Sex Differences in Muscle Damage Following Resistance Exercise at Low or High Intensity

Purpose: To investigate the impact of exercise load 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 to perform resistance exercise with lighter loads, i.e. \<70% one repetition maximum (1RM). Low-load resistance training has shown to induce comparable gains in muscle mass and strength to high-load (≥70% 1RM), while being perceptively less exerting. Low-load resistance exercise may place less mechanical stress on muscle fibres and accordingly, its impact on muscle damage has been investigated. While several studies have reported less severe muscle damage, muscle soreness, and functional impairments with low-load resistance exercise compared to high-load, others have found no differences. Further, there is a lack of studies conducted solely in females or comparing between sexes. 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 performed with low or high loads.

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 low-load (30% 1RM) or high-load (80% 1RM) exercise condition.

Three weeks later, participants will complete a resistance exercise session at their allocated intensity 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' 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 less severe in the low-load exercise condition. It cannot be ascertained whether males and females will have the same responses to the exercise.


Eligibility Criteria:
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, anti-oxidant supplements, polyunsaturated omega-3 fatty acids (and other substances that may alleviate muscle damage) 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
* 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
* Unwilling to provide blood samples, perform resistance exercise, or abstain from use of NSAID's and other substances (stated above)
* Unwilling to abstain from other forms of exercise during the experimental period