1. Introduction
Decreased muscle mass in older people is attributed to impaired skeletal muscle ability to respond to anabolic stimulation and to increase the activity of proteolytic signaling pathways [1]. According to studies, low exercise intensities like 30% of 1-repetition maximum (1RM) to voluntary fatigue are effective in stimulating muscle protein synthesis and hypertrophy [2]. In other words, in fatigue conditions, increased activity of the motor unit leads to increased activity of high-threshold motor units that innervate type II fibers and increases stimulation for hypertrophy and muscle strength [3-5]. Since hormone changes and growth factors are involved in the effect of resistance training with different loads on muscle hypertrophy, this study aimed to investigate the impact of 8-week resistance training to volitional fatigue with 30% and 80% of 1RM on testosterone, insulin-like growth factor 1 (IGF-1), IGF binding protein-3 (IGFBP-3), and functional adaptations in older women.
2. Materials and Methods
The study participants were 28 older women living in Mashhad City, Iran who were randomly assigned into three groups of resistance training with low load (RT+LL), 30% 1RM (n=12), resistance training with high load (RT+HL), 80% 1RM (n=8), and control (n=8). The training was conducted 3 sessions per week for 8 weeks. The training protocol included warming up, training to fatigue with low and high loads (30% and 80% of 1RM), and cooling down. The minimum number of repetitions was 20 for the RT+LL group and 8 for the RT+HL group.
To estimate the maximum strength of the participants, we used a weight by which the subject could perform training correctly up to 10 times. Using the Brzeski Equation (Formal 1), the maximum strength of the subjects was achieved in performing bench press and knee flexion to determine the maximum upper and lower body muscle strength [6].
1. 1RM= weight / [1.0278- (0.0278×number of repetitions)] 
The Shapiro-Wilk test was used to check the normality of the data distribution. The paired t test was used to examine within-group differences, and one-way ANOVA and Bonferroni post hoc test were used to determine the differences between groups. The significance level was set at P<0.05.

3. Results
Five milliliters of blood samples were taken from the vein of subjects in the fasting time before the intervention and eight weeks after training to measure their biochemical factors. All factors were measured by enzyme-linked immunosorbent assay (ELISA) method (Table 1).

The results showed that the resistance exercise had no significant effect on testosterone, IGF-1, and IGFBP-3 levels (P>0.05). Both types of low-load and high-load training significantly increased lower body muscle strength and endurance (P<0.05), and there was no significant difference between the two groups (P>0.05) (Table 2).

4. Conclusion
The results showed that eight weeks of low- and high-load resistance training to fatigue had no significant effect on IGF-1, IGFBP-3, and testosterone, but both types of exercise increased muscle strength and endurance in older women. Low-load resistance training significantly improved muscle function; also, it had a positive but not significant effect on some biochemical factors related to muscle strength and muscle hypertrophy. Although higher intensity resistance exercises may have more beneficial results, they can put too much pressure on the musculoskeletal and cardiovascular system, and besides their potential unpleasant consequences, they can make the elderly feel tired and drained [8]. Therefore, it is recommended to use low-load resistance training where it is not possible to use high-load resistance training (e.g. in rehabilitation, chronic diseases, or physical disability).
Ethical Considerations
Compliance with ethical guidelines
This study received its ethical approval from the Research Ethics Committee of Hakim Sabzevari University.
Funding
The present paper was extracted from the PhD. thesis of first Author, Esmat Rashidi, in Department of Sport Physiology and Sport Management, Faculty of Sciences Sport, Hakim Sabzevari University.
Authors' contributions
All authors contributed in preparing this article.
Conflicts of interest
The authors declared no conflict of interest.
 

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