"Convincing evidence from numerous studies indicates that carbohydrate feeding during exercise of about 45 minutes or longer (Jeukendrup 2004, 2008; Jeukendrup et al. 1997) can improve endurance capacity and performance. Studies have also addressed questions of which carbohydrates are most effective, what feeding schedule is the most effective, and what amount of carbohydrate to consume is optimal. Other studies have looked at factors that could possibly influence the oxidation of ingested carbohydrate, such as muscle glycogen levels, diet, and exercise intensity. Mechanisms by which carbohydrate feeding during exercise may improve endurance performance include the following.
* Maintaining blood glucose and high levels of carbohydrate oxidation. Coyle et al. (1986) demonstrated that carbohydrate feeding during exercise at 70% of V.O2max prevents the drop in blood glucose that was observed when water (placebo) was ingested. In the placebo trials, the glucose concentration started to drop after 1 hour of exercise and reached extremely low concentrations (2.5 mmol/L) at exhaustion after 3 hours. With carbohydrate feeding, glucose concentrations were maintained above 3 mmol/L, and subjects continued to exercise for 4 hours at the same intensity. Total-carbohydrate oxidation rates followed a similar pattern. A drop in carbohydrate oxidation occurred after about 1.5 hours of exercise with placebo, and high rates of carbohydrate oxidation were maintained with carbohydrate feeding. When subjects ingested only water and exercised to exhaustion, they were able to continue again when glucose was ingested or infused intravenously. These studies showed the importance of plasma glucose as a substrate during exercise.
* Glycogen sparing in the liver and possibly muscle. Carbohydrate feedings during exercise “spare” liver glycogen (Jeukendrup et al. 1999), and Tsintzas and Williams (Tsintzas et al. 1998) discussed a potential muscle glycogen sparing effect. Generally, muscle glycogen sparing is not found during cycling (Jeukendrup et al. 1999), but it may be important during running (Tsintzas et al. 1995).
* Promoting glycogen synthesis during exercise. After intermittent exercise, muscle glycogen concentrations were higher when carbohydrate was ingested than when water was ingested (Yaspelkis et al. 1993). This finding could indicate reduced muscle glycogen breakdown. But the ingested carbohydrate was possibly used to synthesize muscle glycogen during the low-intensity exercise periods (Keizer et al. 1987a).
* Affecting motor skills. Few studies have attempted to study the effect of carbohydrate drinks on motor skills. One such study investigated 13 trained tennis players and observed that when players ingested carbohydrate during a 2-hour training session (Vergauwen et al. 1998), stroke quality improved during the final stages of prolonged play. This effect was most noticeable when the situations required fast running speed, rapid movement, and explosiveness.
* Affecting the central nervous system. Carbohydrate may also have central nervous system effects. Although direct evidence for such an effect is lacking, the brain can sense changes in the composition of the mouth and stomach contents. Evidence, for instance, suggests that taste influences mood and may influence perception of effort. An interesting observation provides support for a central nervous system effect. When a hypoglycemic person bites a candy bar, that person’s symptoms almost immediately decrease, and the person feels better again long before the carbohydrate reaches the systemic circulation and the brain. The central nervous system effect may also explain why some studies report positive effects of carbohydrate during exercise on performance lasting approximately 1 hour (Jeukendrup et al. 1997). During exercise of such short duration, only a small amount of the carbohydrate becomes available as a substrate. Most of the ingested carbohydrate is still in the stomach or intestine. Studies in which athletes rinsed their mouths with carbohydrate (but did not ingest any) during 1-hour time trials showed performance improvements similar to those that occurred when the athletes ingested the carbohydrate (Carter et al. 2004). Others (Pottier et al. 2008) recently confirmed these findings.
Whether the central nervous system effects of glucose feeding are mediated by sensory detection of glucose or perception of sweetness is not known, although studies with placebo solutions with identical taste to glucose solutions suggest that sweetness is not the key factor. Brain imaging studies also show that increased brain activity is specific to carbohydrates. Feeding Strategies and Exogenous Carbohydrate Oxidation
A greater contribution of exogenous (external) fuel sources (carbohydrate) spares endogenous (internal) sources, and the notion that a greater contribution from exogenous sources increases endurance capacity is enticing. The contribution of exogenous substrates can be measured using stable (or radioactive) isotopic tracers. The principle of this technique is simple: The ingested substrate (e.g., glucose) is labeled, and the label can be measured in expired gas after the substrate has been oxidized. The more the ingested substrate has been oxidized, the more of the label (tracer) will be recovered in the expired gas. Knowing the amount of tracer ingested, the amount of tracer in the expired gas, and the total CO2 production enables us to calculate exogenous substrate oxidation rates.
The typical pattern of exogenous glucose oxidation rates is shown in figure 6.6. The labeled CO2 starts to appear 5 minutes after ingestion of the labeled carbohydrate. During the first 75 to 90 minutes of exercise, exogenous carbohydrate oxidation continues to rise as more and more carbohydrate is emptied from the stomach and absorbed in the intestine. After 75 to 90 minutes a leveling off occurs, and the exogenous carbohydrate oxidation rate reaches its maximum value and does not increase further. Several factors have been suggested to influence exogenous carbohydrate oxidation including feeding schedule, type and amount of carbohydrate ingested, and exercise intensity."
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Here's an excerpt from Swimming Anatomy with permission of the publisher, Human Kinetics.
"As the hand enters into the water, the wrist and elbow follow and the arm is extended to the starting position of the propulsive phase. Upward rotation of the shoulder blade allows the swimmer to reach an elongated position in the water. From this elongated position, the first part of the propulsive phase begins with the catch. The initial movements are first generated by the clavicular portion of the pectoralis major. The latissimus dorsi quickly joins in to assist the pectoralis major. These two muscles generate a majority of the force during the underwater pull, mostly during the second half of the pull. The wrist flexors act to hold the wrist in a position of slight flexion for the entire duration of the propulsive phase. At the elbow, the elbow flexors (biceps brachii and brachialis) begin to contract at the start of the catch phase, gradually taking the elbow from full extension into approximately 30 degrees of flexion. During the final portion of the propulsive phase the triceps brachii acts to extend the elbow, which brings the hand backward and upward toward the surface of the water, thus ending the propulsive phase. The total amount of extension taking place depends on your specific stroke mechanics and the point at which you initiate your recovery. The deltoid and rotator cuff (supraspinatus, infraspinatus, teres minor, and subscapularis) are the primary muscles active during the recovery phase, functioning to bring the arm and hand out of the water near the hips and return them to an overhead position for reentry into the water. The arm movements during freestyle are reciprocal in nature, meaning that while one arm is engaged in propulsion, the other is in the recovery process.
Several muscle groups function as stabilizers during both the propulsive phase and the recovery phase. One of the key groups is the shoulder blade stabilizers (pectoralis minor, rhomboid, levator scapula, middle and lower trapezius, and the serratus anterior), which as the name implies serve to anchor or stabilize the shoulder blade. Proper functioning of this muscle group is important because all the propulsive forces generated by the arm and hand rely on the scapula’s having a firm base of support. Additionally, the shoulder blade stabilizers work with the deltoid and rotator cuff to reposition the arm during the recovery phase. The core stabilizers (transversus abdominis, rectus abdominis, internal oblique, external oblique, and erector spinae) are also integral to efficient stroke mechanics because they serve as a link between the movements of the upper and lower extremities. This link is central to coordination of the body roll that takes place during freestyle swimming.
Like the arm movements, the kicking movements can be categorized as a propulsive phase and a recovery phase; these are also referred to as the downbeat and the upbeat. The propulsive phase (downbeat) begins at the hips by activation of the iliopsoas and rectus femoris muscles. The rectus femoris also initiates extension of the knee, which follows shortly after hip flexion begins. The quadriceps (vastus lateralis, vastus intermedius, and vastus medialis) join the rectus femoris to help generate more forceful extension of the knee. Like the propulsive phase, the recovery phase starts at the hips with contraction of the gluteal muscles (primarily gluteus maximus and medius) and is quickly followed by contraction of the hamstrings (biceps femoris, semitendinosus, and semimembranosus). Both muscle groups function as hip extensors. Throughout the entire kicking motion the foot is maintained in a plantarflexed position secondary to activation of the gastrocnemius and soleus and pressure exerted by the water during the downbeat portion of the kick."
Despite the author's name, Jack Daniels, you don't have to like whiskey to enjoy this book. It "provides you with his proven VDOT formula to guide you through training at exactly the right intensity to become a faster, stronger runner."
"A good measure of how much work you’re doing as a runner is how much distance you’re covering. It costs just about the same amount of energy to run eight miles in 40 minutes as it does to run eight miles in 60 minutes; you’re doing the same amount of work--only the rate is different. However, the amount of work (mileage) that you’re performing represents only part of the stress to which you’re subjecting yourself. Slower runners spend more time accumulating the same mileage covered by faster runners, and more time on the road means more footfalls, more landing impact, and a greater chance for increased fluid loss and elevated body temperature. Thus, although mileage achieved is a logical starting point, it’s also useful to keep track of total time spent running.
Keep track of your weekly mileage so that you can use this record as a basis for how much of the various types of quality work you do and so that your training is consistent. Just as you use your current VDOT or (based on current racing ability) to guide your training intensities, you can use your current weekly mileage to set limits on quality sessions--but use time spent running to log points accumulated at various intensities of running.
In the case of weekly mileage, remember the principles of stress and reaction (principle 1, page 8) and diminishing return (principle 5, page 12) I discussed in chapter 1. Stay with a set amount of mileage for at least three weeks before increasing your mileage. This gives your body a chance to adjust to and benefit from a particular load before moving on to a more demanding one. When it’s time to increase your mileage, add to your weekly total as many miles (or one and a half times as many kilometers) as the number of training sessions you’re doing each week, up to a maximum of a 10-mile (15-kilometer) total adjustment. For example, after at least three weeks of 20 miles per week spread over five training sessions, your maximum increase should be 5 miles or 7.5 kilometers--1 mile (or 1.5 kilometers) for each of the five sessions you’re doing each week. In this case, you would be moving from 20 to 25 miles per week.
A runner who’s doing 10 or more workout sessions per week could increase his or her weekly total by 10 miles, after spending at least three weeks at the previous amount. Let a 10-mile (15-kilometer) weekly increase be the maximum mileage change, even if you’re running two or more daily sessions seven days a week. Another way of dealing with increases in weekly training load is to add to the weekly total the lesser of 60 minutes per week or 6 minutes multiplied by the number of training sessions you undertake each week.
I think that two hours a day of running is quite a lot, and it’s unusual for even elite runners to run more than three hours a day (about 30 miles a day for an elite distance runner). Remember that stress is a function of time spent doing something, and that’s why a 20-mile run is more stressful for a slow runner than for a faster one. It’s not just the 20 miles but the time spent completing those 20 miles. The increased number of steps can wear you down, and the extra hour in the heat or on slick roads can take its toll. To avoid overtraining and injury, slower runners might have to run less total mileage than faster runners."
"ABC Running Drills
Other than with strength training, how can running form and performance be improved? Because running has a neuromuscular component, running form can be improved through form drills that coordinate the movements of the involved anatomy. The drills, developed by coach Gerard Mach in the 1950s, are simple to perform and cause little impact stress to the body. Essentially, the drills, commonly referred to as the ABCs of running, isolate the phases of the gait cycle: knee lift, upper leg motion, and pushoff. By isolating each phase and slowing the movement, the drills, when properly performed, aid the runner’s kinesthetic sense, promote neuromuscular response, and emphasize strength development. A properly performed drill should lead to proper running form because the former becomes the latter, just at a faster velocity. Originally these drills were designed for sprinters, but they can be used by all runners. Drills should be performed once or twice a week and can be completed in 15 minutes. Focus on proper form.
The A motion (figure 3.2; the movement can be performed while walking or more dynamically as the A skip or A run) is propelled by the hip flexors and quadriceps. Knee flexion occurs, and the pelvis is rotated forward. The arm carriage is simple and used to balance the action of the lower body as opposed to propelling it. The arm opposite to the raised leg is bent 90 degrees at the elbow, and it swings forward and back like a pendulum, the shoulder joint acting as a fulcrum. The opposite arm is also moving simultaneously in the opposite direction. Both hands should be held loosely at the wrist joints and should not be raised above shoulder level. The emphasis is on driving down the swing leg, which initiates the knee lift of the other leg.
The B motion (figure 3.3) is dependent on the quadriceps to extend the leg and the hamstrings to drive the leg groundward, preparing for the impact phase. In order, the quadriceps extend the leg from the position of the A motion to potential full extension, and then the hamstrings group acts to forcefully drive the lower leg and foot to the ground. During running the tibialis anterior dorsiflexes the ankle, which positions the foot for the appropriate heel landing; however, while performing the B motion, dorsiflexion should be minimized so that the foot lands closer to midstance. This allows for less impact solely on the heel, and because the biomechanics of the foot are not involved as in running, it does not promote any forefoot injuries.
The final phase of the running gait cycle is dominated by the hamstrings. Upon impact, the hamstrings continue to contract, not to limit the extension of the leg but to pull the foot upward, under the glutes, to begin another cycle. The emphasis of this exercise (figure 3.4) is to pull the foot up, directly under the buttocks, shortening the arc and the length of time performing the phase so that another stride can be commenced. This exercise is performed rapidly, in staccato-like bursts. The arms are swinging quickly, mimicking the faster movement of the legs, and the hands come a little higher and closer to the body than in either the A or B motions. A more pronounced forward lean of the torso, similar to the body position while sprinting, helps to facilitate this motion."