Where do Your Strength Gains Come From? Muscle Activity > Hypertrophy > Initial Strength - 3/5 Candidates Matter

Find out what's taking you from PR to PR, is it an increase in muscle size or activation on how important is how strong you already are?

Have you ever asked yourselves why you've been adding 20lbs of weight to your squat and your legs still don't look any bigger? If you're a man you're probably not happy about that. I am not sure if the insights into Balshaw et al.'s recent study provide into the mechanism behind the resistance-training-induced strength gains will help you will make you happy/-ier, but certainly smarter ;-)

The British scientists got to the bottom of your gains by assessing the individual and combined contribution of the adaptations in neural (agonist quadriceps EMG, antagonist hamstring EMG) and morphological (quadriceps muscle volume and θp, the fascicle pennation angle) variables.
Based on the data from their 12-week study in twenty-eight healthy young men, who had not completed lower body RT for >18 months and were not involved in systematic physical training, the scientists were able to calculate the individual contribution of the previously named variables and the trainees' baseline strength on the effect of the following workout:

"After a brief warm-up of submaximum contractions of both legs, participants completed four sets of ten unilat eral isometric knee extensor contractions of each leg; with sets alternating between dominant and non-dominant legs. Each set took 60 s with 2 min between successive sets on the same leg" (Balshaw 2017). 

To differentiate potential interference with explosive vs. sustained contractions, the participants were further randomized to two groups:

• the explosive contraction group completed short, explosive contractions with participants instructed to perform each contraction “as fast and hard as possible” up to ≥80% MVT for ~1 s, and then relax for 5 s between repetitions.
• the sustained contraction group completed prolonged contractions at 75% MVT, with 2-s rest between contractions. 

For both groups, the scientists provided a computer monitor that displayed the rate of torque development (10-ms time epoch) and a target torque trace 2 s before every contraction, in the explosive and sustained contraction group, respectively.
Each subject performed the above isometric knee extensor RT thrice a week (3/week). Before and after isometric maximum voluntary torque (MVT) as well as the neural drive to the agonist (QEMGMVT) and antagonist (HEMGANTAG) were assessed simultaneously. In addition, QUADSVOL was determined with MRI and QUADSθp with B-mode ultrasound.

Figure 1: Relationships of percentage change (∆) in knee extension may. voluntary torque (MVT) and ∆ quadriceps muscle volume (QUADSVOL; r = 0.461, P = 0.014), b ∆ quadriceps muscle fascicle pennation angle (QUADSθp; r = −0.207, P = 0.291), after 12 wks of resistance training. Solid and dashed lines indicate the trend of the relationship between variables and 95% confidence intervals, respectively. Black triangles denote sustained-contraction resistance training participants (n = 15); white circles denote explosive-contraction resistance training participants (n = 13 | Balshaw 2017)

I have to say that the way the scientists plotted the data is a bit odd - with the strength gains being the actual outcome variable of interest, I would expect it to be placed on the vertical, not the horizontal axis... but anyway. Figures 1-2 tell you that...

• hypertrophy contributes quasi-linearly to the gains (Figure 1 A) - I would estimate the reciprocal of the slope of the linear regression line to be ~2.5, meaning for each 1 % increase in muscle volume there was a 2.5% increase in maximal voluntary torque;
• agonist activity changes contribute quasi-linearly to the gains (Figure 2 A) - I would estimate the reciprocal of the slope of the linear regression line to be "only" ~0.8, meaning for each 1% increase in muscle activity there was a 0.8% increase in maximal voluntary torque;
• pre-training strength negatively predicts the strength gains (Figure 2 C) - What may sound odd, initially, is actually only logical. The stronger you are at baseline, the lower your strength gains are going to be. For this relationship, I would estimate the slope of the linear regression analysis to be approx. -1.5, which means that for each extra Newton-metre (nM) of pre-training maximal voluntary torque, the increase in response to training will be reduced by 1.5%;
• pennation angle and antagonist activity changes do not contribute clearly to the strength gains (Figure 1 B, Figure 2 B) - you know that because there was no clear correlation between the corresponding variables in the regression analysis the scientists did

Figure 2: The relationships between the percentage change (∆) in knee extension maximum voluntary torque (MVT) and: (A) ∆ quadriceps EMG at knee extension MVT (QEMGMVT; r = 0.576, P = 0.001); (B) ∆ antagonist hamstrings EMG during knee extension MVT (HEMGANTAG; r = 0.298, P = 0.123) and (C) pre-training knee extension MVT (r = −0.429, P = 0.023), after 12 weeks of resistance training (Balshaw 2017).

The pennation angle, and antagonist activity (here the hamstring) on the other hand appear to contribute only marginally to the increase in strength gains the previously untrained subjects saw over the course of the 12-week study.
References:

• Balshaw, Thomas G., et al. "Changes in agonist neural drive, hypertrophy and pre-training strength all contribute to the individual strength gains after resistance training." European Journal of Applied Physiology (2017): 1-10.
• Erskine, Robert M., Gareth Fletcher, and Jonathan P. Folland. "The contribution of muscle hypertrophy to strength changes following resistance training." European journal of applied physiology 114.6 (2014): 1239-1249.