A–B. Active Living and Movement Competence · Active Living and Movement: Train with Purpose, Play with Strategy

Technique Lab: Phase Checkpoints and the Physics of Movement

B1.3 B1.4 1.2About 75 minutes

Learning goals

I can use checkpoints for the preparation, execution and follow-through phases to analyse and refine my technique.
I can extend all my joints at contact to produce maximum force in a serve.
I can bend my knees to lower my centre of mass and widen my base to improve balance, as in wrestling.
I can explain impulse (force × time) and use it to absorb force when landing and catching.
I can use video and feedback to correct my own performance.

Words to know

checkpoint
A specific thing to look for in one phase of a skill, such as "elbow high" in preparation.

joint extension
Straightening a joint (for example, the elbow or knee) to reach farther or apply more force.

centre of mass
The point where the body's mass is balanced; also called centre of gravity.

impulse
Force multiplied by the time it acts. The same change in motion can come from a large force for a short time or a smaller force for a longer time.

angle of release
The angle at which an object leaves the hand or implement.

constructive feedback
Specific, helpful comments about what to keep and what to change.

Remember from Grade 9

In PPL1O you learned the three phases of movement (preparation, execution, follow-through) and movement principles: stability, summation of forces (using body parts in sequence), Newton's third law (push against the ground), direction of force, levers (a straight arm moves the hand faster), and controlling spin.

Grade 10 goes further: you'll analyse skills with checkpoints on video, and add three more principles: joint extension at contact, lowering your centre of mass for stability against an opponent, and impulse for absorbing force.

Hook: same player, two serves

Keisha serves a volleyball with a bent elbow, contacting the ball beside her ear. It barely clears the net. Her coach says one thing: "Reach up and hit it with a straight arm, in front of your hitting shoulder." The next serve flies deep into the court.

She didn't get stronger in ten seconds. She used physics.

What B1.3 and B1.4 ask of you

B1.3: demonstrate an understanding of the phases of movement (preparation, execution, follow-through) and apply this understanding to refine your movement skills in a variety of activities.
B1.4: apply appropriate movement principles to refine skills. The curriculum's Grade 10 examples:
extending all joints to produce maximum force during the striking phase of an overhand or underhand serve in a net/wall activity;
bending the knees to lower the centre of mass and so increase stability in wrestling.

Phase checkpoints: the overhand serve

A checkpoint is one specific thing to look for in each phase. Pick 2 or 3 per phase:

Preparation

Opposite foot forward (left foot for a right-handed server), weight on the back foot.
Ball held in the non-hitting hand, in front of the hitting shoulder.
Hitting elbow back and high, hand near the ear.

Execution

A low, steady toss (or lift) about an arm's length above and slightly in front of the hitting shoulder.
Step onto the front foot and rotate hips then shoulders (summation of forces).
Contact with the arm fully extended, firm hand (heel of the hand or palm), in front of the hitting shoulder.

Follow-through

Hand continues toward the target.
Weight finishes on the front foot, ready to move into the court.

Three phases of an overhand serve. The key Grade 10 checkpoint is in execution: contact with the arm fully extended, above and in front of the hitting shoulder, after a step and rotation.

Principle 1: extend the joints for maximum force

When you extend (straighten) the elbow, wrist and body at contact:

the hitting arm acts as a longer lever, so the hand moves faster for the same body rotation;
force from the legs, hips and trunk passes through a firm, straight arm instead of being lost in a bending elbow;
you contact the ball higher, giving a better angle over the net.

The same idea applies to the underhand serve (swing with a straight arm like a pendulum), and to many skills: shooting a basketball, a tennis serve, a standing long jump (extend ankles, knees and hips together).

Principle 2: lower centre of mass and wider base for stability

You're harder to move when:

your centre of mass is low (bend your knees);
your base of support is wide (feet apart, often staggered);
your centre of mass is over the middle of your base;
your stance is wide in the direction of the force (a staggered stance if the push comes from the front).

Wrestlers use a low, wide, staggered stance so an opponent can't easily push or pull them off balance. The same principle helps a goalkeeper, a basketball defender, a lacrosse player in a faceoff or a hockey player battling along the boards.

Two stances. With a low centre of mass over a wide, staggered base, the athlete on the right is much harder to push off balance than the one on the left.

Principle 3: impulse (force × time)

To stop a moving object (or your own body), you need a certain amount of impulse = force × time. You can choose how to share it out:

Stiff: a large force over a short time: a hard, jarring landing or a ball bouncing out of stiff hands.
Soft: a smaller force over a longer time: bend your ankles, knees and hips when landing; let your hands "give" when catching.

This is why landing mats are soft, why goalies "give" with the ball, and why you roll after a fall in martial arts. It also works the other way: to produce more speed, apply force over a longer time (a longer arm swing, a full leg drive).

Force during two landings from the same height. The area (impulse) is the same, but the soft landing spreads it over a longer time, so the peak force on the joints is much lower.

Living skill: adaptive skills (1.2)

The curriculum's 1.2 example for movement is explaining how a positive attitude, practice and constructive feedback help you correct your own performance. When a classmate gives feedback, say "thanks" and try it right away. Change one checkpoint at a time; trying to fix everything at once usually fixes nothing.

Worked example

Let's analyse a serve from video

Diego films three of his overhand serves. Pausing the video, he notices: (1) his left foot is forward and his elbow is back; (2) he tosses the ball behind his head; (3) at contact his elbow is bent at about 90°; (4) after contact his arm stops at shoulder height. Use phase checkpoints to diagnose and plan one fix.

Preparation: good (opposite foot forward, elbow back).
Execution, toss: a toss behind the head forces him to lean back and contact the ball behind his shoulder.
Execution, contact: a bent elbow means a shorter lever and lost force (B1.4: extend all joints).
Follow-through: stopping early often means he's slowing the arm before contact.
Plan: fix the toss first (low, in front of the hitting shoulder); a good toss lets him reach up and contact with a straight arm, and the follow-through usually improves too. Film again after 10 practice serves.

The toss is the root problem. Fixing it lets Diego extend his arm at contact, giving a stronger, more accurate serve. One fix at a time, then re-film.

Worked example

Let's compare landings with impulse

Two students land from the same jump. Both need the same impulse to stop: 300 newton-seconds (N·s). Student A lands stiffly, stopping in 0.05 s. Student B bends and gives, stopping in 0.30 s. Find the average force on each.

Impulse = force × time, so force = impulse ÷ time.
A: 300 ÷ 0.05 = 6000 N.
B: 300 ÷ 0.30 = 1000 N.
Compare: 6000 ÷ 1000 = 6, so A's average force is 6 times larger.

Bending and giving (B) cuts the average force from 6000 N to 1000 N: much less stress on ankles, knees and hips.

Hands-on activity

Technique lab: serve, stance and soft landings

You need: a soft volleyball or playground ball, a wall outside (or a rope tied between two trees as a net), chalk or tape for a target, a phone to film (with permission), a partner, and a grassy area or mat.

Warm-up (5 min): jog, arm circles, shoulder rolls, wall push-ups, squats.

Serve checkpoints (15 min): stand 4 to 6 m from a wall with a target about 2.5 m high. Do 10 overhand (or underhand) serves. Your partner watches one checkpoint at a time (toss, extended arm at contact, follow-through) and gives one piece of feedback. Film 3 serves, fix one checkpoint, and film 3 more.
Stability test (8 min): stand tall with feet together. Your partner gives a gentle, slow push on your shoulder from the front. Then take a low, wide, staggered stance and repeat. Which was harder to move? Take turns. (Gentle pushes only, on grass or a mat.)
Quiet landings (7 min): do 5 small jumps in place landing stiff-legged but small (just a few centimetres), then 5 landing softly (bend ankles, knees, hips). Listen: soft landings are quieter because the force is spread over more time.
Catch with give (5 min): play catch; first with stiff hands, then letting your hands give back toward your body. Count drops.

Safety: keep the stiff landings very small (no high jumps landing stiff). Pushes are gentle and controlled; stop if anyone feels unsteady. Use a soft ball and check that nothing breakable is near the wall.

Just for fun

Newton never played volleyball, but every serve still has to follow his laws. He's the only person who's on every team and never comes to practice.

Your turn

Name the three phases of movement.

Give one checkpoint for each phase of the overhand serve.

Why does extending the arm at contact produce more force?

Give two ways to increase stability in wrestling.

Write the formula for impulse.

How does bending your knees when landing reduce force?

Why should you change only one checkpoint at a time?

More practice: warm-up

Which phase includes the toss in a serve?

Is a wide or narrow base more stable?

Should your centre of mass be high or low for stability?

In a soft catch, do your hands move toward or away from the ball?

Name one tool for analysing technique.

More practice: on your own

A basketball player shoots with her elbow still bent at release. Which principle is missing and what is the result?

Impulse 240 N·s. Stopping time 0.04 s vs 0.24 s. Find both forces.

Explain why a hockey defender in front of the net uses a wide, low stance.

A friend's underhand serve goes straight up instead of over the net. Which checkpoint would you check?

Why do gymnasts land on thick mats?

Challenge

A volleyball player jumps for a spike. Use joint extension, summation of forces and impulse to explain three ways to hit harder.

A 50 kg student lands from a jump moving at 4 m/s. Impulse needed = mass × speed. Find the impulse, then the average force if she stops in 0.1 s versus 0.4 s.

Design a 3-step video analysis plan for a classmate's standing long jump, with one checkpoint per phase and one principle you would look for.

Check your understanding

Quiz

In the curriculum's example, extending all joints during the striking phase of a serve produces:

Wrestlers bend their knees mainly to:

Impulse equals:

Bending your knees on landing reduces force because it:

Contact with the ball in the overhand serve happens in which phase?

A server tosses the ball behind her head. The best first fix is:

For parents and teachers

Big idea: Expectations B1.3 and B1.4 ask students to understand the phases of movement and apply movement principles to refine skills. Grade 10 examples: extending all joints to produce maximum force in a serve, and lowering the centre of mass for stability in wrestling. This lesson also uses impulse (force × time) to explain soft landings and catches.

How to help at home:

Film your teen's serve or throw (with their permission) and help them pause at each phase.
Be the gentle partner for the stability test.
Ask them to explain why bending the knees makes landings quieter and safer.

Common difficulties:

Trying to fix everything at once. One checkpoint at a time.
Confusing "harder" with "more effort". Technique (extension, sequence, timing) adds power.

Safety: keep stiff-landing comparisons tiny, pushes gentle and slow, and use a soft ball near walls. The impulse math connects to Grade 10 math and to physics in Grade 11 and 12.