Research Blog

Glycogen Starvation in Muscle Training


Glycogen efficiently stores glucose in a branched, dense polymer. A study released in 2004 suggests that glycogen starvation during training increases endurance performance.

Glycogen is polymerized glucose. For athletes, glycogen has many advantages. When glycogen is broken down into pyruvate during glycolysis it results in a net gain of 3 ATPs. ATP is our body's and muscle's high-energy delivering molecule. When glucose is broken down during glycolysis it produces just 2 ATP. Thus, glycogen produces 50% more ATP during glycolysis than glucose. When the cell is in hypoxia (oxygen depletion) it is unable to break down the glycolysis product, pyruvate, further. Thus, for anaerobic exercise, glycogen breakdown results in a considerable energy benefit. In addition, as the image above shows, glycogen is branched every ten glucose monomers. This allows more glucose molecules to be available for glycolysis (only glucose molecules at the end of a chain can be cleaved). Over the course of animal evolution, glycogen has developed into the primary energy-storage molecule because of its denseness and efficient energy-release.

Glycogen has long been known to be beneficial to athletes; hence carbohydrate loading (carbo loading's affect on performance is debatable, but that is another story). Is glycogen beneficial during training? That is the question an elegant Danish study sought to answer. The study found that carbohydrate depletion during training was beneficial to performance (Skeletal Muscle Adaptation: training twice every second day vs training once daily; 2004; Anne K. Hansen, et al.)

The study took 7 healthy, untrained men and put them on a strict training schedule for ten weeks. The training consisted of leg extensions. Each participant worked out both legs for an hour, wait two hours under a fast, and workout one of the legs (low-glycogen protocol) for an hour. The next day, the participant would workout the other leg (high-glycogen protocol) for an hour.  On this two-day sequence both legs got two hours of work, but one at lower levels of glycogen than the other. This is because following the first hour bout of exercise, glycogen levels in the muscle drop. The two-day sequence was repeated for ten weeks. The experimental sequence can be seen in the diagram below.

Schematic overview showing the design of the study.

In response to 10 weeks of training, maximum power output increased significantly, being the same in the two legs. The endurance at 90% of this new maximum power output was markedly increased for both legs, but time until exhaustion was twice as long for low-glucose protocol trained legs compared with high-glucose protocol trained legs. In accordance, the actual work performed by low-glucose protocol trained legs was also markedly larger compared with high-glucose protocol trained legs. The results can be seen in the table below.                                        

Maximal power output and time until exhaustion at 90% of maximal power output before and after 10 weeks of training and total work before and after 10 weeks of training.

Muscle glycogen levels were raised significantly in low-glycogen protocol trained legs at the conclusion of 10 weeks of training. The authors of the study speculated that this may have been the result of a change in GS enzyme, a regulator of glycogen production. Mitochondria enzymes CS and HAD were both significantly higher in low-glycogen protocol trained legs.

The results of this study seem to support this hypothesis regarding glycogen: train low, perform high. However, these results must be treated with caution when applied to athletes. Other factors could be at play; training twice every other day compared to once daily may affect other pathways besides glycogen.  In addition, glycogen starvation would potentially limit the amount of training one could conceivably do.

In summary, this study suggests, but does not prove, that glycogen starvation during training may increase endurance.

Mechanics of Ankle Sprain Prone Runners

muddy running shoes

Research shows that individuals who do and do not develop chronic ankle instability after an initial ankle sprain have different running kinematics.

Ankle sprains are a common injury for runners. Generally, ankle problems persist after an individual sprains their ankle. Ankle instability following an initial sprain is called chronic ankle instability (CAI). Two types of chronic ankle instability are possible: mechanical instability and functional instability. Both types of chronic ankle instability are characterized by complaints by individuals of their ankles giving way, but individuals with mechanical instability display lax lateral ankle ligaments. A study published in the August 2011 issue of the Journal of Sports Medicine looked at the mechanics of individuals who suffered from chronic ankle instability, both functional instability and mechanical instability, as well as individuals who suffered  from an ankle strain, but were lucky enough not to develop chronic ankle instability. Individuals who managed to avoid developing chronic ankle instability after an ankle sprain were appropriately labeled "copers" (Foot clearance in walking and running in individuals with ankle instability, 2011, Cathleen Brown).

The study found that individuals with chronic ankle instability had a lower minimum metatarsal height during the terminal swing. A graph comparing the metatarsal heights of copers, individuals with mechanical ankle instability and individuals with functional ankle instability as a function of time can be seen below.  

Metatarsal height of copers compared to individuals with functional ankle instability and mechanical ankle instability during running.

Individuals with a lower foot height while running should see more ankle strains due to inadvertent toe strikes that would result. The individuals suffering from mechanical ankle instability had a significantly higher external foot rotation than the functional instability and coper groups. The mechanical instability group was found to be less plantar flexed during foot contact and were less everted.  Differences in gait were also found. The copers had far less frontal gait, possibly a sign of more control in their stride.

Similarly, differences were seen in the mechanics of the copers and chronic ankle instability suffering individuals while walking. The copers had less external foot rotation than the individuals suffering from chronic ankle instability. However, no significant difference was found in dorsiflexion while walking.

The results here are preliminary, only 11 male individuals were in each cohort.  Thus, the study did not have much statistical power. However, the results do suggest that rehabilitation for individuals suffering from ankle strains can focus on the kinematics of their running or walking.

Synchronizing the Body to Music

women jogging with iPod

Music can affect exercisers several ways. Through its motivational, relaxing and synchronization properties, music has been shown to aid the exerciser psychologically, physiologically and recoverably.

Most people enjoy working out with music. Whether at the gym or jogging trail, iPods are a common sight. So why do music and working out go together so well? What does science say about this relationship?

Not surprisingly, studies show that motivational music makes working out more enjoyable and may boost performance. In addition, music has been shown to have a positive effect on recovery.

Human movement and rhythmical perception both occur at the same frequency of 120 beats per movement. 120 beats per a minute (or 2.0 Hertz) is consistent with a study that used a metronome to measure participants walking rhythm and another study that observed the frequency of participants' finger tapping. An analysis of 70,000 pop songs found that the beat was on average 120 beats per minute. The human physiological rhythm rises to an elevated frequency of 2.6-2.8 Hz while running on a treadmill for optimal performance. Interestingly, when participants running on a treadmill selected music to accompany them the frequency of the music was 2.6-2.8 Hz.

The effect of music on performance has been shown by some studies to increase performance and other studies to not affect peak performance. A study done in 2006 found that participants elicited a faster 400 meter sprint with motivational music than during a no music control (Simpson, S.D. and Karageorghis, C.I. 2006. The effects of synchronous music on 400-m sprint performance). A more recent study done in 2012 found that elite triathletes were able to go longer on a submaximal treadmill test done to exhaustion while subject to two different music conditions, motivational music and neutral music (Terry , P.C. , Karageorghis , C.I. , Mecozzi Saha , A. , & D'Auria , S. 2012 . Effects of synchronous music on treadmill running among elite triathletes). Interestingly, time to exhaustion was increased 12% with music. Motivational, but not neutral, music elicited a more positive mood following the test. Blood lactate levels were not changed by music, but oxygen consumption was about 3% lower with the music conditions than the no music control. The authors suggest that the synchronization of running and music produces a stronger response through physiological changes than the psychological motivation of the motivational music.

Existing theories believe that at higher exercise intensities attention to external stimuli such as music wanes. In 2009 a study of participants on indoor cycles did not increase their tempo in response to music, but they reported finding the cycling more pleasurable while cycling to music (Shaulov, N. and Lufi, D. 2009. Music and light during indoor cycling). Females and participants under age 26 were found to be more affected by music than their male or older counterparts (Ergogenic and psychological effects of synchronous music during circuit-type exercise, 2011, Priest, et al.). Participants did report more positive feelings during the repetitions with motivational music, but actual performance was not significantly better. The differences in gender can be seen in the figure below. 

Total repetitions of simple exercises such as sit-ups, jumping jacks, step-ups, squats and heel raises.  The conditions were motivational music, oudeterous (neutral) music and a metronome control.  Women were more strongly influenced by the music.

Research has also found music impacts post-wokout recovery. A study done in 2008 found that music increased recovery compared to a no-music control (Jing, L. and Xudong, W. 2008. Evaluation on the effects of relaxing music on the recovery from aerobic exercise-induced fatigue). Thirty healthy college males were assigned to pedal at 50 rev/min on a cycle ergometer until fatigue. Half were assigned to 15 minutes of rest with relaxing music, while the other half were a no music control. The music rest group showed significantly greater decreases in heart rate, urinary protein and ratings of perceived exertion after 15 minutes than the no music control. This suggests music aids in rehabilitation of the both psychological and physiological pathways following a workout.

The research is definitive in that music aids us psychologically during exercise. However, peak performance may or may not be affected by music. For the recreational exerciser looking for a boost of motivation and the doctor encouraging adherence to an exercise regime, music may provide that answer. A great review on the effects of music on exercisers can be found by reading Music in the exercise domain: a review and synthesis by Costas I. Karageorghis & David-Lee Priest.

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