Sex Differences in Muscle Damage Following Resistance Exercise With or Without Milk Protein Ingestion

Sex Differences in Resistance Exercise-induced Muscle Damage: The Impact of Milk Protein Ingestion

Purpose: To investigate the impact of milk protein ingestion on resistance exercise-induced muscle damage in untrained males and females.

Rationale: Unaccustomed resistance exercise can cause muscle damage, presenting as muscle soreness and reduced muscle function - such as loss of strength, power, and flexibility - for several days after the exercise bout. Therefore, individuals may require longer recovery periods before performing another exercise bout, and their performance may be impaired. Further, muscle soreness may reduce exercise compliance, particularly in novice individuals. Over time, this may compromise the gains in muscle mass and strength achieved through exercise training. Therefore, strategies to reduce the severity of exercise-induced muscle damage and/or to enhance post-exercise recovery processes are advantageous for exercising individuals.

One such strategy is the consumption of dietary protein before or after muscle-damaging exercise, which has shown to alleviate muscle soreness, improve blood markers of muscle damage, and reduce the decline in maximal force and flexibility. In particular, consuming 20-gram doses of milk protein in the days after resistance exercise can improve the recovery time of muscle soreness and maximum force, and also lower levels of damage markers in the blood. However, most studies have been conducted with male participants who are well-trained in resistance exercise. It has been suggested that males and females respond differently to muscle damage, and therefore, this research aims to provide a sex comparison in the muscle damage response to an acute bout of resistance exercise with or without milk protein feeding.

Therefore, 40 healthy, young (18-35 years) adults (20 males, 20 females) will be recruited to participate in this randomised controlled trial. Maximal leg strength and body composition (by dual-energy X-ray absorptiometry; DXA) will be conducted at baseline. In females, all primary outcome measures will be obtained during the late follicular phase of the menstrual cycle. Participants will then be randomised to a protein (dairy yoghurt) or placebo (oat-based yoghurt) dietary condition.

Three weeks later, participants will complete a high-intensity resistance exercise session on leg extension and leg curl machines to induce muscle damage. Various measures of muscle damage (blood biomarkers, muscle soreness, flexibility, and swelling) will be obtained before, immediately after, and 24, 48, 72, and 168 h after the exercise protocol. The maximal strength test will be repeated 72 and 168 h after the exercise. Participants will consume the protein or placebo yoghurt 4 times per day (every 3-4 hours) on the day of the exercise bout and the following 3 days. Participants' habitual activity and dietary intake will be monitored and controlled throughout the study period.

Expected outcome: It is expected that the resistance exercise protocol will induce muscle damage, which will be attenuated with the ingestion of milk protein. It cannot be ascertained whether males and females will have the same responses to the exercise or to protein ingestion.

Inclusion Criteria:

• BMI 18.5 - 25.0 kg/m2
• Untrained in resistance exercise
• No known chronic disease or current acute illness
• No current or recent (past 3 months) musculoskeletal injury
• No frequent use (2x per week for past month) of non-steroidal anti-inflammatory drugs and compliant to abstain from use during experimental period
• No recent or current engagement in massage or cryotherapy and compliant to abstain from use during experimental period
• No current use of protein or antioxidant supplements
• Are able to consume dairy products (i.e., not lactose-intolerant or vegan)
• Females will be eumenorrheic (regular menstrual cycle) >12 months
• Absence of pregnancy and breast-feeding

Exclusion Criteria:

• Underweight
• Overweight/obese
• Resistance trained
• Current or recent injury
• Pregnancy or breast-feeding
• Lactose intolerant
• Unwilling to provide blood samples, perform resistance exercise, or abstain from use of NSAID's and protein supplementation (unless instructed as part of the research)
• Unwilling to abstain from other forms of exercise during the experimental period