in the skeletal muscle .
- The human spleen as an erythrocyte reservoir in diving-related interventions. Kurt Espersen, Hans Frandsen, Torben Lorentzen, Inge-Lis Kanstrup, Niels J. Christensen. Journal of Applied PhysiologyMay 2002,92(5)2071-2079;DOI: 10.1152/japplphysiol.00055.2001
- Lautt WW, Greenway CV. Hepatic venous compliance and role of liver as a blood reservoir. Am. J. Physiol. 1976 Aug;231(2):292-5. PubMed PMID: 961879.
- Michael J. Gregory, Ph.D. The Circulatory System. Licensed under CC-BY-NC-SA-3.0
- Richard E. Klabunde, PhD. Cardiovascular Physiology Concepts. Active Hyperemia. Available from http://www.cvphysiology.com/Blood%20Flow/BF005.htm (accessed 03.08.2014)
- Blaber AP, Hinghofer-Szalkay H, Goswami N. Blood volume redistribution during hypovolemia. Aviat Space Environ Med. 2013 Jan;84(1):59-64. PubMed PMID: 23305001.
How does extra blood come from to fill your muscles during exercise?
Blood pumps (blood) and sucks (lymph). There are many pumps in our body
- thoracic pump
- smooth musculature
- respiratory pump
which work together to provide the blood to the peripheral circulation. These pumps provide us Pulse, Vasomotor tone and Respiratory waves which when act together can lead to local hyperemia or congestion; see my answer only about these waves here in the thread What does irregular heartbeat mean in simple language.
By Starling law, anything that goes inside the heart goes out from there. See this answer about elevated position on the venous return which is similar to the the exercise on the venous return. Smooth musculature accommodates blood to the venous return.
Why your muscle gets bigger during exercise?
There are many reasons which can be explained by Frank-Starling principle and Fick's law. They lead either to
- hyperemia (increased local blood flow) or
- protein intake before the exercise
- increased sympaticus (decreased parasympaticus; yes, sympathetic tone in digestive truct and some other systems is down-regulated) and blood flow through the muscle (see the above thread)
- unstretched muscle can have congestion during exercise which is felt as hard muscle, also long after the training
- too regular exercising without rest can also lead to congestion
in the spaces described in my earlier answers like here about What can cause the swelling in high protein diet of Whey proteins:
where explanations in my earlier answer.
Where does the extra blood come from to fill your muscles during exercise?
From your body (See Cornelius' answer for specific locations)
- veins 50-60%
under physiologic fine regulation where heart is the main pump (positive gradient) while its sucking effect (negative gradient; lymph) keeps homeostasis stable for exercising muscles to maintain hyperemia supported by rapid adaptive smooth musculature, vasomotor tone and respiratory pumps (three types of waves).
If you stand up too quickly, you get a head rush. One way to counteract the symptoms of the head rush is to contract your leg muscles really intensely. This forces blood out of your legs and into the rest of your circulation, including your head. This is actually a tip my family doctor told me that I've always remembered.
As mentioned in the links below working out a muscle group causes excess blood flow to these areas which could cause the blood vessels to dilate, causing the muscles in these areas to swell and appear larger. I now realize this has nothing to do with the head rush item I mentioned above, but still may be a useful piece of advice if you every get a head rush!
1. Your Muscles Tear
You know that sore feeling you get the day after you hit the gym for the first time in months? Well, that soreness is actually your muscles working to repair all the little rips you created during your last workout. Evidently, when you start working out more, your muscles actually form tiny tears which help them grow bigger and stronger as they heal. So basically, even when you're safely lifting weights, you're tearing your muscles — but it's good for you. Strange, right? If you're struggling with that soreness, though, the Mayo Clinic recommends staying hydrated, properly fueling, and making sure you're getting adequate rest, so your body has time to heal.
How Much Protein Do I Need?
The amount of protein you require depends on your weight and your daily caloric intake. Most Americans consume more than enough protein in their daily diets. A few specific groups of people are at risk for being protein-deficient, including elderly women and people with illnesses or eating disorders. A protein deficiency is defined as eating 50% to 75% of the recommended amount of daily protein, Butterfield explains.
Ideally, you should consume 0.36 grams of protein for every pound of body weight, according to recommended daily allowances (RDA) set by the Food and Nutrition Board. So if you weigh 170 pounds, you need about 61 grams of protein each day.
Protein should also make up approximately 15% of your total daily caloric intake, also according to the RDA. In a diet of 1,800 calories a day, for example, about 270 of those calories should come from protein.
Why is working out so painful?
During the month of January, the gym is place to be. According to the New York Times, new gym and health club memberships may as much as double during the first month of every year when many resolve to get fit or lose weight. But Time Magazine reported that by mid-February, gym attendance is back to its usual levels in other words, all those resolutions have been put on the back burner.
Exercise is known to have all sorts of benefits: According to the Mayo Clinic, exercise can increase your energy and bone density, better your mood, help you sleep and even improve your sex life. But as any athlete will tell you, there’s a reason coaches like to use the phrase, “No pain, no gain!” Whether you’re taking your first pair of running shoes out of the box or powering through your fourth marathon, chances are your workouts will cause you some discomfort. It seems a little counterintuitive that an activity that benefits us could come with unwanted side effects. But there’s a reason for the struggle: Our bodies are learning to operate under a little bit of strain that will ultimately benefit them.
All cells in our bodies need oxygen to convert sugar into ATP, our bodies’ main source of energy. But our muscles need extra oxygen when we’re stretching and contracting them during exercise. Ever find yourself breathing more heavily with a faster heart rate after walking up a flight of stairs? This is because your muscles in your arms and legs signal to your heart and lungs that they need more oxygen to make more energy. Automatically, you start breathing more quickly to get more of this precious gas (which only makes up about 20 percent of the air).
Your heart, another muscle, takes the oxygen from your lungs and supplies it to your blood. As the heart keeps pumping, oxygen-enriched blood spreads through the arteries and veins in your body to reach your muscles (as well as all your other organs and tissues). This is highway of vessels called your cardiovascular system, and as you exercise more and more regularly, it becomes more efficient at delivering the goods.
Walking, running, swimming, and other exercises that raise your heart rate and are sustainable for more than a few minutes are all what scientists call aerobic activity, and what gym-goers call cardio. The Centers for Disease Control and Prevention recommends that you get 75 minutes of cardio three to five days a week to keep your heart and lungs healthy.
Sometimes, though, even the oxygen circulating isn’t enough to get your muscles to do what you want them to. When you’re lifting weights or doing movements like lunging, squatting or pushups, your cardiovascular system doesn’t have time to deliver oxygen to your muscles to make ATP. Instead, they use sugar to create a substance called lactate, or lactic acid, in a process called anaerobic activity (literally, without air). Anaerobic activity isn’t sustainable for very long, and it’s not as efficient as aerobic exercise. Furthermore, lactic acid is responsible for creating that stinging sensation in your muscles when you’re “feeling the burn.” While it may not be pleasant, lactic acid is our body’s way of protecting itself from damaging muscles. Theoretically, the pain is a signal to us that we need to ease up on the exercise so we don’t overexert them. Once you come down from that anaerobic level, the body begins to get rid of any residual lactic acid and your muscles go back to their preferred aerobic activity.
But then there’s the after-effect of working out. While many feel refreshed and less-stressed after hitting the gym, your muscles may feel tender the next day. This is called delayed onset muscle soreness, or DOMS, and it can last for up to 72 hours. Scientists used to think that this was because of the lactic acid buildup, but it’s actually a side effect of repairing tiny tears in your muscles. Rebuilding muscle tissue is actually what helps muscles grow, and sometimes the second day after intense exercise may feel worse than the first.
Muscle soreness isn’t particularly dangerous — the pain may even subside when you start exercising again, although it may return once more during the recovery. But if you’re having trouble getting out of bed the morning after a tough workout, you should probably give your body a break for the day to let it recover, and make sure to eat plenty of protein — foods like meat, eggs or soy — to help your muscles build themselves up again.
It’s hard to say whether muscle pain is the reason so many New Years’ resolvers quit after the first month or so. It could be that people don’t have the same amount of time they did during the holidays, or that they just haven’t found an exercise that’s fun for them. Of course, there’s no reason exercise has to be limited to the gym: Walking or running outside are excellent workouts, and there are many effective routines you can do at home without taking up too much time. So, whether you decide to join a gym or not, push through the pain! It’s all just a part of biology.
Summary and recommendations
Muscle growth seems to occur best when training with relatively higher volumes, close to muscle fatigue, and with shorter rest periods between sets/reps.
- When training, 6 – 12 repetitions per set is the optimal range for muscle growth.
- Train towards contraction failure.
- Take relatively short rest periods — 30 – 90 seconds. Rest-pause techniques can also be effective.
- Perform 12 – 20 sets per muscle group. Supersets can help add volume and improve efficiency.
- Be consistent with training.
- Consume enough energy (calories), with a minimum of 12 – 15% of calories from protein or 1.0 gram of protein per kilogram of bodyweight.
- Sleep 7 – 9 hours per night.
Maintaining Homeostasis of Blood Glucose Levels
Your body breaks down carbohydrates into glucose to meet immediate energy needs. It stores extra glucose as glycogen in your liver and muscle cells. When blood glucose levels drop during exercise, you can experience weakness and dizziness, so you rely on glycogen stores to increase your blood glucose levels. You can maximize your glycogen stores by eating a high carbohydrate diet, which is about 60 percent of your total calories. Focus on consuming more fruits, vegetables and grains. The Academy of Nutrition and Dietetics recommends a snack after a moderate- to- high-intensity workout. Consume foods with easily-digested carbohydrates and a little protein, such as Greek yogurt and berries, a banana with peanut butter or a glass of chocolate milk.
Core Temperature Responses to Exercise
During muscular exercise, core temperature initially increases rapidly and subsequently increases at a reduced rate until heat loss equals heat production, and essentially steady-state values are achieved. At the initiation of exercise, the metabolic rate increases immediately however, the thermoregulatory effector responses for heat dissipation respond more slowly. The thermoregulatory effector responses, which enable sensible (radiative and convective) and insensible (evaporative) heat loss to occur, increase in proportion to the rise in core temperature. Eventually, these heat loss mechanisms increase sufficiently to balance metabolic heat production, allowing achievement of a steady-state core temperature.
During muscular exercise, the magnitude of core temperature elevation is largely independent of the environmental condition and is proportional to the metabolic rate (Gonzalez et al., 1978 Nielsen, 1938, 1970). This concept was first presented by Nielsen (1938) who had three subjects perform exercise at several intensities (up to approximately 3.0 liters oxygen per minute) in a broad temperature range (5° to 36ଌ with low humidity). Figure 3-1 presents the heat exchange data for one subject during an hour of cycle exercise at a power output of 147 watts and at a metabolic rate of approximately 650 watts. The difference between metabolic rate and total heat loss represents the energy used for mechanical work and heat storage. The total heat loss and, therefore, the heat storage and elevation of core temperature were constant for each environment. The relative contributions of sensible and insensible heat exchange to total heat loss, however, varied with environmental conditions. In the 10ଌ environment, the large skin-to-ambient temperature gradient facilitated sensible heat exchange, which accounted for about 70 percent of the total heat loss. As ambient temperature increased, this gradient for sensible heat exchange diminished, and there was a greater reliance upon insensible heat exchange. When the ambient temperature was equal to skin temperature, insensible heat exchange accounted for almost all the heat loss. In addition, when the ambient temperature exceeded the skin temperature, there was a sensible heat gain to the body.
Heat exchange data averaged over 1 hour for one subject performing constant intensity exercise in a variety of ambient temperatures. The difference between metabolic rate and total heat loss is the sum of mechanical power (147 watts) and mean rate of (more. )
Nielsen's finding that the magnitude of core temperature elevation is independent of environmental conditions is inconsistent with the personal experience of most athletes. For example, a runner will experience greater hyperthermia if he or she competes in a 35ଌ environment (Robinson, 1963). Lind (1963) showed that the magnitude of core temperature elevation during exercise is independent of the environment only within a certain range of conditions or a ''prescriptive zone.'' Figure 3-2 presents a subject's steady-state core temperature responses during exercise performed at three metabolic intensities in a broad range of environmental conditions. The environmental conditions are represented by the "old" effective temperature, which is an index that combines the effects of dry-bulb temperature, humidity, and air motion. Note that during exercise the greater the metabolic rate, the lower the upper limit of the prescriptive zone. In addition, Lind found that even within the prescriptive zone there was a small but significant positive relationship between the steady-state core temperature and the "old" effective temperature. It seems fair to conclude that throughout a wide range of environmental conditions, the magnitude of core temperature elevation during exercise is largely, but not entirely, independent of the environment. During exercise with a substantial metabolic requirement, the prescriptive zone might be exceeded, and there is a further elevation of steady-state core temperature.
Relationship of steady-state core temperature responses during exercise at three metabolic rates to the environmental conditions. Source: Sawka and Wenger (1988), used with permission. Redrawn from Lind (1963).
As stated, within the prescriptive zone, the magnitude of core temperature elevation during exercise is proportional to the metabolic rate (Nielsen, 1938 Saltin and Hermansen, 1966 Stolwijk et al., 1968). Although the relationship between metabolic rate and core temperature is strong for a given individual, it does not always hold well for comparisons between different individuals. Åstrand (1960) first reported that the use of relative intensity (percentage of maximal oxygen uptake), rather than actual metabolic rate (absolute intensity), removes most of the intersubject variability for the core temperature elevation during exercise.
How to improve respiratory muscle performance during exercise
Unloading the respiratory muscles during exercise by using low-density gas mixtures (such as heliox), mechanical ventilators or supplemental oxygen is neither practicable nor allowed for healthy athletes. What can be done in order to improve the fatigue resistance and mechanical efficiency of respiratory muscles is training. Although there is still no definitive evidence as to whether it is possible to improve exercise tolerance, reliable recent studies showed that respiratory muscle training has a small but probable and significant effect on endurance exercise performance. What needs to be determined is the mechanism or combination of mechanisms by which respiratory muscle training improves exercise performance: relief of respiratory muscle fatigue relief of limb muscle fatigue attenuation of the respiratory muscle metaboreflex and relief of the discomfort associated with high levels of respiratory muscle work [21–23].
Phase 4: Resolution (You snuggle, fall asleep, or go again.)
Excellent job, team! Here's what happens in the comedown phase of sex.
23. It’s pretty simple, really: With some time, basically everything from your heart rate to your breasts to your labia goes back to its usual color, size, and state.
While this resolution phase usually leads to a refractory period for people with penises (as in, a time when they physically can't have sex), that's not necessarily so for people with vaginas. You might be able to reach orgasm again quickly, the Cleveland Clinic explains. Time for round two?