[Hayden Giuliani recently finished her Master’s degree at the University of North Carolina Chapel Hill, where she now works as a research coordinator. She is currently in the Coaching Mentorship Program at Athletic Lab.]
So what do we really know about strength training? What happens beneath the surface to increase our strength and power “that ever so desired outcome” Strength training has been growing in popularity not only for sport but also for the general population, but sometimes we don’t always understand the benefits it provides for our body. Neuromuscular changes underlie what happens when you strength train, i.e. growth of muscles, increased strength and power and increased anaerobic capacity. We will be discussing both what happens within the muscles and also the brain’s communication to them.
Before we proceed, I want to be clear about the kind of strength training I am talking about. I am designing this post at the type of strength training that high-level athletes and advanced lifters use. It is designed to impose a progressive overload, with high intensities and athletic movements. I would like to emphasize that strength training should be an 1) overload and 2) progressive in nature. (It should be considered that not all research studies follow this guideline, but the ones mentioned here generally did.) With an organized strength training plan, we typically see increases in strength (either tested as a 1- repetition maximum or maximal voluntary contraction), increases in power and speed, and greater muscle size (hypertrophy). The extent to which these adaptations occur can vary, based on age, gender, and training age. In addition, training load/ intensity, volume, and frequency also play critical roles that I will discuss throughout.
In a long-term study by Ahtiainen and colleagues (2003), muscle strength and size increased in both trained and untrained individuals, with a greater degree of improvement found in the untrained group. For example, lower extremity isometric force increased 21% compared to 4% and 1- repetition maximum (1RM) increased 19% v 7% over 21 weeks. Interestingly, although muscle cross-sectional area (CSA) of a quadriceps muscle only increased in the untrained group, the trained individuals showed a greater isometric force to CSA ratio before and after the training protocol. The authors implied that this might mean that the trained individuals may have greater activation of the agonist muscles, and there is greater capacity for improvements for untrained individuals in other neuromuscular adaptations. This study lays the groundwork, showing that there are large improvements available to be made, whether an individual comes in trained or relatively untrained.
Underlying mechanisms that contribute to increases in muscle strength, power, and size have been further investigated in many studies, particularly a few classic ones by a group of researchers led by Per Aagaard (2001,2002). The first of these mechanisms that I will discuss is muscle architecture, which is typically defined as the physical arrangement of muscle fibers that determines a muscle’s mechanical function and broken down into pennation (fascicle) angle and fascicle length. After 14 weeks of strength training, it was found that the pennation angle increased by 36%. An increase in pennation angle corresponded to an increase in muscle CSA and force-generating capacity. A separate study by Blazevich and colleagues (2007) investigated both pennation angle and fascicle length. The authors found that strength training resulted in both increased pennation angle and increased fascicle length after 5 weeks. Fascicle length was shown to have a close association with the shift in the torque-angle relationship, which corresponds to a greater force output at a wider range of motion. (Figure 1) This same study also showed there was no difference between concentric training and eccentric training on muscle architecture in the lower extremity. Eccentric training has traditionally been shown to be more highly associated with improvements in fascicle length. Based on this, it seems that the training range of motion and speed of contraction, rather than the training mode, is associated with greater fascicle lengths and greater force, but eccentric training may produce changes more quickly. (Narici 2007) The roles of these two factors should be looked at collectively because both play a significant role into the improvements of strength and power. While a greater pennation angle may allow for greater torque development by creating more torque, and fascicle length may contribute more to velocity, strength training can improve each and will, therefore, improve performance.
Though the changes in muscle architecture indirectly contribute to muscle hypertrophy, there are changes within the muscle that more directly contribute to hypertrophy. A common method of investigating the changes in overall muscle size is ultrasound (similar to the ultrasound used to look at a baby within a mother’s womb), which can measure cross-sectional area (CSA). As mentioned earlier from the Ahtiainen study (2003), CSA increases with strength training. Aagaard and colleagues (2001) have shown increases in CSA of the lower extremity muscles up to 10%, while another study by Hakkinen (1998) showed 12% in just 10 weeks. These same studies investigated changes within the muscle fibers that may explain the change in CSA, by using muscle biopsies. Muscle fibers are simply categorized type I or type II, which can be broken down further into type IIa and type IIx. Type I is associated with slower twitches or more endurance types of activities, while type II tends to associate with faster twitches or strength/power. With strength training, both types of muscle fibers increase in size, but type II fibers show a greater capacity for enhancement. For instance, in the Aagaard study (2001), the average increase for all muscle fibers was 16%, but type II increased 18% alone. With an increase in the size of the individual fibers, there is an inherent increase in the overall size of the muscle. Some muscles may respond more readily than others in regards to this mechanism, but this occurs to some effect (significantly or non-significantly) in almost all instances of strength training. I will explain later the instance in which this may not occur. Some people may believe that muscle growth will only occur when a number of sets and repetitions fall within a certain hypertrophic range (e.g. 3-4 sets, 8-10 reps), but that isn’t necessarily the case. Traditionally, volume has been thought of as the primary influence on hypertrophy, but it seems more recent reviews show that intensity (load) can influence this greatly, meaning that load, sets, reps, rest intervals, and type of contraction (concentric or eccentric) can be varied to produce similar responses. (Wernbom 2007) (Hypertrophy is influenced by the amount of muscle damage, and subsequently, the hormonal response to a bout of exercise and chronically within an individual, which is beyond the scope of the present article.)
We have discussed the muscular side of the neuromuscular adaptations, so now it is time to dive into what happens within our nervous system, which may hold even more important implications. For untrained individuals, neurological adaptations occur earlier and more quickly than muscular adaptations, with strength training, which most likely explains why there were differences between groups in the Ahtiainen et al. study mentioned earlier. Basically, the rapid improvements noted in untrained individuals can be attributed to neurological adaptations, rather than muscular ones. The simplest way to monitor neurological changes is the change in EMG (electromyography) activation. EMG demonstrates the frequency of the signals from the brain to the muscle as it contracts. Neural activation will occur before motor activation, and there are a few factors that contribute to increased neural activation. This includes increased motor unit activation, increased motor unit firing rate, improved synchronization of the motor units, and a decrease in antagonist muscle activation. Though more invasive techniques are necessary to establish which of these factors contributes primarily to a more efficient neural pathway, the increase in EMG amplitude seen after training establishes that there are clear adaptations occurring. By training a specific motor pattern as with strength training, those specific motor units are activated and increase in synchronization, which is really just muscle memory. The breakdown of EMG signals can paint a very general picture of which of these factors are improving. (Example: Figure 2)
For example, Aagaard and colleagues (2001) found a significant increase in EMG amplitude (73-143%) in one of the vastus lateralis muscle, which would correlate to the increased motor unit activation and increased use of the muscle’s capacity. The authors also found an increase in the rate of force development (RFD), from 15-21%, which is associated with increased firing rate and synchronization. RFD is incredibly important not only for explosive movements in sports but also for the prevention of falls in aging. Similarly, contraction impulse can tell us not only RFD characteristics but also contraction time, which has also been shown to increase with strength training. (Aagaard 2002) Lastly, the rate of EMG rise, which has also been shown to improve significantly by 54-106%, implies a greater firing frequency and plays a large role in the increase in RFD. (Aagaard 2001). Past all the numbers and various terms, there are many contributors of neural adaptations, but luckily, the more strength training is performed, the better they all work together to provide the desired response, which is greater power and overall strength. If an individual can activate their muscle more quickly, then theoretically they can produce more force, and more force quickly.
All this information can become fairly technical and complicated the more you dive into the research and this article only scratches the surface, but here are some take-away s. With strength training, microscopic adaptations, such as the growth of the muscle fibers, improving the alignment of the fibers to allow for mechanical advantages, and increased neural activation, contribute to the macroscopic changes in strength and power. Although strength training is beneficial for all people, here are some additional implications for different types of athletes. For strength and power athletes it may seem pretty obvious that they should spend time focused on strength training. It improves RFD and mechanical power, enhances sport-specific skill by working the necessary neural pathways, and enhances general athletic movement while decreasing injury risk. (Suchomel 2016) As mentioned above, there are multiple factors that play a role in deciding what the training program will look like, and there is not “one-size-fits-all”. Intensity/load, repetitions, sets, intra- and inter-set rest periods, and frequency can all be adjusted based on the type of athlete and the goals of training. It is particularly important, though, to be consistent with strength training, as that is how many of these adaptations occur. Also, strength training will improve general sense all factors, but the specificity of neural adaptations should be one reason for training sport-specific pathways and systems in the weight room.
For the endurance athlete, it may not seem like the obvious option, but in fact, it can be beneficial for them too. It has been shown, specifically through a review of the literature by Aagaard and Andersen (2010), that strength training improves both short-term and long-term aerobic capacity. Five-kilometer times and long distance time trials improved after the addition of strength training, while still performing normal endurance training. Even more interesting is that the high muscle loading intensity (i.e. working at 85-95% of maximum) and a large volume of strength training showed more benefit than lighter loads. Increased RFD, maximal force output and a relative contribution of type II fibers contribute to an increased capacity to do work at higher outputs, for instance, cycling at a greater wattage for a longer period of time. The best part is that all these improvements come without the muscle’s hypertrophic response, which could hinder endurance performance. Endurance athletes could simply replace 30% of their aerobic training with strength training, or explosive-type training, and be able to improve endurance performance. Sounds like a good addition to me.
References
Ahtiainen JP, et al. (2003) Muscle hypertrophy, hormonal adaptations, and strength development during strength training in strength-trained and untrained men. Eur J Appl Physiol, 89:555-563.
Aagaard P, et al. (2002) Increased rate of force development and neural drive of human skeletal muscle following resistance training. J Appl Physiol, 93: 1318-1326.
Aagaard P, et al. (2001) A mechanism for increased contractile strength of human pennate muscle in response to strength training: changes in muscle architecture. J Physiol, 532.4:613-623.
Blazevich AJ, et al. (2007) Influence of concentric and eccentric resistance training on architectural adaptation in human quadriceps muscles. J Appl Physiol, 103: 1565-1575.
Seynnes OR, et al. (2007) Early skeletal muscle hypertrophy and architectural changes in response to high-intensity resistance training. J Appl Physiol, 102:368-373.
Hakkinen K, et al. (1998) Changes in muscle morphology, electromyographic activity, and force production characterstics during progressive strength training in young and older men. J Gerontol, 52A-6: B415-B423.
Suchomel TJ, et al. (2016) The importance of muscular strength in athletic performance. Sports Med, DOI 10.1007/s40279-016-0486-0.
Aagaard P & Andersen JL. (2010) Effects of strength training on endurance capacity in top-level endurance athletes. Scand H Med Sci Sports, 20:39-47.
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