Skeletal Muscle Glycogen Depletion and Recovery During Four ConsecutiveDays of Prolonged Lift and Carry Exercise

A substantial portion of the nation’s working population has jobs that involve lifting and carrying heavy objects. Muscles metabolize carbohydrate stores to accomplish such work. Little is known about how muscles replenish carbohydrates from day to day during the workweek. Objective This study documents muscle glycogen depletion and recovery in two muscles routinely used in extended lifting and carrying exercise, and determines the extent to which four days of such exercise affects muscle glycogen levels. Methods Ten subjects (5 M, 5 F) were studied; age 25±4y M, 22±2y F, weight 92±8kg* M, 62±5kg F, and height 185±3cm* M, 170±2cm F. Subjects recorded their diet before and during the protocol. On four consecutive days subjects were asked to squat to floor level and lift a 30kg box, carry it 3m, and place it on a shelf 132cm high. This was repeated 3X/min over a three hour period (540 lifts) or until the subject could no longer continue. Subjects were allowed five minutes rest every 30min. Exercise was performed at the same time of day, allowing nineteen hours of recovery between bouts. The protocol was not normalized for subject gender or size. Natural abundance C-13 NMR was performed on the left quadriceps and left biceps brachialis immediately before and after each exercise bout. Ability to complete the prescribed protocol, dietary intake before and during the protocol, and muscle glycogen levels before and after exercise were recorded and compared. Results Subjects differed significantly by gender in their ability to complete the four-day protocol (12 hours total protocol: 10.8±0.9hr M, 6.4±1.6hr F, p=0.0366). Dietary intake did not differ during the four-day protocol versus prior to the study (2109±256kcal/da M prior, 2107±87kcal/da M during, 1657±136kcal/da F prior, 1755±331kcal/da F during). In the biceps brachialis (both genders combined) pre-exercise glycogen levels rose significantly over the four-day protocol (vs day one) [62.3±3.6mmol/L D1, 68.5±4.6mmol/L (p=0.0437) D2, 75.1±4.9mmol/L (p=0.0019) D3, 81.9±5.4mmol/L (p=0.0003) D4, paired analysis vs D1]. In the quadriceps a similar pattern was seen [92.2±9.0mmol/L D1, 101.3±8.9mmol/L (p=0.0107) D2, 110.3±10.2mmol/L (p=0.0089) D3, 115.9±9.8mmol/L (p=0.0003) D4 paired analysis vs D1]. Conclusions We conclude that male and female muscle glycogen is similarly super compensated between each day of four consecutive days of prolonged exercise, in the absence of increased dietary intake.