“Heterogeneity in Resistance Training-Induced Muscle Strength and Mass Responses in Men and Women of Different Ages”, Juha P. Ahtiainen, Simon Walker, Heikki Peltonen, Jarkko Holviala, Elina Sillanpää, Laura Karavirta, Janne Sallinen, Jussi Mikkola, Heli Valkeinen, Antti Mero, Juha J. Hulmi, Keijo Häkkinen2016 ()⁠:

Physical activity recommendations for public health include typically muscle-strengthening activities for a minimum of 2 days a week. The range of inter-individual variation in responses to resistance training (RT) aiming to improve health and well-being requires to be investigated.

The purpose of this study was to quantify high and low responders for RT-induced changes in muscle size and strength and to examine possible effects of age and sex on these responses. Previously collected data of untrained healthy men and women (age 19–78 years, n = 287 with 72 controls) were pooled for the present study.

Muscle size and strength changed during RT are 4.8 ± 6.1% (range from −11 to 30%) and 21.1 ± 11.5% (range from −8 to 60%) compared to pre-RT, respectively. Age and sex did not affect to the RT responses. 14% and 12% of the subjects were defined as high responders (>1 standard deviation (SD) from the group mean) for the RT-induced changes in muscle size and strength, respectively.

When taking into account the results of non-training controls (upper 95% CI), 29 and 7% of the subjects were defined as low responders for the RT-induced changes in muscle size and strength, respectively. The muscle size and strength responses varied extensively between the subjects regardless of subject’s age and sex.

Whether these changes are associated with, eg. functional capacity and metabolic health improvements due to RT requires further studies.

Figure 2: Histogram of muscle strength (a) and size (b) changes (relative to baseline) in men and women in the training group. Black bars denote responses of men, while grey bars denote responses of women
Figure 3: Heterogeneity of muscle strength (a) and size (b) training responses in relation to the baseline value of different age groups. Black bars denote responses of men, while grey bars denote responses of women
Figure 4: Heterogeneity of muscle strength (a) and size (b) training responses in relation to the baseline value in the training and control groups. ‘High’ and ‘low’ responders in the training group were denoted by the vertical dotted lines. Individuals with training response below the upper 95% CI of control group were defined as ‘low responders’. Individuals with training response beyond 1 SD from the mean of the training group were defined as ‘high responders’. Black bars denote responses of men, while grey bars denote responses of women.