Electric muscle stimulation (EMS), also known as neuromuscular electrical stimulation (NEMS) or percutaneous electrical stimulation (PES), is a method of eliciting muscle activity through applied electrical current. Your muscles naturally contract in response to electrical signals sent from your brain, and EMS replicates this with electrodes placed on the skin and a current run through them from a power source (Figure 1).
Figure 1. An example of an EMS unit with the electrodes (pads) placed on the quadriceps muscles
There are a couple of interesting physiological differences between a voluntary contraction through the central nervous system and an involuntary contraction through EMS. First, while a voluntary contraction will recruit smaller motor units and slow-twitch, Type-I fibers first and then activate the Type-II fibers as needed, an EMS contraction reverses this order. Because the involuntary contraction bypasses this neurological coordination, and because the applied current flows more easily through the larger neurons of the fast-twitch fiber, they are activated immediately, a response that is impossible to achieve through our own volition. Second, a voluntary contraction activates individual fibers in relays in order to conserve energy and not tire too quickly; EMS activates all of the motor units in the area at the same time, contracting all of the fibers with no holding back.
For much of the 20th century, EMS was used for orthopedic rehabilitation and physical therapy, specifically neuromuscular reintroduction and prevention of atrophy. However, it wasn’t until a Soviet scientist presented to the West in 1973 what Communist Bloc countries had been doing for 2o years: EMS as a method of strength training. Dr. Y. Kots of the Central Institute of Physical Culture in the USSR claimed to see up to 30-40% strength gains in already-trained individuals using specific methods of EMS, and while these results are quite extreme and have not been replicated in Western studies, the years of research since have shown that training with EMS leads to greater increases in isokinetic peak torque, maximal isometric strength, and maximal dynamic strength. However, a more useful way of looking at EMS is its effect on athletic performance – can it lead to improved performance, and is it a viable training option?
As seen in Figure 2, many studies have proven the efficacy of EMS in improving strength and jumping ability, and in some cases sprinting ability. The longest-running study from this paper was the 2007 study on elite rugby players, which lasted 12 weeks. A test group of 15 players went through two 6-week bouts (first 3 sessions/week, then 1 session/week) of EMS on the plantar flexors, knee extensors, and gluteus muscles, and a control group of 10 players received no training; both groups performed tests at 0, 6, and 12 weeks. The EMS group showed improvements compared to the controls not only in strength (squat, leg extension) but also in power (squat jump and drop jump height), an attribute more translatable to sport performance. The test group, however, saw no power increases after 6 weeks, only 12, and no improvement in sprint times over 12 weeks. The decision to change the training protocols halfway through the experiment does not discredit the results, but makes it more difficult to clearly see the relationship between protocol and result. The study on tennis players, at 4 weeks long, showed large improvements in maximum voluntary contractile force of the quadriceps and small, yet significant, improvements in sprint times and counter-movement jump heights. The study on soccer players, at 5 weeks long, showed the greatest improvements in strength and kicking power (measure by ball velocity) to come between weeks 3 and 5, with no improvements before week 3. It, however, demonstrated EMS to cause no changes to sprint ability.
Figure 3. A professional soccer player exerting force onto a soccer ball
While these studies and many others confirm the ability of EMS intervention to improve strength and in many cases other measures of power that relate to on-field performance, they do not compare EMS directly to a voluntary training program. The rugby study’s control group underwent no additional training, and the tennis and soccer studies had no control groups. They each could have had a group that underwent voluntary strength training in parallel to the other groups. The argument against this, though, is that a voluntary training program requires more time and effort that is in short supply for busy athletes; the tennis study mentioned the virtue of EMS for players with busy competitive schedules who don’t have the time for voluntary strength training. In this regard, it appears that EMS can be used as a tool to enhance athletic performance. However, the most valuable questions right now concern how to incorporate EMS with regular training programs, at different periods of athletes’ competitive schedules, to reap the greatest benefits of sport-applicable muscle function.
Barbells have been used for strength training for centuries, and the basic design of those used today was invented in 1928, yet they remain one of the most popular and effective exercise tools out there. From the main power lifts of bench, squat, and deadlift, to the olympic lifts of clean, jerk, and snatch, and limitless other movements, a barbell can be used to target any muscle group to improve strength and power. However, it must retain its shape. Through countless loading cycles, years of use, and sometimes extreme bending stresses, a barbell needs to be ready to be picked up and used again right away, and that means it cannot yield, or permanently bend – this would make it more difficult to use, change its motion patterns, and put it at risk of breaking. While typical use of a barbell for most people would not push it to its mechanical limits (Figure 1), those who compete in weightlifting often place so much weight on the ends of the bar that it indeed bends very much (Figure 2). A barbell must be constructed of the proper material to withstand the loads it is placed under and bend without becoming permanently bent – or, in engineering terms, deform elastically but not plastically.
Figure 1. Use of a loaded barbell to perform a deadlift
Figure 2. Use of an extremely heavily loaded barbell to perform a deadlift
When it comes to competition weightlifting, there are actually different dimensions and specifications required of barbells used for different lifts – read about it here. I decided to focus on a barbell for deadlifting because it’s the movement that can be done with the most weight and is not dynamic like olympic lifts. I borrowed dimensions from the most commonly-used barbell for deadlifting, the Texas 7-1/2″ Bar (Figure 3).
Figure 3. Dimensions of the most commonly-used barbell made for deadlifting
I also decided to design for preventing yield failure rather than fatigue failure because it is a more pressing design concern; it would make more sense to constrain for yielding and optimize for fatigue life rather than the other way around.
The world record deadlift is 500 kg (1,102.3 lbs) by Eddie Hall, so I used a weight of 453.6 kg (1,000 lbs), as events involving more weight than this are so infrequent that yielding in that case would not be of particular concern. This weight is divided into two evenly distributed loads at the ends of the bar, treated as a point load at the center of the distribution, while the opposing forces act where the hands would be placed (I assumed this to be the middle of the knurled portion as seen in Figure 3) [Figure 4].
Figure 4. Lifting of a barbell designed as a beam deflection problem
However, the problem can be simplified to fit a common pattern of loading/support (Figure 5), allowing for a few simple hand calculations to find the stress in the bar. This requires ignoring the weight of the bar itself (which, because of its even distribution and relative lightness, is not crucial anyway) and placing the loads at the very ends of the bar. In the end these assumptions will skew the estimate towards a slightly higher stress, giving an even safer design constraint.
By calculating the bar’s moment of inertia, the distance from the neutral axis, and the section modulus of the cross section of the beam, the maximum bending stress can be found to be 587 MPa (Figure 6).
Figure 6. Simplified representation of a loaded, held barbell and calculation of stress
Therefore, the barbell must be made of a material with a yield strength greater than 587 MPa. A look at a plot of materials’ yield strengths shows that metals, ceramics, and composites are all possibilities (Figure 7).
Figure 7. A plot of different materials’ yield strengths compared to their densities (from the text Materials Engineering, Science, Process and Design by Ashby et. al, 2007)
Metals make the most sense, however, because of their density and ductility. Composites’ light weight means they would be difficult, or impossible, to make into regulation-weighted-and-dimensioned barbells. Ceramics are also very brittle, meaning they break before bending at all; it is usually safer for a product to give warning before breaking, in the form of bending, making a ductile metal a better choice. Given its cost compared to titanium alloys, steel is easily the best choice for a barbell.
There is a dizzying amount of different steel mixtures and grades, but based on searching through tables and information sheets such as this and this, it is a safe bet that molybdenum-alloyed steels (steel alloy 4140/4340, yield strength 655/852 MPa) , cold worked austenitic stainless steels (stainless steel grade 301/304/310, yield strength 470-1310 MPa), and martensitic stainless steels (stainless steel grade 410/420/431, yield strength 415-1895 MPa) are all appropriate choices for a barbell that would not suffer permanent deformation even under the most weight a human has ever (dead)lifted.