Metabolic Responses to Weight Lifting
Authors: Nelson Arnold 
Published: December 2017

Editor's Note,
The ability to lift heavy loads while performing multiple repetitions is not only highly correlated with muscle mass or the total number actomyosin interactions, but also metabolic functions that includes substrate concentrations and by-product removal. Muscles use adenosine triphosphate (ATP) in at least three locations during exercise; to run the actomyosin interaction, operate sarcoplasmic reticulum calcium pumps, and operate sarcolemma sodium and potassium pumps. Weight lifting sessions are considered to be an intermittent activity that includes only a few second bursts of high force and/or velocity movements followed by rest periods of up to several minutes. Therefore, the anaerobic pathways such as the phosphagen and glycolytic systems are the initial pathways to respond due in part to the ability to match the increased rates of ATP depletion by increasing ATP production.
After the initial resting ATP stores are used up, the phosphagen system starts contributing to ATP replenishment. This system consists of reactions from the creatine kinase (CK) pathway and the adenylate kinase (AK) pathway. However, the CK pathway can only work at max capacity for a short period for resting phosphocreatine (PCr) concentrations are only about 4-6 times the amount of resting ATP stores. Once the PCr concentrations are depleted, the AK reaction will begin by using two adenosine diphosphate (ADP) to form one ATP and one adenosine monophosphate (AMP). Although ATP is produced in this pathway, this production of ATP does coincide with an increased concentration of AMP. This is problematic because increased AMP levels will in turn stimulate the adenylate deaminase reaction, which will produce ammonia (NH3). This conversion of AMP into NH3 will result in the muscle cell having a net loss of total adenine nucleotides available to resynthesize ATP.
Glycolysis is the next reaction in line, which increases its role in ATP replenishment as PCr stores become depleted and aerobic metabolism cannot meet ATP demands of work being performed. During intense exercise, the demand of glucose as a substrate can increase and consequently stimulate hepatic glucose output (via glycogenolysis) and glucose uptake in the working muscle (via increased GLUT4 translocation). Pre- and post- activity measurements of blood glucose concentrations (BG) or muscle glycogen via muscle biopsies are the most common method of detecting changes in glucose metabolism. Due to timing and the problems of differentiating between the rates of BG appearance and clearance, it is difficult to determine the direct role of glucose metabolism during the actual resistance training (RT) protocols. In addition, the studies that show muscle glycogen depletion following resistance training use as many as 6-10 sets to failure with a loads ranging from 30%-80% of the one repetition maximum (1RM). Moreover, these same research studies were usually was limited to a single movement (e.g. knee extension). Thus, the applicability of the glycogen and glucose utilization studies results to normal RT training programs which employ multiple activities and joint movements is questionable.
In addition to measuring the usage of PCr and glucose, the impact of the anaerobic pathways upon RT protocols can be seen from either creatine or carbohydrate supplementation studies. The consensus of these supplementation studies suggest that the use of PCr has a greater impact upon RT than glucose usage. The benefits of creatine supplementation are seen following both immediate post supplementation and following a training in the supplemented state. For instance, the number of repetitions of a task consisting of 5 sets to failure at the 10 repetition maximum increased after six days of creatine supplementation without any resistance training during the six days. Similarly, isokinetic power has been shown to increase after 5 days of creatine supplementation without any resistance training. Also, other research has found an increased peak torque after 5 days, and an increased 1RM after 28 days of creatine supplementation without any resistance training. Additionally, when creatine supplementation is combined with a resistance training program, numerous studies have reported that individuals achieve much greater strength gains in the form of an increased 1RM, or greater number of repetitions to failure at a given percentage of the 1RM.
The influence of glucose feedings upon resistance training is equivocal. For example, one study showed that while glucose feedings reduced muscle glycogen loss when doing 3 sets of 10 repetitions of back squats, speed squats, and one-leg squats, the glucose feeding did not influence performance. Likewise, four days of a high carbohydrate diet did not improve the power output during multiple sets of maximal jump squats when compared to a lower carbohydrate diet. Also, the total number of completed sets of 5 repetitions of squats using 85% 1RM did not differ between glucose supplementation and control. On the other hand, in a study where individuals did morning and afternoon workouts where the participants did as many complete sets as possible of 10 repetitions of squats at 55% 1RM, it was found that that consuming a 0.3 g·kg body mass-1 carbohydrate supplement after each set allowed the individuals to perform more complete sets in the afternoon workout. In another study, when measuring the work done by individuals performing 16 sets of 10 repetitions of maximal isokinetic knee flexion/extension at 120o·s-1, the individuals performed more work following consumption of a 1.0 g·kg body mass-1 carbohydrate supplement pre-exercise and 0.51 g·kg body mass-1 carbohydrate supplement after sets 1, 6, and 11. Finally, total work as well as time to exhaustion increased when the participant preceded a static exercise protocol consisting of 20 s of work at 50% of maximal voluntary contraction with the ingestion of 1.0 g·kg body mass-1 carbohydrate coupled with additional carbohydrate supplements of 0.7 g·kg body mass-1 every 6 min.
In conclusion, substrate availability can have significant effects on resistance training, particularly with high volume workouts that consist of multiple exercises targeting a single joint movement. Thus, in addition to ensuring the proper number of sets, repetitions, and loads, the athlete should take steps to ensure that their dietary practice before, during, and after training sessions are properly monitored in order to keep energy stores at optimal levels. Since the aforementioned studies consisted of primarily a single exercise, further research is needed to determine the effect of glucose and creatine concentrations and/or supplementations on resistance training sessions which are composed of several different lifts. It would also be a benefit if the research into glucose and creatine concentrations and/or supplementations, investigated how these feedings effect other factors such as metabolite concentration/accumulation, performance variables such as peak force/velocity and time to peak force, recovery time, and the speed of strength, power and muscle mass gains.
Arnold Nelson,
PhD, Louisiana State University, USA