Tennis Swing Faults: The 16 Technique Errors and How to Fix Them

01 What counts as a "swing fault"

A swing fault is a specific, measurable deviation between what your body did and what the shot needed — not a vague feeling that something is off. That distinction matters, because a measurable fault can be checked, tracked and confirmed fixed, while "your forehand feels wrong" cannot.

Every fault below is defined the same way: a named body measurement, a healthy range, a threshold at which it is worth flagging, and the correction. These are the exact definitions and thresholds TEKRISE’s measurement layer uses on your video — the numbers underneath the coaching, published so you can check your own footage against them rather than take a verdict on trust. If you measure 138° at your elbow at contact, this page tells you what that means and what good looks like.

02 How each fault is measured

TEKRISE tracks 12 body joints frame by frame from a phone video — shoulders, elbows, wrists, hips, knees and ankles. Pose tracking runs on the device (ML Kit on Android, Apple Vision on iOS); on Android the racquet and ball are located on-device too. Those joint positions are then turned into the coach-readable measurements below: joint angles, rotation separations, distances normalised to your own torso length, and speeds.

Normalised to your torso, not to centimetres. A phone camera does not know how far away you are, so absolute distances are meaningless. Instead, widths and travel distances are expressed as a multiple of your own torso or hip width — a 1.3×-torso stance means the same thing whether you filmed from 5 metres or 15.

Eleven checks run on every swing. Sixteen distinct fault labels can come out of them, because several checks produce a shot-appropriate variant — a weak coil reads as shoulder_turn on a forehand but no_shoulder_turn_1hbh on a one-handed backhand, with different coaching copy.

You will not get sixteen corrections at once

Every check runs on every swing, but the coaching you read back is deliberately short — it names the few things worth changing next. Handing a player sixteen corrections is how you guarantee none of them get worked on. Fix the top one, re-record, and see what comes back.

03 Faults at contact

Contact is the highest-value frame in the swing: it is where technique becomes ball flight. Two checks run here.

FAULT ID · arm_extension · serve_contact_too_low

Reaching — arm still bent at contact

Straighten your hitting arm — reach out to meet the ball.

The angle at your hitting elbow (shoulder–elbow–wrist) on the contact frame. A bent arm at contact means you are meeting the ball too close to your body, which costs power and pushes the contact point behind you. On a serve the same fault means you are striking from too low, losing both pace and net clearance.

MEASURED: Elbow angle at the contact frame, in degrees.
HEALTHY: 155–175° on groundstrokes; 160–180° on a serve. (Not 180° — a fully locked arm is neither normal nor desirable.)
FLAGGED: Flagged below 150°; treated as a major fault below 135°.
HOW TO FIX: Set your contact point further in front and further from your body. Shadow-swing and freeze at contact: if you cannot straighten the arm from that position without shuffling, you are letting the ball get too close. On serve, think "hit the ball at the top of your reach", not "hit it hard".

FAULT ID · wrong_stance_type

Feet and shoulders badly misaligned

Line your feet up under your shoulders — avoid an over-rotated base.

The angle between the line through your ankles and the line through your shoulders at contact. A large mismatch means the base you are hitting from is twisted relative to your upper body, which makes the shot hard to balance.

MEASURED: Ankle line vs shoulder line at contact, in degrees.
HEALTHY: Roughly aligned — under 35° of difference.
FLAGGED: Flagged above 35°, and only when the camera is roughly face-on.
HOW TO FIX: Reset your feet before you swing rather than during it. If you are consistently over-rotated, you are probably turning your feet to reach a ball you should have moved to.
NOTE: Low confidence by design. A single 2-D camera cannot know where the incoming ball came from, so this only fires on extreme misalignment, and is skipped entirely on angled or side-on footage.

04 Rotation and timing faults

This is where most easy power comes from, and where most club players leave it. Both checks below use hip–shoulder separation: how far your shoulders turn past your hips during the wind-up. Coaches call the effect "coil" or "the X-factor" — your torso stores elastic energy the way a wound spring does, and releases it into the ball.

FAULT ID · shoulder_turn · no_shoulder_turn_1hbh · no_turn_on_overhead

Not enough coil

Coil more — turn your shoulders back while your hips stay set.

Peak hip–shoulder separation across the swing. If your shoulders barely turn past your hips, there is no stored energy to release, and the shot becomes all arm. This is the single most common source of "I am swinging hard but the ball goes nowhere".

MEASURED: Peak separation between the shoulder line and hip line, in degrees.
HEALTHY: 45–55°. Above 40° is reported back to you as a strong coil.
FLAGGED: Flagged below 35°; major below 22°.
HOW TO FIX: Turn earlier and turn with your shoulders, not your arm. A useful check: at the end of your take-back, can you see your back shoulder in your peripheral vision? On a one-handed backhand, keep the non-hitting hand on the throat of the racquet through the turn — it forces the shoulders to do the work.
NOTE: This check is not disabled on angled footage. Its thresholds already account for the way a camera flattens rotation, so gating it would lose more real detections than it would prevent false ones.

FAULT ID · late_preparation · late_prep_backhand

Preparing too late

Turn and take the racquet back sooner — be ready before the bounce.

How far before contact your coil peaks. This fault only fires when your coil was actually good — a weak coil is the previous fault, not this one. The problem here is purely timing: you get into a strong position, but so late that the swing has to be rushed.

MEASURED: Milliseconds between peak coil and contact.
HEALTHY: Peak coil at least 120 ms before contact.
FLAGGED: Flagged under 120 ms; major if the coil peaks at or after contact.
HOW TO FIX: Split-step and start the turn as the ball leaves your opponent’s strings, not as it crosses the net. The cue "racquet back before the bounce" fixes this for most players. Nothing about your swing needs to change — only when it starts.

05 Base and footwork faults

FAULT ID · stance_width

Base too narrow

Set your feet wider — about shoulder-width and a bit more.

The distance between your ankles at contact, relative to your own body. Feet close together means you tip off balance as soon as the ball has any pace on it, and you cannot push off into the shot.

MEASURED: Ankle separation, normalised to torso length (face-on footage) or to hip width (angled footage).
HEALTHY: ≈1.3× torso length face-on; ≈1.6× hip width from an angle.
FLAGGED: Face-on: flagged below 0.95×, major below 0.7×. Angled: flagged below 1.2× hip width, major below 0.9×.
HOW TO FIX: Widen your ready position and land wider on the last step into the shot. Most narrow-base players are not choosing a narrow base — they are arriving late and taking one step fewer than the ball needed.
NOTE: Skipped entirely on side-on footage, where stance depth dominates and both width ratios stop meaning anything. The angled-view thresholds are provisional and still being calibrated against real footage.

FAULT ID · feet_not_moving

Static feet

Keep adjusting with small steps — do not watch the ball land.

How far your body centre travels across the whole clip. Very little travel means you planted and reached rather than moving to the ball, which is the root cause of a lot of faults further up this page.

MEASURED: Total body-centre travel, normalised to torso length.
HEALTHY: At least 0.6× torso length of travel across the clip.
FLAGGED: Flagged below 0.6×.
HOW TO FIX: Small adjusting steps between shots, and a split-step every time your opponent hits. If your stance width and contact point are both flagged too, fix the feet first — those two often resolve on their own once you are arriving on balance.
NOTE: A deliberately rough proxy: it measures movement, not good movement. A clip filmed during a stationary drill will trip it legitimately.

06 Weight and posture faults

FAULT ID · trunk_tilt

Leaning back, or standing too upright

Drive forward — do not fall away from the ball.

How far your torso leans forward at contact. Falling backwards at contact is the more serious version: it leaks power and is a classic cause of balls sailing long. Being merely very upright is the milder version of the same problem.

MEASURED: Torso lean at the contact frame, in degrees (positive = forward).
HEALTHY: 5–15° forward.
FLAGGED: Flagged below 3°; major below −5°, i.e. actually leaning back.
HOW TO FIX: Take the ball earlier and move into it rather than waiting for it. Leaning back is usually a symptom of being crowded by the ball, so the fix is often footwork rather than posture.

FAULT ID · poor_weight_transfer

Weight stays back

Push off the back foot and move your weight into the shot.

How much your torso lean changes across the swing. Little or no change means your weight never travelled forward — the shot was generated by the arm alone.

MEASURED: Change in torso lean from the start of the swing to contact, in degrees.
HEALTHY: At least 4° of forward shift through the swing.
FLAGGED: Flagged below 4°.
HOW TO FIX: Feel the back heel lift as you swing. Practise the shot from a step-in position, then rebuild the full movement around that sensation.
NOTE: Suppressed when the previous fault already fired, so you do not get two cards for one "no weight in the shot" root cause.

07 Swing speed and finish faults

FAULT ID · no_racquet_head_speed

Pushing rather than swinging

Let the racquet go — accelerate through contact, do not guide it.

Peak racquet-head speed through the swing. A low peak means the racquet is being steered to the ball rather than swung through it. This is where easy pace and spin are lost.

MEASURED: Peak racquet-head speed, in image units per second.
HEALTHY: A clear acceleration into contact.
FLAGGED: Flagged only when the swing is clearly slow, to avoid false alarms.
HOW TO FIX: Swing at a target beyond the ball, not at the ball. Relax the grip — a tight grip is the most common cause of a guided, decelerating swing.
NOTE: The threshold here is framerate- and zoom-dependent and is still being calibrated, so it is set conservatively: it fires rarely and only on obvious cases.

FAULT ID · no_follow_through · no_serve_follow_through

Swing stops at the ball

Swing all the way through — wrap the finish, do not stop at the ball.

The length of your racquet path after contact compared with before it. Stopping at the ball means the racquet was already decelerating through contact, which costs both spin and control. On a serve it also puts avoidable load on the shoulder.

MEASURED: Post-contact wrist path length divided by pre-contact path length.
HEALTHY: A ratio of at least 0.5 — the finish should be at least half the length of the swing into the ball.
FLAGGED: Flagged below 0.5.
HOW TO FIX: Pick a finish position and swing to it every time — over the opposite shoulder on a forehand, or down and across on a serve. The follow-through is not decoration; it is the evidence that you were still accelerating at contact.
NOTE: Skipped when the clip was trimmed close to contact — fewer than 3 frames after contact and there is nothing to judge, so it stays silent rather than guessing.

08 Serve-specific faults

FAULT ID · no_trophy_position

No trophy position or racquet drop

Set a trophy pose, then let the racquet drop behind your back.

How much your hitting wrist travels vertically before contact on a serve. Little vertical travel means the racquet never loaded up behind your back — you went straight from the take-back to the ball, skipping the step where the power is stored.

MEASURED: Vertical range of the hitting wrist, normalised to torso length.
HEALTHY: At least 0.3× torso length of vertical travel.
FLAGGED: Flagged below 0.3×.
HOW TO FIX: Pause at the trophy position: tossing arm up, hitting elbow high, racquet head pointing up. From there let the racquet fall behind your back before you drive up. Most players who skip the drop are rushing the toss.

These numbers are current, not frozen

Every threshold on this page is tuned as we see more real footage, and the four camera-view and angled-stance cutoffs are still being calibrated. The values here are the ones in force as of 7 September 2026.

09 What these measurements cannot tell you

Being straight about the limits is part of the point of publishing the numbers.

It is one camera, not a motion-capture lab. Every measurement here is a 2-D estimate from a single phone. It is reliable for spotting patterns and tracking change over time; it is not a laboratory measurement, and small frame-to-frame differences are not meaningful.
Camera angle changes what can be measured. Widths and stance measurements are projections, so they compress as the camera moves off-axis. TEKRISE classifies your camera view and simply skips the checks that stop being trustworthy rather than reporting a confident wrong number.
Some thresholds are still provisional. The camera-view cutoffs, the angled-view stance ratios and the racquet-speed floor are being calibrated against real footage. They are set conservatively so they under-report rather than over-report.
A fault is not a verdict on the shot. Plenty of effective shots break one of these rules deliberately — a defensive slice on the run is not supposed to look like a textbook drive.
Volleys and slices are the weakest fit. The rotation and follow-through checks were written around groundstrokes, so treat coil and finish findings on a punch volley or a defensive slice with more scepticism than the rest.
None of this replaces a coach. Measuring a swing is a small slice of what coaching is. It cannot feed you balls, read your opponent, fix your grip mid-rally, or tell you when to be patient.

10 Which fault to fix first

When several faults fire at once they are usually not independent, and fixing them in the wrong order wastes weeks. A reliable order:

Order	Fix this	Because
1	Footwork — `feet_not_moving`	Arriving late causes narrow stances, late preparation and leaning back. Fix it and several other faults stop firing on their own.
2	Timing — `late_preparation`	Costs nothing technical to change. You already have the swing; it just needs to start earlier.
3	Coil — `shoulder_turn`	The largest single source of free power, and it needs the two above to be in place first.
4	Contact point — `arm_extension`	Now that you are arriving early and turning, you can meet the ball out in front.
5	Finish — `no_follow_through`	Usually a symptom of the four above. It often resolves without direct work.

This is also roughly the order TEKRISE works through them, because severity and root-cause order tend to agree.

Frequently asked

How many tennis swing faults can TEKRISE detect?

Eleven checks run on every swing, producing sixteen distinct fault labels — several checks emit a shot-appropriate variant, so a weak coil reads differently on a forehand, a one-handed backhand and an overhead. The coaching you get back is deliberately short: it names the few things worth changing next, not every deviation it can measure.

What is hip-shoulder separation and why does it matter?

It is how far your shoulders turn past your hips during the wind-up, measured in degrees. It stores elastic energy in your torso that is released into the ball, which is why it is the largest single source of effortless power. A healthy peak is 45–55°; TEKRISE flags anything below 35°.

What is the correct arm extension at contact in tennis?

Around 155–175° at the hitting elbow on groundstrokes, and 160–180° on a serve. Not a fully locked 180° — that is neither normal nor desirable. Below 150° means you are meeting the ball too close to your body and losing power and control.

Which swing fault should I fix first?

Footwork, then timing, then coil, then contact point, then the finish. Faults are rarely independent: arriving late to the ball causes narrow stances, late preparation and leaning back, so fixing the feet often clears several other faults at once.

Can a phone camera really measure this accurately?

It is accurate enough to spot technique patterns and track change over time, which is what matters for improvement. It is not a motion-capture lab: every value is a 2-D estimate from a single camera, small frame-to-frame differences are not meaningful, and measurements that depend on camera angle are skipped rather than guessed at when the angle makes them unreliable.

Keep reading

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