Just about every volleyball player wants to know how to jump higher and hit the ball harder. The best volleyball players have a huge jump and a whip of an arm swing to hit balls through the floor so its understandable.
Technique is always going to be the foundation to success and that comes from hours of on the court. Still, there is more you can do to get that explosive vertical jump.
This video demonstrates two exercises every volleyball player should include into their workouts to help them dominate on the court or beach. Coach Rett Larsen should know what he talking about. He was the performance coach for the Gold Medal team in Womens Volleyball at the 2016 Rio Olympics!
Band-Resisted Vertical Jump – part one
One of my favorite ways to increase vertical jumps is to use a band resistance. This forces the athlete to overcome a resistance to create a more efficient and forceful jump, and over time, jumping without the bands will be easier to perform.
Velocity’s Speed Formula is proven to get results for athletes at all levels of sport. Developed by World Famous Track Coach Loren Seagrave, it’s used in elite sports around the world to make sure athletes get faster.
While it appears simple, it’s based on complex biomechanics and motor control theory. By improving these 4 elements, you can improve your speed too.
When you speak about strength or being strong, what do you imagine? An athlete hoisting a barbell loaded with heavy weight in a Squat or Bench Press? How about an Olympic weightlifter explosively moving 400 pounds from the floor to over his head in a single movement?
These types of things are often considered “strong,” but what about other sporting actions? How about sprinting at full speed, jumping high, or throwing and kicking? Most people become unsure whether or how strength is part of these movements.
What is strength in general and specifically for athletes? Strength is all about physics, and we are talking about Newton’s 2nd Law of Motion: in a nutshell, Force is equal to Mass multiplied by Acceleration.
Strength is a way of talking about the application of force. An athlete can apply force to the ground, to an opponent, to a ball or other piece of sports equipment, or even internally to his or her own body.
Mass & Magnitude
The mass in this equation is what’s being moved. As an athlete that could be things like:
a ball or stick in your hands, to
your own body weight (jumping, sprinting and cutting)
a 300-pound linemen
500 pounds on a barbell
Acceleration and Time
One thing most people recognize is that in sports, doing things quicker is usually an advantage. Athletes don’t have unlimited time to apply force.
Acceleration is how fast something increases its speed. The faster the acceleration, and thus the speed, the shorter the time.
In sprinting or agility, your foot is in contact with the ground for a limited time. In jumping, there is limited time, and doing it faster than your opponent can be key. When throwing or kicking a ball or swinging a racket, bat or stick, you want it moving as fast as possible.
Speed of movement matters.
In physics, force is what we call a “vector.” This means it has a magnitude (how much?) and a direction (which way?). Direction matters because forces can be applied in different directions for different effects.
One thing to consider about direction is whether the muscle is lengthening or shortening during the contraction. When it’s contracting and getting shorter (e.g., bringing the bar up in a Bicep Curl), it’s called a “concentric” action.
If you’re applying force while the muscle lengthens (e.g., while slowly lowering the bar in the 2nd half of the Bicep Curl), it’s called an “eccentric” action.
Types of muscle contractions:
CONCENTRIC = Shortening
ECCENTRIC = Lengthening
Eccentric and concentric strength are not the same. The same muscles may be used, the same structures and contractile proteins, and the same lints moved. Yet, the brain uses different motor control strategies. For the same action concentrically or eccentrically the motor control is different.
Physiology & Motor Control
Another important thing to understand about strength for athletes is where it comes from. Often people equate strength with bigger muscles. This is for good reason, because they are related, although not perfectly and not for all types.
Generating force with your body is a combination of the structure of your muscles (size and biological content) and your neuromuscular control. The muscle is your engine to develop horsepower, but your brain is the driver that decides how hard you push the pedal.
When we analyze an athlete in his or her sport, we observe various forms of movement. Speed, agility, jumping, throwing, kicking, hitting, twisting, landing and so on are movement caused by how an athlete generates force.
It follows that all types of athletic movement are based on how you generate and apply strength.
Still, how can everything be about strength? Is what your muscles do squatting a full barbell different from what they do when you throw a baseball that only weighs ounces?
The answer to understanding strength is actually composed of different combinations of Newton’s 2nd Law. Force = Mass multiplied by Acceleration
Playing with the Equation
In different movements we manipulate the 3 parts of the equation—Force, Mass and Acceleration (Speed & Time). The we consider the direction of contraction (eccentric or concentric). Now we have a way to analyze sports movements and strength types.
We use this movement-based approach to simplify complex biomechanics into 6 specific types of strength.
6 Types of Strength
This is the basic capability of the muscle to produce a forceful contraction. In application it also involves coordinating multiple muscle groups across multiple joints. The amount of force that can be generated regardless of the time it takes to develop and apply it is called max strength. This is what we call this type of strength even when he or she is under sub-maximal loads.
Using a car analogy, imagine a big industrial dump truck. It may not move fast, but it can move big loads.
As mentioned before, motor control is different if the action is concentric or eccentric. The capacity to develop high levels of eccentric force is key in sports. Actions such as landing from a jump, stopping, changing direction, winding up to throw a ball and swinging a bat are all eccentric in nature.
When we come to cars, think brakes. Eccentric strength is like having great brakes on a car to handle those high speeds. An F1 racer has to have great brakes so he or she can go into turns as fast as possible before braking.
Most sports applications of force involves doing it quickly. Faster is usually better. This is where power comes in. Power is equal to the velocity times the force. Increasing either force or the speed its applied will lead to more power.
When an athlete applies force rapidly to a larger load (e.g., blocking another lineman or pushing a bobsled), it’s what we term Strength-Speed Power. “Strength” is first in the name because it’s the bigger component in generating the power. This is like a NASCAR racer who can apply a lot of torque (force), moving the car even at high speeds.
Here it’s the “speed” of movement (or short time of force application) that is the larger factor in generating the power. Think of an athlete swinging a bat, throwing a ball, or applying force to the ground during high velocity sprinting.
The racing analogy is more akin to motorcycle racing—still applying force at high speeds (like NASCAR), but against much lighter loads.
Rate of Force Development
This is the drag racer. In a drag race, the goal is to go from 0 mph to full speed in as little time as possible. This is the same quality that creates quickness in an athlete. Rapid movement of the limbs, a quick release of the ball throwing or a shot in hockey, fast feet for soccer. Being able to rapidly generate force, regardless of whether the force level is high is known as Rate of Force Development.
A drag racer coming off the line and getting up to speed as fast as possible is a good car analogy.
This one’s a combo. It’s a fast eccentric action coupled with a high RFD force. Think of rapid footwork, or a quick step to change direction and juke an opponent. Or the second quick jump when a basketball player comes down and goes back up quickly to get a rebound.
We use a motocross bike as the analogy. Because it has high Rate of Force Development with eccentric-type landings of bumps that gives it that “springy” quality.
Developing Strength that’s Functional
At the end of the day, athletes want the type of strength that will help them perform at the highest level and gives them the resilience to stay healthy.
Every athlete needs a base across all six types of strength. While it seems to make sense to go straight to the specific type of strength for your sport, it’s not the best strategy.
Doing that actually limits development and long term potential. During early stages of strength training, a broad base of strength is important. Even at the elite levels of sport, athletes mix strength types during different parts of the year.
As you progress in your development and level of competition, you begin to focus on the specific qualities. The strength types more important to your sport, your position and even your individual genetics and style of play.
Strength is much more than how much you can lift on the barbell.
Lots of people want to lose bodyfat and get leaner. Many hope to get lean enough to show off six-pack abs. If so, you better understand the fundamentals of fat loss.
Unfortunately with all the conflicting information out there it can be hard to know what works and what’s important. Nutrition, keto, paleo, supplements, cardio, strength, HIIT, CrossFit…? Its a long list of confusing information.
If you want a six pack, better understand the fundamentals of fat loss. Sports Performance Director Tim Hanway from Velocity Norwood, shares some of the fundamentals of fat loss.
Food is what fuels your human machine. Choosing the right kind of fuel can go a long way towards optimizing your performance. This article focuses on things to avoid before your workout.
Too much protein
Protein is, of course, an important part of any athlete’s diet. Getting the right timing around your workout is important. Too much of it before a workout or a game can leave you feeling too full or lethargic – neither of which you want.
Fried and Fatty Foods
Any food with a lot of saturated fat should be avoided. This type of fat molecule takes a long time for your body to digest. It stays in your digestive system longer, and can lead to bloating, cramping, or diarrhea.
Foods high in fiber
Even though your doctor might want you to up your fiber intake, don’t do it pre-workout. Doing so right before you want to break a sweat might have some unpleasant consequences. Fiber certainly gets the digestive system moving, but you don’t want that during a hard training session.
There’s a lot more to be said about nutrition before, during, and after training or competition. Check back for more in the future, and if the topic interests you, start researching!
Almost every sport is about more than just running fast or a huge vertical. Pick one, and we’ll bet that most of the action happens around changing direction. For the majority of the athletes with whom we work at Velocity around the country, this means they have to be just as good at stopping as they are at starting. Without good brakes, they simply can’t control their speed.
Three of our coaches have chosen their favorite drill to help their athletes have strong, fast brakes so that they can stop on a dime.
Level Lowering Ladder
One of the most basic skills an athlete needs to change direction is the ability to maintain proper position during deceleration. One of the tools we like to use at Velocity is the agility ladder because it helps focus the athlete on foot position and accuracy in addition to whatever skills we choose to address that day.
To do these drills, athletes first need to have the coordination to perform basic ladder drills well, such as swizzle, scissor switches, and the icky shuffle. Once the athlete can perform each of these without difficulty, they can modify the drill and pause as they drop their center of mass, stopping themselves in the proper position. The most basic, and therefore most important, positions in sports are the square, staggered, and single leg stance. A mini-band can be placed around the athlete’s knees to create awareness of proper knee position. If the athlete adds a medicine ball into the drill, they can work on more ballistic/dynamic eccentric movement with a different stimulus.
The athlete needs to lower his/her center of mass to create “triple flexion” in lower extremity joints: hip, knee, and ankle. The center of mass, knee, and ground contact must be in a good alignment to keep the movement safe and efficient.
Most importantly, the athlete must achieve proper hip hinge and dorsiflexion of the ankle. The vast majority of non-contact injuries occur during deceleration, often at knees or ankles. Learning how to absorb (load) force with proper body position (hip hinge, stable knee, and dorsiflexed ankle) will help prevent these injuries.
Springs and ShocksLadder
The agility ladder is a great tool to help our athletes develop their shocks and springs.
When it comes to speed, athletes need to be springy and quick off the ground. When we talk about “springs,” we mean our athletes’ ability to be faster by using the elastic properties of their muscles.
“Shocks” means having the ability to absorb impact and force so our athletes can stop safely and quickly. This drill emphasizes both abilities and applies to any sport.
How to do the drill:
through the ladder try to be a quick as you can off of the ground. This is where we focus on our springs. When we land we want to land and be under control. The more control we have when decelerating the safer our body will be when changing direction. Most important part of the landing is keeping the body in proper position and not allowing a valgus knee.
Important details to watch are: position and control. We want an athlete to be able to develop the strength and control through the proper range of motion. This is especially important as we begin to add not speed or distance. Do not let athletes progress unless they can properly and effectively let control their landing for at least 2 seconds.
Resisted Deceleration March Series
Slowing down is often the most challenging aspect of changing direction and requires the athlete to absorb more force than at any other phase of the movement. This series of drills teaches athletes to keep good posture and body-alignment during deceleration. When we add a concentric movement (explosiveness) immediately followed by a deceleration phase the drill also develops reactive strength and power in the athlete.
How to do the drills:
Position the athlete in a good athletic base with a resistance band or bungee cord around their waist. The partner holding the band increases resistance by pulling toward the direction where deceleration needs to occur.
The athlete controls their posture while moving toward “the direction of pull”. Their shin is a very important detail and must point away from the direction of pull. This helps their foot dig into the ground and resist the momentum that is trying to keep them moving in their original direction.
The ground contact, knee, and athlete’s center of mass should be in alignment and proper posture maintained.
If you want to incorporate an explosive moment, have the athlete perform any form of change-of-direction movement, such as a lateral push, crossover step, or jump.
Important details to watch are:
Make sure the athlete understand the basic athletic base position. Hip-hinge and dorsiflexion of the ankles are very important.
The level resistance needs to be appropriate to their strength and ability. You may adjust this by using a different size resistance band or the distance between the athlete and partner.
Ground contact, shin angle, knee position, and the athlete’s center of mass stay aligned (away from the direction of pull).
Make sure the athlete is not leaning on the band.
Eccentric control first, then concentric! Make sure your athletes understand how to use the brakes before they hit the gas pedal.
Kettlebells are a great tool which have been around for decades but have become popular again. And it’s for good reason; they’re versatile and dynamic. We surveyed some of our coaches to find out what their favorite exercises are with a kettlebell.
The Double Kettlebell Clean+Squat+Push-Press is a full-body exercise complex that gives you a lot of bang for your buck. When done correctly, you develop power through the Clean, leg strength in the Squat, vertical pressing strength in the Push Press, and core strength throughout the entire movement.
Execution: Before beginning, you must keep your core rigid through the entire movement to ensure you don’t hurt yourself.
Start with the two kettlebells of the same weight 1-2 feet in front of you and feet slightly wider than your shoulders.
With knees slightly bent, keep your back flat and push your hips back to the wall behind you and grab the kettlebells tightly.
Take a good breath and “hike” the kettlebells backward between your legs
Stand up as fast you can to snap the kettlebells up and forward into the rack position.
Clean the kettlebells to the rack position.
Take a new breath, slowly squat down with the kettlebells as low as you can then drive up as fast as possible.
Start to press the kettlebells above your head as you reach the top position, using the momentum of your squat to help finish the movement.
Once the kettlebells are straight above your head, take another good breath and slowly pull the kettlebells down to the Rack position.
Once in the rack position, reset with a good breath and prepare yourself for another repetition. Instead of starting with kettlebells on the ground, carefully let the kettlebells “fall” (while still holding them) and again hike them back through your legs and repeat the exercise for as many reps as prescribed.
If your goal is to develop all-around strength, use a heavy set of kettlebells for 3-6 reps. If your goal is to get a solid metabolic workout, go with a lighter set with which you can get in 8+ challenging reps.
Misao’s Pick: Halo
The Kettlebell Halo improves upper body mobility and stability. It is an overhead pattern that requires core stability as well as mobility and stability of the shoulders and shoulder blades.
In a kneeling or standing position, hold the kettlebell with both hands by the horns
Brace your core and hold the bell in front of your chest.
Slowly circle the bell around your head clockwise, then counter-clockwise. The movement must be slow and under control.
The weight of the bell needs to be light enough so your torso does not sway side to side or arch.
You can easily progress this exercise by changing the way you hold the bell. Holding the weight with the bell pointing down is easier as the weight stays securely in your palms. If you grip it upside down (with the bell on top) it becomes more challenging because the weight travels farther away from your body, increasing the strain on the muscle due to a longer lever.
Coach Kenny’s Pick: Turkish Get Up
The Turkish Get-Up is great for shoulder stability, overall strength, and just plain toughness. It also can help develop a sense of body control and awareness and test an athlete’s focus.
Start laying on your back with your right knee bent and your left arm extended out to the side. Your kettlebell should be on the ground next to your right arm.
Grasp the bell with your right hand and press it up so your right arm is completely straight and perpendicular to the ground.
Keep your eyes on the bell throughout the entire movement.
Roll up onto your left elbow, and then to your left hand.
Push your hips up towards the ceiling as high as you can.
Slide your left leg under your body and come up onto your left knee.
To get back down, simply reverse the movement.
Come down to your left knee.
Place your left hand on the ground
Slide your left leg out from underneath you so it’s totally straight, keeping your hips pressed up.
Let your hips come to the ground.
Lower on your left elbow.
Completely lower yourself to the ground so you are laying flat.
When bringing the kettlebell back to the ground, be sure to use your free hand to help guide it. Safety first.
Now do the same thing on the other side.
Coach Yo’s Pick: Bottom-Up Overhead Press
This series is great for shoulder stability, grip strength, elbow joint health, and core strength and stability based on athlete’s positioning. The Bottom-Up series gives the athlete a different stimulus since the load (kettlebell) is in an unstable position. This will improve overall proprioception (your level of awareness of where your body is in space), and by using different base positions ( ½ Kneeling, Tall Kneeling, Standing, Single Leg, etc), allows athletes to develop core strength and stability. It is a very unique exercise, and the kettlebell is an ideal tool for its execution.
Execution: Before you start, make sure that you have proper overhead mobility and stability and can do basic overhead press exercises with dumbbells or barbell. Once you have that skill, you can start by holding the kettlebell upside-down (bottom-up) right in front of shoulder. Make sure you the weight you use is not more than you can control with your grip alone. You can check holding the bell upside-down in a static position for a while without letting it drop. Ensuring you have basic stability before adding movement is always a good idea and will prevent needless injuries.
Performing the exercise in different positions will work on different elements of core strength and stability.
Coach Gary’s Pick: Split Squat KB Complex
The Split Squat KB Complex addresses muscle activation patterning, neuromuscular control, and dynamic stability of the trunk and lower extremities. This complex will challenge any athlete while reducing the likelihood of lower extremities injuries. This is valuable because more than 50% of injuries in college and high school athletics are knee injuries according to the American College of Sports Medicine. This 4-phased complex also allows coaches to progress athletes based on ability, making it excellent for novices and experienced athletes alike.
Starting position – Kneel with your right foot flat and the right knee directly over the heel. Start with the bell on the ground in front of your left knee.
Inhale and lift the kettlebell with your left hand to the level of your forward (right) thigh.
Level your hips by pressing the hips forward and Press the forward (right) heel into the ground.
The upper body should remain tall and erect with the chest up and out and the shoulders level and stacked over the hips.
Phase 1: Split Squat – Stand up on both legs while driving your front heel into the ground. Once your legs are fully extended, reverse the motion and lower the body back to the starting.
Phase 2: Clean to Split Squat – Quickly thrust the body upward and bring the bell to the front of your shoulder. With the bell in this position, extended both legs to stand up, again driving your front heel into the ground. Once fully extended reverse the motion and lower the body back to the starting position.
Phase 3: Jerk to OH Split Squat – Quickly thrust the body upward and Jerk the bell overhead with the upper arm tight to the ear. With the bell in this position extended both legs to elevating the body upper, think about driving your front heel into the ground. Once fully extended reverse the motion and lower the body back to the starting position but remaining on the feet.
Phase 4: OH Split Squat to knee drive – With bell overhead and the upper arm tight to the ear, extend both legs to stand up. Once fully extended, quickly drive the back knee up and in front of body then back to the same spot on the ground. Once ground contact is made lower the body back to the starting position but remaining on the feet.
Exercise 6: Pistol Squat
The Pistol Squat is a great way to test balance and overall hip and glute strength. It also gives you a clear interpretation of your strength to bodyweight ratio. If you can easily perform the movement as a bodyweight exercise, add a kettlebell.
Front rack the kettlebell of your choice. Hold the bell with whichever hand is opposite from your “down” leg.
Load your weight over one leg and slowly lower yourself to the ground on a single leg.
Extend your “up” leg in front of your and keep it from touching the ground.
Load your bodyweight onto one leg and as you drop down into a squat shift the loading glute back and extend the opposite leg forward in an attempt to keep it from touching the ground.
If you want to challenge yourself further, try performing the same movement while standing on some type of balance pad to give your foot an unstable surface to manage.
Coach Rob’s Pick: Single Arm Kettlebell Swing
The Swing is certainly the most ubiquitous use of the kettlebell. Once you have mastered it, try moving onto the single arm swing. This variation adds an anti-rotational component to the explosive hip drive inherent to the Swing.
Start with your feet hip-width or slightly wider. The kettlebell should be on the ground about a foot in front of you. Remember that during any weightlifting exercise, it is crucial that you keep your core tight and your back flat. Failure to do so, especially during a ballistic movement like a kettlebell swing is asking for injury.
The weight you select should be lighter than you think you need until you get the feel for the exercise.
Drop your butt towards to floor while keeping your chest up, grasp the bell firmly with one hand and “hike” it behind you, keeping your wrist tight to your body.
Stand up quickly and let the bell rise up to about shoulder height. This part of the movement should be snappy and crisp.
Keep your grip on the bell and let it fall, swinging back behind you while you keep it tight to your body.
Repeat this movement for as many reps as prescribed.
Once you have this movement down, you can challenge yourself by switching hands every rep. To achieve this, let the bell swing up to its highest point, at which time it should be weightless for a brief moment. Have your opposite hand ready to grab the handle as soon as you let go with the swinging hand.
Whether you alternate hands or not, the Single Arm Swing is sure to get your heart rate up, make you sweat, and develop leg strength and core stability. Have fun!
Sports Performance Director
Velocity Sports Performance - Norwood
As a professional coach, I have written extensively on a multitude of topics related to strength and conditioning. Whether I am talking about programming, the emotional aspects of training, or the nuts and bolts of coaching, I always come back to the importance of strength.
Strength is, in my opinion, the single most important physical attribute that an athlete can possess; it is quite literally the precursor to all expressions of athleticism. Speed, agility, quickness, explosiveness, and endurance all require strength in different forms. Strength training greatly enhances all of these qualities, which is why adopting a strength-training program that utilizes upper- and lower-body compound movements is perhaps the most effective path to athletic success and longevity.
One of the biggest challenges athletes encounter with strength and conditioning programs is that all the benefits they gain from training are reversible. All the hard work and performance gains an athlete makes during the off-season or pre-season can evaporate when this type of training is not maintained for prolonged periods of time.
The realities of In-season:
People are often surprised and have trouble accepting that they can lose their gains, especially young parents and athletes. The cold, hard truth is that more often than not, practices are simply not focused or intense enough during the season to stress a young athlete’s body enough to develop or maintain strength and fitness levels.
A head coach is, more often than not, focused on his or her own “one thing” during the season: winning. Simply put, priorities change once the season starts! Head coaches are instead more focused on tactics, plays, and improving whatever deficiencies were revealed in the team’s last game than they are on fitness and strength gains.
Let’s consider a basketball team: If they did not recover enough rebounds during the last game, that coach is definitely going to have the athletes work on lots of ‘box out’ drills in order to re-enforce technique and try to remedy the situation. Likewise, if the team’s offense wasn’t functioning properly, chances are that same coach is going to spend a significant amount of time in practice that week walking through all the plays at a moderate pace in order to “iron out the kinks” and fix any confusion.
What does this mean from an observational and practical standpoint? Most likely, the five starters on the team will go through the plays at a moderate intensity (at best) while the remaining 10 players stand around and watch from the sideline for prolonged periods of time. The truth is, almost any team’s in-season practice is going to consist of a lot of standing around, talking, and direction from the coach, with much less time dedicated to all-out scrimmages or drills attempting to simulate game-day conditions. This is supported by a scientific study conducted by Wellman and colleagues (2007) that compared the differences between pre-season and in-season practices and game times among NCAA Division I football players.
Whether discussing the height of collegiate sport or your average middle-school or high school team, studies like this one show that athletes simply do not experience the same kind of workloads during the in-season period compared to pre-season. The result is that players get weaker – literally losing strength.
In a study performed on elite male rugby and football players, McMaster and colleagues (2013) found that strength levels have a tendency to decrease after a three-week period when no form of strength activity is maintained. In addition, according to Meylan and colleagues (2013), the decay rates of strength parameters for youth athletes can show an even more marked difference, especially for those athletes who have not yet hit their growth spurt. According to the researchers, these athletes lost more strength and lost it even more quickly as compared to their peers who had already hit their growth spurt.
The Good News:
There are some very practical solutions that athletes can employ in order to mitigate the negative effects of the paradoxical in-season strength and fitness loss. If the mantra ‘use it or lose it’ applies – and it does – the simple solution is to ‘use it’ by strength training in-season. This does not mean that an in-season strength program should be the same as an off- or pre-season program. We know that athletes are spending a lot of time in practices and games, all of which require physical resources and take a toll on the body.
In a study conducted on male handball players (Hermassi et al. 2017), researchers found that as few as two sessions per week were sufficient for athletes to maintain their performance gains, while another study found that so long as intensity was kept high, athletes were able to maintain their performance gains with as little as one session per week (Bell et al. 1993).
Call to Action:
What can you do to safeguard and maximize your son or daughter’s performance gains that they worked so hard for during the off- and pre-season?
The answer is this:
Maintain an in-season strength and conditioning routine that can be executed in a little as one hour per week.
Our experience – and the experience of the athletes who train with us – confirms that this is all it takes to make sure they finish the season just as strong as they were at the start. In addition to meaning these athletes perform well during the season it also means that their strength improvements do not have to be regained at the end of each season, effectively accelerating their performance at a rate greater than their peers.
Bell, G. J., Syrotuik, D. G., Attwood, K., & Quinney, H. A. (1993). Maintenance of Strength Gains While Performing Endurance Training in Oarswomen. Canadian Journal of Applied Physiology,18(1), 104-115. doi:10.1139/h93-010