Kettlebell Training - Swinging your way to fitness! - By Paul Hemsworth
Kettlebells, and more importantly, the kettlebell style of lifting is one of the most effective overall training styles you will find. What makes the kettlebell such an effective tool is the versatility of improving strength, power, cardiovascular fitness, and joint stability/mobility. Expensive multi-station machines cannot even begin to compete with this space-saving, versatile training tool. Whether you want to workout at the park, the beach, at the gym, or in your basement, the kettlebell can be taken anywhere.
The first time I saw kettlebells, I felt the same way I did when I first saw those fat-melting vibrating belts: skeptical. At first glance, kettlebells appear to be a modified dumbbell at best. Nothing could be further from the truth. Kettlebell lifting is an art; a style of lifting that uses the unique shape of the bell to optimally groove the patterns and slings of the body. They are not meant for bicep curling or chest pressing, but rather technically sound, fully integrated movements. You see, kettlebell lifting is an endurance sport; it is about how long you can go for.
Hip Hinging and Activation for Strength & Power
The first time I was trained with kettlebells, my hamstrings and glutes were screaming at me for the next four days. It was truly one of the most well-rounded workouts I had ever been through. At the forefront of kettlebell lifting is the art of hip-hinging. This technique teaches you to use your glutes and core to preserve and protect your spine. For those of you who have trouble with the deadlift, the kettlebell swing, is a great way to learn how to create a hip hinge with proper gluteal activation. This hinge is the cornerstone of a strong and powerful deadlift.
Cardiovascular Benefits
During that first kettlebell lesson, I quickly realized how metabolically taxing this style of lifting is. For those of you who are looking to lose weight, I assure you that you very few modes of exercise will keep a sustained elevated heart rate the way kettlebell training does. And, the more muscles that are used during an exercise, the more calories will ultimately be burned. Kettlebell training uses all major muscle groups. Furthermore, interval training (alternating running hard and easy as an example), has been proven more effective in improving VO2max, anaerobic threshold, and excess post-oxidative consumption (EPOC) than slow, steady, long runs. Kettlebell lifting, for time, will lead to the same cardiovascular adaptations. You will be challenged through intervals using full body movements, while encouraging joint-sparing technique.
Stability & Mobility
Another aspect of kettlebell training that is hard to beat with other training modes is core stability. During the kettlebell swing for example, the muscles surrounding your spine need to act as a brake by bracing at the apex of the lift so that you do not go into excessive lordosis, or create a huge arch in your lower back. Because of this, it grooves a fantastic sequence of driving with your gluteals and braking with the core. Other lifts such as the Turkish Get-up require maximal stability to perform the lift properly and safely. The stability doesn’t stop at the core, however. Because of the off-set center of gravity of the weight, the bell allows the shoulder to find its own path for optimal force – or joint – closure. When performing an overhead push-press, the shoulder doesn’t have to grind through the movement, thus a safer alternative to barbells or dumbbells, especially for those with shoulder problems. Keeping with the shoulder, poor thoracic spine mobility limits many people in the amount of shoulder mobility or strength work that they can do. Both the kettlebell itself and kettlebell lifting techniques cater very well to these issues by allowing the chest to open up while keeping the shoulder “packed” into the socket.
I was privileged to sit down with RKC Level 2 & AKC certified coach Jim Talo to ask him a few questions regarding kettlebells.
Paul: Jim, to some people, kettlebell lifting may look unsafe. What can you say about this?
Jim: Often perspective limits how we accept or approach situations. Awareness of our capabilities for a given activity is much more important in my mind. An example of unsafe could be overweight, de-conditioned men playing golf or de-conditioned teenagers snowboarding. Without awareness and due respect, squatting in a power rack is unsafe. With a new tool, it is best to follow the instructions and use it in a manner for that which it is most beneficial. As with anything, become technically sound first.
Paul: Do you recommend getting a DVD and learning that way?
Jim: In my experience with new kettlebell lifters, DVDs' are a great resource as a reference along with some hands-on instruction with a coach. Most people need some adjustments to rediscover their primal movement patterns in order to move with the kettlebell. Most of the time, people try to lift (muscle) the kettlebell rather than initiate hip drive to push and swing the bell.
Paul: If you were to recommend three exercises for beginners to learn and get familiar with, what would they be?
Jim: The swing movement teaches one to initiate hip drive, the clean movement for transitioning into front squat, push press and eventual overhead lifting, and the turkish get-up. The get-up is not necessarily a kettlebell specific lift, however it is fantastic for promoting shoulder mobility and rotator cuff stability for overhead lifting that is prevalent in kettlebell work.
Paul: What are some common mistakes that people present with when lifting kettlebells?
Jim: The most common mistake is the kettlebell being lifted (muscled) like a dumbbell, rather than being projected by the hips. Another equally common issue is the need to teach people to move and hinge at the hips rather than at the waist. The advantage of kettlebells is that they tend to move in arcs, allowing the body to adjust mid-movement as needed. Barbells and dumbbells tend to be moved through linear paths close to the body. Kettlebells move through arcs and spirals allowing joints to find a 'groove' for particular movements.
Paul: How many times a week do you recommend lifting kettlebells and how would you supplement them into you regular training regime?
Jim: Kettlebell lifting methodology allows one to be very creative and may be attuned to one's goals. Kettlebells may be used to directly affect the posterior chain of the body and to address imbalances of shoulder strength, or they may be used as a cornerstone of a strength and conditioning program. The methodology would be in the prescription determined by one's goals. Many experienced and avid kettlebell lifters train 5 to 6 days a week.
Paul: Could you describe a quick kettlebell workout if crunched for time?
Jim: For someone just starting kettlebell work: kettlebell swings - 1 minute each hand; clean and push press - 5-10 reps per side; clean and front squat - 5-10 reps per side...rinse and repeat. If I had only 15 minutes: kettlebell snatches, double bell clean and jerks with some double front squats sprinkled in.
If this style of training sounds like it would suit you I highly urge you to educate yourself with the varying styles of lifting that kettlebell gurus Pavel Tsatsouline, Steve Cotter, and Valery Federenko have to offer. If you can’t learn from them directly, make sure that you get hands on training from a certified kettlebell lifter for the best results. You can contact Jim Talo for questions regarding kettlebells, or enquire about his workshops through the Human Motion website at http://www.humanmotion.com/. Jim’s has also co-authored a book called The Great Kettlebell Handbook. Go to http://www.productivefitness.com/ to order yours now!
Saturday, September 27, 2008
Sunday, September 14, 2008
Analyzing Core Stabilization Techniques - Part 1: Bridging the Gap
As most of you know, the world of core stabilization has yielded as much attention as Paris Hilton buying a new Chihuahua. The difference: core stabilization warrants most of the attention it gets. I say “most” because as with many catchy terms in the fitness industry, it can be abused with the content that goes into defining these terms. However, for the sake of this article I am going to review what I feel to be the more logical techniques that are involved in stabilizing that snake-like structure we call the spine.
What is Core Stabilization?
That’s the million dollar question isn’t it? If you asked 100 different sport scientists that question, you would get 100 different answers. To me, core stabilization is the ability to create uncompromising stiffness around the spine as to not allow any “energy leaks” during various static or dynamic tasks. You may agree or disagree with me on that definition, but the bottom line is this: Whether you are an elite athlete, construction worker, or receptionist, chances are you will probably go through some sort of back pain in your life. So throw the 6-pack talk out the window for now and start thinking about the spine. If we can ensure the athlete is a column of strength with no kinks in the chain, then we can ensure optimal power with minimal force loads on the spine.
That’s the million dollar question isn’t it? If you asked 100 different sport scientists that question, you would get 100 different answers. To me, core stabilization is the ability to create uncompromising stiffness around the spine as to not allow any “energy leaks” during various static or dynamic tasks. You may agree or disagree with me on that definition, but the bottom line is this: Whether you are an elite athlete, construction worker, or receptionist, chances are you will probably go through some sort of back pain in your life. So throw the 6-pack talk out the window for now and start thinking about the spine. If we can ensure the athlete is a column of strength with no kinks in the chain, then we can ensure optimal power with minimal force loads on the spine.
Abdominal Anatomy
Abdominal Wall
Internal & External Obliques (IO & EO): Involved in flexion, as their forces are redirected to the rectus abominis (RA) to enhance the flexor potiential. They are involved in lateral bending, twisting, and stabilization of the lumbar spine (McGill, 1991a, 1991b, 1992; Juker, McGill, and Kropf, 1998). Lastly, they are involved in active expiration (Henke et al., 1988).
Transverse Abdominis: Rotates thorax from side to side, increases interthoracic pressure, and is involved in defecation, urination, childbirth. The TA is also an anticipatory muscle.
Rectus Abdominis (RA): The major flexor of the trunk. It forms a continuous hoop around the spine by transferring the forces from the obliques. The upper and lower RA are activated together and at similar rates during flexion (Lehman & McGill, 2001): So throw your “upper and lower abdominal exercises” out the window.
Internal & External Obliques (IO & EO): Involved in flexion, as their forces are redirected to the rectus abominis (RA) to enhance the flexor potiential. They are involved in lateral bending, twisting, and stabilization of the lumbar spine (McGill, 1991a, 1991b, 1992; Juker, McGill, and Kropf, 1998). Lastly, they are involved in active expiration (Henke et al., 1988).
Transverse Abdominis: Rotates thorax from side to side, increases interthoracic pressure, and is involved in defecation, urination, childbirth. The TA is also an anticipatory muscle.
Rectus Abdominis (RA): The major flexor of the trunk. It forms a continuous hoop around the spine by transferring the forces from the obliques. The upper and lower RA are activated together and at similar rates during flexion (Lehman & McGill, 2001): So throw your “upper and lower abdominal exercises” out the window.
Rotatores: Have a high number of muscle spindles and thus serve more as a spinal positioner than a rotator of the spine (Nitz & Peck, 1986). They are most active when trying to resist the rotation of the spine that the obliques and latissimus are likely causing.
Extensors 
Longissimus & Iliocostalis: Have thoracic and lumbar components. These are the major back extensors.
Multifidus: Extension of the spine but only through the correcting of spinal joints that are enduring stress. Line of action actually contributes to shearing forces of superior vertebrae.
Longissimus & Iliocostalis: Have thoracic and lumbar components. These are the major back extensors.
Multifidus: Extension of the spine but only through the correcting of spinal joints that are enduring stress. Line of action actually contributes to shearing forces of superior vertebrae.
Quadratus Lumborum (QL): Bilateral support wall or stabilizer for the lumbar spine. The QL is active during flexion, extension, and lateral bending of the spine and maybe one of the few muscles that doesn’t turn off during the flexion/relaxation phenomenon.
Psoas: Major hip flexor. May assist in some stabilization due to its orientation (Origin is T12-L5).
Psoas: Major hip flexor. May assist in some stabilization due to its orientation (Origin is T12-L5).
Core Stabilization Mechanisms: Abdominal Hollowing vs. Abdominal Bracing
Abdominal Hollowing
The abdominal hollowing technique was essentially developed from a group of Australian sport scientists (Richardson et al. 1999). This “Queensland group” determined that the transverse abdiminis (TA) and multifidus (MT) muscles in particular, were very important muscles for motor patterning. They found that following injury to the back, the TA and MT underwent motor disturbances that had profound effects on the motor patterning of the body. Because further injury would just add to these effects leading to a chronic state of poor patterning and pain, the Queensland group argued that only specific abdominal activation techniques could break this poor programming. Thus was born the abdominal hollowing technique: This technique involves the drawing in of the abdomen in an attempt to isolate the TA, while relaxing the surrounding musculature (RA, IO, EO).
Abdominal Hollowing
The abdominal hollowing technique was essentially developed from a group of Australian sport scientists (Richardson et al. 1999). This “Queensland group” determined that the transverse abdiminis (TA) and multifidus (MT) muscles in particular, were very important muscles for motor patterning. They found that following injury to the back, the TA and MT underwent motor disturbances that had profound effects on the motor patterning of the body. Because further injury would just add to these effects leading to a chronic state of poor patterning and pain, the Queensland group argued that only specific abdominal activation techniques could break this poor programming. Thus was born the abdominal hollowing technique: This technique involves the drawing in of the abdomen in an attempt to isolate the TA, while relaxing the surrounding musculature (RA, IO, EO).
Abdominal Bracing 
The abdominal bracing technique was primarily developed - or more appropriately, coined - by Canadian biomechanist Stuart McGill. This technique involves the co-activation of all the muscles surrounding the spine (RA, IO, EO, TA, MT, Latissimus, QL, and the extensors) in an attempt to create 360 degrees of stability. While bracing, the individual doesn’t draw in or push out, but rather “braces” or widens the trunk. If you think about what you would do if someone was to punch you in the stomach: You would set or brace for the punch and effectively create stability all the way around the spine. (For more on abdominal bracing, see Ultimate Back Fitness & Performance by Stuart McGill).
The abdominal bracing technique was primarily developed - or more appropriately, coined - by Canadian biomechanist Stuart McGill. This technique involves the co-activation of all the muscles surrounding the spine (RA, IO, EO, TA, MT, Latissimus, QL, and the extensors) in an attempt to create 360 degrees of stability. While bracing, the individual doesn’t draw in or push out, but rather “braces” or widens the trunk. If you think about what you would do if someone was to punch you in the stomach: You would set or brace for the punch and effectively create stability all the way around the spine. (For more on abdominal bracing, see Ultimate Back Fitness & Performance by Stuart McGill).
To Brace or Hollow: That is the question
Much of the data that came out of the Queensland research was misinterpreted. Because they were working with injured individuals with malfunctioning motor patterns, the techniques they came up with were an attempt to disrupt the faulty patterns and educate the patients on abdominal control. Moreover, the TA anticipates trunk, upper and lower limb movement as well as protects the spine (Hodges, 1999). This anticipatory and protective function can be lost with acute or chronic low back pain. However, many clinicians took this information and regarded the techniques as a way of creating optimal core stability during various tasks. Thus, abdominal hollowing seems to be the preferred choice of many physiotherapists, strength coaches, chiropractors and kinesiologists for core stabilization.
Enter Stuart McGill! Not dismissing the importance of these muscles in their role as intra-abdominal pressure creators and stabilizers, McGill and others have since argued that this is simply not enough to endure tasks of even moderate intensity. Furthermore, during athletic events, unpredictable forces from all directions occur in almost any sport. Specifically, if a posterior perturbation – or unsuspected push from behind - occurs on the spine (lets say a defensive stiff-arm as you lean into a defender in basketball), abdominal hollowing produces the same resistance to the force that no activation does and results in an increase in spinal flexion (vs. 43% reduction of spinal flexion when bracing is used) (Vera-Garcia et al. 2007). As kettlebell lifter and educator Brett Jones says, if you took a cardboard box on its side and loaded it from the top, the box would crumble. Just ask Human Motion’s Cliff Harvey what would have happened if he drew his stomach in while attempting world record lifts in weightlifting: He too would have crumbled. Furthermore, it is almost certain that if you try to contract only the TA, you will have activity in the IO and EO as well.
When the muscles surrounding the spine co-contract, they create a stiffness that is greater than the sum of the individual muscle stiffness (McGill, 2006). Thus, during the hollowing procedure, you are actually inhibiting the potential for optimal stiffness, ultimately limiting performance. You would think that in order to brace properly and ensure “superstiffness” that you would need to have an all out contraction during most activities. However, this doesn’t seem to be the case as the first 25% of a maximal abdominal contraction creates sufficient stiffness for most activities (Brown & McGill, 2005). During 1RM lifts such as Cliff’s world record attempts however, a maximum voluntary contraction (MVC) of all the surrounding musculature is necessary to withstand the massive force.
Much of the data that came out of the Queensland research was misinterpreted. Because they were working with injured individuals with malfunctioning motor patterns, the techniques they came up with were an attempt to disrupt the faulty patterns and educate the patients on abdominal control. Moreover, the TA anticipates trunk, upper and lower limb movement as well as protects the spine (Hodges, 1999). This anticipatory and protective function can be lost with acute or chronic low back pain. However, many clinicians took this information and regarded the techniques as a way of creating optimal core stability during various tasks. Thus, abdominal hollowing seems to be the preferred choice of many physiotherapists, strength coaches, chiropractors and kinesiologists for core stabilization.
Enter Stuart McGill! Not dismissing the importance of these muscles in their role as intra-abdominal pressure creators and stabilizers, McGill and others have since argued that this is simply not enough to endure tasks of even moderate intensity. Furthermore, during athletic events, unpredictable forces from all directions occur in almost any sport. Specifically, if a posterior perturbation – or unsuspected push from behind - occurs on the spine (lets say a defensive stiff-arm as you lean into a defender in basketball), abdominal hollowing produces the same resistance to the force that no activation does and results in an increase in spinal flexion (vs. 43% reduction of spinal flexion when bracing is used) (Vera-Garcia et al. 2007). As kettlebell lifter and educator Brett Jones says, if you took a cardboard box on its side and loaded it from the top, the box would crumble. Just ask Human Motion’s Cliff Harvey what would have happened if he drew his stomach in while attempting world record lifts in weightlifting: He too would have crumbled. Furthermore, it is almost certain that if you try to contract only the TA, you will have activity in the IO and EO as well.
When the muscles surrounding the spine co-contract, they create a stiffness that is greater than the sum of the individual muscle stiffness (McGill, 2006). Thus, during the hollowing procedure, you are actually inhibiting the potential for optimal stiffness, ultimately limiting performance. You would think that in order to brace properly and ensure “superstiffness” that you would need to have an all out contraction during most activities. However, this doesn’t seem to be the case as the first 25% of a maximal abdominal contraction creates sufficient stiffness for most activities (Brown & McGill, 2005). During 1RM lifts such as Cliff’s world record attempts however, a maximum voluntary contraction (MVC) of all the surrounding musculature is necessary to withstand the massive force.
Let’s hug it out: We are dealing with apples and oranges
There seems to be a lack of understanding as to the different techniques used between physios and strength coaches for core stabilization and activation. When a patient is seeing a physio, they are exactly that – a patient. Most of the time they are coming from an injury and have consequently obtained faulty patterns within their muscle sequencing. On the other hand, they could have had years of overuse injuries or poor gait biomechanics that has led to muscular imbalances. Thus, abdominal hollowing seems to be the technique of choice to help create that control that probably was never there even before the “injury” brought them to rehab. THIS IS PERFECTLY FINE. This is our group of apples. Our group of oranges are either these same patients coming from physio or our uninjured group of individuals who need to get stronger. Once these individuals are able to withstand heavier forces and are loaded up with weights, abdominal hollowing is no longer sufficient to lift this kind of weight, while sparing the spine. Thus, the abdominal brace must be taught. Herein lies the problem. We are constantly nagging each other (various health care practitioners) about the different techniques used. We need to remember that it is the needs of the client/patient that is our primary concern. WE NEED TO EDUCATE AND PREPARE THEM FOR THE NEXT STEP. Physios: Inform the patient that if they are an athlete or they are going to be lifting weights in the future, they will have to learn both techniques. Strength coaches: Actually integrate both techniques into your training. Isolate then integrate. It is a great way to allow the client to achieve initial success (abdominal hollowing) and then allow them to see the big picture of lifting heavier loads (abdominal bracing).
An integrated team approach can produce great success for the athlete, however, all members need to be on the same page even if their philosophies differ. Work with each other to produce the best results for the client/patient. Your athlete will ultimately be stronger, safer, and less confused in the process!
There seems to be a lack of understanding as to the different techniques used between physios and strength coaches for core stabilization and activation. When a patient is seeing a physio, they are exactly that – a patient. Most of the time they are coming from an injury and have consequently obtained faulty patterns within their muscle sequencing. On the other hand, they could have had years of overuse injuries or poor gait biomechanics that has led to muscular imbalances. Thus, abdominal hollowing seems to be the technique of choice to help create that control that probably was never there even before the “injury” brought them to rehab. THIS IS PERFECTLY FINE. This is our group of apples. Our group of oranges are either these same patients coming from physio or our uninjured group of individuals who need to get stronger. Once these individuals are able to withstand heavier forces and are loaded up with weights, abdominal hollowing is no longer sufficient to lift this kind of weight, while sparing the spine. Thus, the abdominal brace must be taught. Herein lies the problem. We are constantly nagging each other (various health care practitioners) about the different techniques used. We need to remember that it is the needs of the client/patient that is our primary concern. WE NEED TO EDUCATE AND PREPARE THEM FOR THE NEXT STEP. Physios: Inform the patient that if they are an athlete or they are going to be lifting weights in the future, they will have to learn both techniques. Strength coaches: Actually integrate both techniques into your training. Isolate then integrate. It is a great way to allow the client to achieve initial success (abdominal hollowing) and then allow them to see the big picture of lifting heavier loads (abdominal bracing).
An integrated team approach can produce great success for the athlete, however, all members need to be on the same page even if their philosophies differ. Work with each other to produce the best results for the client/patient. Your athlete will ultimately be stronger, safer, and less confused in the process!
References
Brown, & McGill . (2005). Muscle force–stiffness characteristics influence joint stability: A spine example. Clinical Biomechanics, 20(9), 917.
Henke, Sharratt, Pegelow, & Dempsey, (1988). Regulation of end-expiratory lung volume during exercise. Journal of Applied Physiology, 64(1), 135.
Hodges (1999). Is there a role for transversus abdominis in lumbo-pelvic stability? Manual Therapy, 4(2), 74.
Juker, Mcgill, & Kropf, (1998). Quantitative intramuscular myoelectric activity of lumbar portions of psoas and the abdominal wall during a wide variety of tasks. Medicine and Science in Sports and Exercise, 30(2), 301.
Lehman & McGill, (2001). Quantification of the differences in electromyographic activity magnitude between the upper and lower portions of the rectus abdominis muscle during selected trunk exercises. Physical Therapy, 81(5), 1096.
McGill, (1991a). Electromyographic activity of the abdominal and low back musculature during the generation of isometric and dynamic axial trunk torque: Implications for lumbar mechanics. Journal of Orthopaedic Research, 9(1), 91.
McGill, (1991b). Kinetic potential of the lumbar trunk musculature about three orthogonal orthopaedic axes in extreme postures. Spine, 16(7), 809.
McGill, (1992). A myoelectrically based dynamic 3-D model to predict loads on lumbar spine tissues during lateral bending. Journal of Biomechanics, 25(4): 395.
McGill, (2006). Ultimate back fitness and performance. Waterloo, ON: Backfitpro Inc.
Nitz & Peck, (1986). Comparison of muscle spindle concentrations in large and small human epaxial muscles acting in parallel combinations. The American Surgeon, 52(5), 273.
Richardson, Jull, Hodges, & Hides, (1999). Therapeutic exercise for spinal segmental stabilization in low back pain. Edinburgh, Scotland: Chruchill Livingstone.
Vera-Garcia, Elvira, Brown, & McGill (2007). Effects of abdominal stabilization maneuvers on the control of spine motion and stability
against sudden trunk perturbations. Journal of Electromyography and Kinesiology, 17(5), 556.
Brown, & McGill . (2005). Muscle force–stiffness characteristics influence joint stability: A spine example. Clinical Biomechanics, 20(9), 917.
Henke, Sharratt, Pegelow, & Dempsey, (1988). Regulation of end-expiratory lung volume during exercise. Journal of Applied Physiology, 64(1), 135.
Hodges (1999). Is there a role for transversus abdominis in lumbo-pelvic stability? Manual Therapy, 4(2), 74.
Juker, Mcgill, & Kropf, (1998). Quantitative intramuscular myoelectric activity of lumbar portions of psoas and the abdominal wall during a wide variety of tasks. Medicine and Science in Sports and Exercise, 30(2), 301.
Lehman & McGill, (2001). Quantification of the differences in electromyographic activity magnitude between the upper and lower portions of the rectus abdominis muscle during selected trunk exercises. Physical Therapy, 81(5), 1096.
McGill, (1991a). Electromyographic activity of the abdominal and low back musculature during the generation of isometric and dynamic axial trunk torque: Implications for lumbar mechanics. Journal of Orthopaedic Research, 9(1), 91.
McGill, (1991b). Kinetic potential of the lumbar trunk musculature about three orthogonal orthopaedic axes in extreme postures. Spine, 16(7), 809.
McGill, (1992). A myoelectrically based dynamic 3-D model to predict loads on lumbar spine tissues during lateral bending. Journal of Biomechanics, 25(4): 395.
McGill, (2006). Ultimate back fitness and performance. Waterloo, ON: Backfitpro Inc.
Nitz & Peck, (1986). Comparison of muscle spindle concentrations in large and small human epaxial muscles acting in parallel combinations. The American Surgeon, 52(5), 273.
Richardson, Jull, Hodges, & Hides, (1999). Therapeutic exercise for spinal segmental stabilization in low back pain. Edinburgh, Scotland: Chruchill Livingstone.
Vera-Garcia, Elvira, Brown, & McGill (2007). Effects of abdominal stabilization maneuvers on the control of spine motion and stability
against sudden trunk perturbations. Journal of Electromyography and Kinesiology, 17(5), 556.
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